2 YEARS of PARTS CANNON - Part 1 (Chevy Malibu - Intermittent Stall, Crank No-Start)
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- čas přidán 17. 09. 2021
- A viewer towed this 2012 Chevy Malibu from 3 hours away because it wasn't reliable enough to make the trip on its own.
Over the last TWO YEARS he has been battling an intermittent stumble/stall crank no-start issue.
This car has been to the dealer at least 4 times, as well as other local shops.
SO MANY PARTS have been replaced, and it keeps throwing different trouble codes!
Let's see if we can make this possessed car act up, and find the elusive gremlin under the hood.
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Enjoy!
Ivan - Auta a dopravní prostředky
That a customer was willing to tow a car for 3 hours in order to reach Ivan's shop is the most sincere compliment to Ivan's skill as a diagnostician.
I live in Ontario and even considered it myself ngl lmao.
It's a lot faster and cheaper than taking it to the dealer 4 times haha
Rent a Duramax and trailer and drive it across the country
Yeah. How many guys on the east coast could figure out such a dastardly problem? Ivan, Eric O, Scanner Danner, bribe Keith "the guru" DeFazio to make a housecall? Maybe a dozen other guys?
Ivan is super
"... everything seems to be happy... Maybe they replaced all the right parts." :-) Always satisfying to see a successful Keith Tug-Test. Nothing wrong with a good Cliff-Hanger. When the owner arranges for a 3-hour tow, ya gotta believe the problem is real. Nice work.
I had a Malibu Ecotec with similar symptoms, No codes. I noticed they only happened in the hot part of the day. I reached way back in my brain to remember back in the 60s and 70s we called that vapor lock. Well, to fix that we'd install electric fuel pumps as pressurizing the gas raises the boiling point. But the Malibu has the pump in the tank. I realized that the gas we use has alcohol in it which lowers the boiling point of the gas. Switching to alcohol free gas solved the problems. Really only needed it in the summer months. I should note that at the time I was at elevation in the Denver area so that exacerbated it.
That Thinktool scanner looks to be fully featured and fast. I'm tempted but it's a lot of $$$ for just a DIY guy. If I had a weekend business or a fleet then I wouldn't hesitate. Being able to force switches and such is a very powerful troubleshooting tool.
I agree. I would love to have that tool but that's a lot of money to spend on something that would be used infrequently.
check the harness near the oil filter, sharp edge on block. also follow back front o2 sensor wires under coolant tank rub point back there.
Once again you have given enjoyment to an old tech. Gotta say kinda over the heartbeat of China!
Haha, sadly not a single manufacturer can escape that, not even 1.
We want part 2, we want part 2, we want part 2! Lol! All the drama and twice the fun with Ivan!!!
Ivan, thanks so much for making and posting these videos. I can't tell you how much I learn from watching them. My son's car had an intermittent problem which we chased for over a year, and replacing the engine wiring harness finally solved it. Had I known the "tapping" and tracing techniques, I probably could have fixed just one wire!
Good to see your still at it. Keep it up brother.
This has been a busy summer!
This is going to be a good one. Watching as I type... 🤣😂🤣😂
Ivan you work on many different cars under many different circumstances and still manage to get to the problem!! I also say if ya find the problem you can fix it too! Finding the problem isn't always that black and white. Keep on diagnosing and bringing great content to this channel.
Excellent case study, possessed car, lol!!! Waiting for part 2!
I had a Malibu Ecotec with similar symptoms, No codes. I noticed they only happened in the hot part of the day. I reached way back in my brain to remember back in the 60s and 70s we called that vapor lock. Well, to fix that we'd install electric fuel pumps as pressurizing the gas raises the boiling point. But the Malibu has the pump in the tank. I realized that the gas we use has alcohol in it which lowers the boiling point of the gas. Switching to alcohol free gas solved the problems. Really only needed it in the summer months.
Your videos have given me some excellent information.
You should make a few rules of car diagnostics and attach one to each video. I have a few. Keep the parts cannon locked in a secure place. Never replace a part if you cannot verify that it is defective.....
I just did a timing job on a 2012 malibu a month ago. It had a map sensor code in it also. 2 previous shops had replaced the Map sensor in effort of repair. I also replaced it with aftermarket (it was cheap and easily changed) so I took a chance. Car came back 2 days later with Map codes. I replaced it with a used OEM unit and have not had any issue.
