Let's fix this 1991 Ford Ranger with a 2.3L that cranks but won't start and has no spark. Send us a postcard: Watch Wes Work P.O. Box 106 Fulton, IL 61252 Send us an email: mail@watchweswork.com
Don't feel bad. I saw a similar problem on a medium-sized CNC mill. One loose power connection screw (there were six) caused all sorts of faults in the machine. It took a very experienced electrician all day to trace the problem. A fix is a win. 👍
Had an E-stop button switch do that to a CNC years ago. All the panicked hand-smashing of the E-stop all those years loosened the connections just enough that the machine would not alarm out