What I found was the scaling in the aftermarket MAP sensors were not correct.
Yup OEM ONLY for any electrical components is my rule :)
@@PineHollowAutoDiagnostics I for usre used OEM timing components and try to use OEM whenever possible. Not every situation is equal and it was a car lot special car.
Like #424
Thanks for the video!
It's a good morning when PHAD and WWW uploads!
Your experience is showing!
Fascinating. I saw that baro reading at 14 in the beginning and though that was odd. Thanks Ivan!
Hey John! What's wrong with that Baro?
Wow Ivan......chasing it down. GO BIG RED GO.
Hello Ivan from Australia ;) Part 2 oh no I was just getting nicely into that 🤣😂
Can't wait for part 2
NOW you've got my attention! On to part 2!
Now this car is in hands of great great tech mr.ivan now it will do bbq instead of salad of codes😉
Omg I have learned so much from your videos.thank you
You do get the good ones my friend. I'd love to collaborate with you one time with one of these crazy case studies.
Cleared DTCs will often remain stored in generic obd until the appropriate drive cycle passes , i always check this when customers have cleared codes before bringing it to me .
Totally random faults I suspect a intermittent bus short/issue but u never know when so many hands have been in the mix
New video! Just what the doctor ordered!
Enjoy!
I really am not looking forward to when newer cars start getting old. It's not that they're all that difficult to diagnose and repair, they just need a different set of skills. But the technicians we have in the field today just don't know as much as they should. That's how you get these situations where thousands of dollars are blindly thrown at a car hoping one of them sticks.
And now with the rise of EVs, manufacturers are beginning to restrict the sale of parts and service information and are not working with aftermarket scan tool companies either. They're all copying Tesla. ICE vehicles need to have basic OBD2 and need to be J2534 compliant, but that's just because of emissions. EVs don't have any mandated diagnostic protocol anywhere in the world because they're zero emissions. No OBD2. No J2534. Nothing. Teslas used to have an OBD2 port due to an old poorly written law. The port was very rarely used by the techs in the service center and later the law was changed to not require that port for EVs. Other manufacturers are still putting OBD2 ports in their EVs because they also make ICE vehicles and want to keep the same diag equipment and software for simplicity, but their EVs don't need to follow any existing protocols. However, most still do. For example, Ford IDS and GM GDS2 and their existing interfaces are still used for the Mach E and Bolt, but that can change literally at any moment. The Mach Es rolling off the assembly line today could require totally different hardware and software to diagnose if Ford wanted to and literally nothing is stopping them.
We need better training for technicians and we need right to repair laws passed worldwide. We also need governments around the world to come up with and settle on a mandated diag port and protocol for EVs.
I work at a ford dealership, I’m the painter in the body shop. Where I work, in the service department 4 out of 6 techs are ford certified senior manster techs and one of them is our dedicated diesel tech. A very lengthy process (years) to achieve. Not one of those 4 are worth a shit. In fact only one of the 6 techs are. My bosses 6.0 power stroke shit out. My boss told him what was wrong, what needed to be replaced. The tech insisted NO, that’s not the problem. After three weeks of him tossing parts at it with zero diagnosing being done and claiming ford doesn’t have a diag procedure for what he’s trying to diag, he amassed over $10,000 in parts and labor and still hadn’t fixed the problem. Finally he replaced what my manager told him was the issue in the first place and it it fixed it! Needless to say, my manager had a lengthy bitch fest with the owner, general manager and service manager. In the end, all he paid for was the cost of the part that fixed it and the labor to the tune of $2,300. The dealership ate all that $10,000 in needless parts.
@@chadvalliere8697 that level of "lack of knowledge" is very prevalent these days. No one is taught basic electrical systems in school, so unless you go out and pursue your own education they just find it "black magic". The warranty repair people for appliances just throw whatever part the factory says at it, and don't diagnose problems. That's why i'll never get one of those modern tankless furnaces... I know the out of warranty support isn't going to be there and i won't be able to get parts.
2 years ago i got a free 1700 refrigerator because the techs couldn't fix the ice maker. I found out that root issue was the ice catcher door switch was stuck so the computer thought the catch tray was out, but the techs replaced 4 other parts and they called it a lemon and i got my $ back.
I since moved and left the fridge, but everything i own now is basic and self repairable, i will not trust big $$$ to these modern appliances with "toss parts at it" repair practices, and 0 parts availability after warranty.
Until the workforce changes and people learn, this will stay. If a code can't tell you where a broken wire is, or the EXACT issue, you better pray you have a good tech!
Don't forget the wonderful flat rate pay system, quite frankly it's an evil way to pay people. I remember back in the day we would diagnose a car and give the estimate to the customer. The customer would bitch it's too high cost wise, the service manager/advisor would cut labor cost(which of course costs the tech money) just to sell the job. Of course the parts department would be asked to bend on their ridiculously high parts prices to no avail. So ultimately the parts department shows profit and the service department gets screwed and the tech looks like they are doing less work when in fact they are not. I do agree better training is needed but the flat rate system isn't helping. If they paid techs what they are worth, much of these problems go away.
My new truck is 12 years old LOL
@@jdtractorman7445 Thanks for saving me the time. You nailed it! It doesn't PAY to hunt the actual issue in most shops. Throw a part at it and move to next vehicle. Or just don't work on those types of repairs. Do enough wrong, they quit giving them to you. I always got them AFTER the parts cannon was loaded. Retired last spring. Take that job and shove it!
the suspense is killin me!
I was going to sell my car and get a new one. I don't think so now. Great video!
Exept for the broken wire found , always
comes to my mind with those intermittent codes , bad grounds with heavy loads on the system ,that stalls the car or ECM defective
Ivan, I have owned my 4th Toyota now. For me personally, I HAVE NEVER NEVER seen wiring issues like this car. The worst issue I had was a melted wire plug for one of the fog lamps on my 13 Tacoma. Our 04 Corolla never had a single wire issue...same for my wife's 16 RAV 4. I ask...what kind of pure unadulterated crap does GM build. About you Ivan....you should work for the FBI or better yet, our own RCMP....Man, your sleuthing skills are off the charts.
Going on the first 5 mins or so of history low voltage and various fault codes etc my guess would
Be battery / charging circuit ground and power issues connector maybe also cold damp conditions may have a bearing but looking forward to finding out lol
Another great video.
We have the 2.4L. Runs much smoother and quieter after cleaning the ThrottleBody.
I do know the invalid serial date code pops up when you have a driveablity related code in the pcm . The first time I ran into that code it sent me out on a hour long wild goose chase LOL.
Same here. I can't believe GM wrote it that way, very dumb.
The Can Bus OFF U0073 was even more weird..
Makes sense the radio is on there, you know u can control security and many other things through their radios
Also the grounds on the 2.4L cars were painted over. That can cause computer issues after a few years. Remove some easy grounds and clean the drit/paint and reinstall.
I see someone beat me to it but I came here to say the reference 5.02 - 1.18 = 3.84V. When I see voltages like that I always try to do some simple arithmetic because it could very well be doing a subtraction so that voltage scales linearly with manifold pressure, instead of inversely. Also I would expect the MAP displayed PSI to match the baro displayed PSI when the engine is not running.
So what is a good reading from a MAP, voltage and pressure? ECM was showing less from the scanner than the back probing with the meter.
@@bobbydelamar606 I think the ECM was doing math on the voltage before displaying the PID value. As far as a good reading your MAP pressure should correspond to manifold pressure which you could check with a vacuum gauge.
With all that stuff my first guess is the engine computer. Miss vid of putting the bushing in the Austin Healy 3000 King Pin I should say.
The A/C stopped functioning in my 2014 model year vehicle earlier this year due to a broken signal wire on the refrigerant pressure sensor. Luckily I was able to find the problem on my own (well, mostly on my own) and repair it for only $22. Auto manufacturers have been using thinner gauge wire recently that breaks too easily when not enough slack is added to the harness connectors. Luckily mine wasn't an intermittent failure like this Malibu or I probably wouldn't have tracked it down on my own.
Ivan, my 05 Pontiac G6 has the same voltage low on the radio. Never looked into it since the radio works fine. Curious if it's a GM thing.
"странный глюк" - это очень удачное и подходящее выражение здесь... Надеюсь во второй части я услышу ответ, почему в режиме Generic OBD отображаются не существующие коды.
wow getting in deep here...what a mess of codes
nodding my head in appreciation for that diagnosis and fix sometimes a good old wiggle and shake finds the issues.
TPS & MAP intermittent errors points to defectice (frayed/corroded) cabling or a basic engine block ground problem.
That was good but I really miss watching the work you used to do on those worn out building site machines !!
I fixed them all and they are surprisingly running fine now for about a year!
@@PineHollowAutoDiagnostics I have an 08 malibu. 3 yrs ago it wouldnt start, crank, turned out the copper band in the fuse box had a crack in it. Had it soldered. 3 yrs later still no problems.
Nice cliffhanger...
Now Ameritrash. Love it.
At least it's not engine-swapped 🤣
I heard you fart at 15:36 LMAO!
Ivan, on GM's at least, anything newer than 2008/2009 keeps emissions related codes historically in the computer under OBD, until they complete a certain number of drive cycles and the problem no longer exists, It will turn off the MIL, but it will stay in the computer. . This was done to prevent people from just clearing codes right before they sell a car to make it look ok.
What you stated on your last sentence, is that the same as when codes go into history? AKA history codes? Thanks.
Hey Ivan , what are your views on the topdon phoenix plus versus the thinktool pro, I currently own the autel maxisys ms908 and its too slow in my opinion. In your opinion, if price wasn't an issue which is the best all round tool. Thank you in advance. Great content keep up the amazing work
I really want to see what the coverage of these thinktool pros are like in australia.
Another GM to keep you busy… Planned obsolescence. Starting about mid 2000s they use the smallest gauge wire possible on all their electronics. What could go wrong? It was cool seeing the throttle body open and close when you wiggled the MAP wire.
Jim, that was at least the second time that GM has reduced the wire size in its harnesses. I remember the first time, in 1968, when the Vietnam war got going in full swing and the military bought so much copper for war materiel that its price went way up. GM immediately reduced the size of the wire in its harnesses by 1 or 2 gauges, and it caused all kinds of problems.
Leave it to the bean counters to F--k up what in the past were decent vehicles
It might be a common problem such as the power supply to the PCM is bad if more than 1 fault code is triggered at once.
It could possibly be caused by a corroded terminal or terminals & it's hard to Pick up unless the wiggle test is done,it was a bit like a 1997 Model Ford EL Falcon Futura which I recently fixed up where the fuel pump relay was recently replaced & the engjne still stalled & it was hard to start in the mornings,I replaced the EFI relay as I tapped on it which would cause the engine to stall & I even soldered the battery cables & I replaced the ECU which had a split resistor in the power supply.part of the ECU,that fixed the stalling issue but not the hard starting issue.
Well,it was time for me to check the fuel pressure which was meant to test at 39 PSI (2.7 bar) but it only tested at 30 PSI or 2 Bar of Pressure,I crimped the return line & it went right up to 40 PSI so I diagnosed it as a bad fuel pressure regulator.
Since Bosch don't make them for that Ford Falcon anymore I decided to order a DR2 style fuel injector rail from a DOHC 2005 model Ford SX Territory which shares the same engine as the BA Falcon,the EL Falcon has the SOHC 4 cylinder engine in it,the fuel rails & fuel pressure regulator are interchangeable as they run a 2.7 bar DR2 style Bosch fuel pressure regulator.
I swear that it must've only been putting out about 130 kilowatts before because it was running lean & lacked balls,now it's putting out more like the factory power level of 157 kilowatts so that has made a huge difference in performance.
It still had a hard starting issue in the mornings but which was due to corroded Hall effect sensor terminals on the TFI (thick film ignition) module so I cleaned them up, applied dielectric grease to the terminals & Reinstalled the TFI module onto the distributor.
Now it starts pretty much first go in the mornings,the thing with the older Fords such as that is that they need a decent voltage to start so if it has a weak battery & it's such a Ford then it may not even start.
I do have a power steering issue & sometimes starting (cranking) when hot with my Holden VY SS Commodore which I put down to the power steering lines, rack & the starter motor solenoid when the engine gets warm.
It wasn't a problem in the factory released cars which weren't fitted with performance exhaust headers but it's been a problem ever since I bought the car in 2008 with steering at least.
The power steering pressure tested o.k. & I rebuilt the power steering rack as it leaked & I rebuilt the power steering pump as it leaked as well & the flow control valve was stuck open.
I freed it up & the pressure was all good but it seems to fail when the engine gets warm,my solution is to wrap up the exhaust headers with that exhaust system bandage to insulate it from the power steering system & the starter motor as the steering is good when the engine is cold !
The harness routing and construction on these chassis are horrendous. Peak GM.
Do these push button ignition switches have any issues with the connector melting?
FCA will be will be introducing a new network topology ( Atlantis High Architecture ) new protocols CAN-FD and a new VCI and MDP . it will also be a cloud based subscriptions and new scanners per priority of course .
Please help where you buy the wiring diagram I bought all data hard disk china every think is blur please tell me where you got it brother I am from Malaysia I always follow you in u tube and learn alot from you thanks
Can we get a little bit of a spoiler like did you find the issue or will there be a part 3 ??????
Another who dun-it episode! I know…the GM engineers!
I have a 2000 Saturn SL, I had the same problem with all kinds of fault codes all at once, it ended up just being a Temperature sensor. Once I put that in, everything else went back to normal. Pretty weird.
Yes really weird 😁
Parts cannon?
Parts battlefield!
Hi Ivan , how are you ? I have a question about the scanner Thinktool pro on the Obdll scanning on a vehicle. The first line match, and the rest of the line is fail . Is that normal ? 🤔 hopefully I will get some update from you Ivan . Thank you for your time.
That's just the protocol search :)
Ivan, i must be loosing my mind with this fangled 1.2, 3.8v etc. cant really keep up as I figure it is inverted or something? 1 bad wire!! wow, i'll see part 2 and see the progress!
also I had to overhaul 2003 dodge ram 3500 Cummins battery and ground system, the pass side battery melted a hole near the + post and the clamps on both sides was so bad! both batteries was not showing even volts causing the alt to overload the batteries, apparently, it is common for dual batts if the PCM can command the alt to keep on charging when both aren't similar volts!! could have had a fire and burned that damn thing to ground! dealer had no parts in stock! had to buy 2 gage wires from audio shop, it was dam thick as hell. i soldered the lug inside for wire to go through, did all of them. repaired all the ring terminals and now it runs sweet and perfect, wish i could show you the pics of the carnage!! client was so happy! haha, great video, Ivan, look forward to pt 2 :-)
Feels good to fix up a mess like that 😉👌
@@PineHollowAutoDiagnostics oh hell, yes! sure does, LOL
I wonder if those wires are too short and get tweaked with the engine moving in the engine bay.
I owned one that ran perfect but from time to time you shut it off and it would not crank, engine would not turn over. Had to take the positive battery cable loose and let it reset after that it would run fine. Dealer took the car several times replaced parts. No fix. Dealer could never get it to act up. I finally traded the car off at a loss.
Awesome
I'm super curious about those communication errors. I'm suspecting we're dealing with two problems. A car with a messed up communication bus is psycho.
The steering wheel horn portion looks like the work of a cat
musical chair codes :)))
I'll guesses ECM failing or can bus issue from wire or other module pooing
What are the numbers written on the engine cover?
"It is what it is"...put that on a t-shirt!
The owner said that some problems started when she turned on the radio. Did you turn on the radio during your test drive?
May be the data line in that conector to the throttle body
Don’t chase or worry about the EBCM false DTC that’s the module way of telling you that it didn’t receive the right or any information needed from the ECM thus setting traction light & service traction control system
When the ECM has a problem and goes into default strategy it doesn’t care about any other module
GM TECH OUT!!..
Yup figured as much. What about the U0073 Comm BUS OFF set by the ECM??
@@PineHollowAutoDiagnostics not sure yet I’m driving to work now 😆
@@PineHollowAutoDiagnostics so reading here “ the control module setting the DTC has attempted to establish communications on the serial data circuits more than 3 times in 5 seconds”
The serial data circuits are serial data buses are used to communicate info between the control modules
Serial data circuits also connect directly to the DLC
The module suspends all message transmission
So I’m thinking it’s just default strategy for the ECM
Good morning
Good morning, David! 🌞
Is it possible the scan tool is the reason for the weird CAN bus errors?
Scan tools do not set codes…. They just report them. I have seen some scanners report codes others haven’t. Thats why you have to have multiple scanners unfortunately
Around 7:05: How would the cat monitor run without the O2 being completed?
Great question!!! Not sure how that makes sense...
Ivan tell us the battery is good... Just asking. 🤣👍
This looks like a very interesting one. Just wanted to note that you do a great job at both diagnosing issues and fixing them, as well as camera work. Very good, and thank you for dong it.
I hope that maybe on this one you will scan the car with a different scan tool (maybe the Autel, if you still have it), to see if you get the same codes, given their strangeness. On one hand, I am wondering if your new tool might be showing non existing codes, or incorrect codes. On the other hand, given the symptoms described by the owner maybe the codes are correct. Another scanner might answer that question as well as help solve the issue..
The MAP sensor, I don't think, could cause all the issues the customer described, and the connector harness which showed at least a issue has a lot more wires running though it than the three (3) MAP sensor wires, maybe crankshaft sensor, or camshaft sensor, or something that might explain the seemly unrelated codes, since they do seem to run to the ECM.
Again, great videos. Keep it up, and thanks again..
that's a big part of the problem. Half the tools are messed up on half the cars and it's never the same half of either!
So bad MAP sensor readings will cause a crank, no start?
On this car apparently yes!
Multimeter on the MAP sensor green wire should read 4.7V with engine off unless you're below sea level atmospheric pressure (on top of a mountain or something) and 1.2V at idle with around 20 in.HG of vacuum (around 4.5 psi absolute pressure). I would ignore what the scan tool shows as voltage because that's probably an inverse and concentrate on what the ECM is getting at its connector. If it's getting correct voltages in both engine off and engine on conditions, then the ECM is busted, which would also explain the serial bus errors.
However the MAP was not showing 4.7V with engine off, but 3.85V, which would correspond to an atmospheric pressure of around 12.2 psi. If Ivan is not high up on a mountain I would say the MAP sensor itself is trash (maybe some Dorman part 😂). A known good MAP is needed here for proper diagnosis imho.
Or maybe it's both the MAP sensor and the ECM, it's a Malibu afterall, they're cursed.
you know something are you a car mechanic?
Where did you get those specs?
@@PineHollowAutoDiagnostics Service info for an european car using the same engine (Opel Antara / Chevrolet Captiva)
Can Bus Off.
A faulty signal was received from the map.
Eng ecu stops can signal to stability ecu.
Therefore can bus off code?
Perhaps?
Don't you love GM, always something just a little messed up
not just gm , all cars
I would say GM and BMW make the best case-study cars!
@@PineHollowAutoDiagnostics Eric O. seems to always complain about Fords. I gotta admit that the fuse box not having anything but numbers for the fuses and relays is very annoying. If you don't have the owner's manual you're screwed. I printed up a cheat sheet and keep it in the console of my F150. What I thought was a headlight hi/low relay problem turned out to be the switch in the stalk.
I would have asked the owner about clearing trouble codes and if the car needed to cool down before restart.
Having a history viewable in the scanner eliminates a few steps.
What a confused obd system.
Maybe the can bus does have a semi short or a device thats ignoring communication rules.
I did wonder if the engine computer has a crusty connector.
I would still run away lol.
Anyone know of a good diag guy in the Delaware, MD, PA area?
Unless you find a broken wire or bad grounds at 124,000 miles they've gotten there money out of it. Time to sell it on to the next sucker / owner.
Which scan tool are you using?
Never mind found out
This is going to be a dilly! 🤣
From the way the harness looks. Someone has been fixin stuff. When I fix my own harnesses. I usually replace the harness covers.
When I diagnosis other vehicles and I see a wiring mess I cringe and not touch anything. I read the codes and see what is ailing the patients.😄.
Then close the hood. Give them the report and RUN LIKE HELL.😄😄
I try my best to fix what I can and if they need a 🐱 I tell them go to a shop for new cats. Somethings like CAN COMM issues. I tell my customer's. I don't have a scope to see what the NETWORKS is doing. Plus that goes over their heads. I just tell them take it to a shop that can see the NETWORKS issues.
Get a scope! CAN issues are fun!
Step one. Fire the parts cannon!
That wiring under the hood is terrible. Is that GM spec or did the dealer leave it that way?
👍
👍👍👍👍
What is that horrible grinding noise?
Malibu p0615 please
Throw away the crappy car battery. I have been down this road. I have seen 3 other vehicles go down this mystery path. It is the battery.
I would not be using the battery negative when checking wiring I would use the ground wire in the connection. Like you found before the ground in the harness could be affected but the computer or other device. If the voltage I’m looking for is incorrect then I would test against chassis ground. If the voltage is different when using chassis ground the issue could be in the control module where you may have an open due to something like a bad solder joint or heck a blown transistor etc in the module. Your not checking this way can lead you to bad conclusions.
GM wouldn’t have near the amount of electrical issues if they would use decent protection on their engine harnesses. The plastic loom they use is always missing from getting brittle and falling apart. These cars are the worst too. Almost every one that comes into the shop has harnesses that are completely bare.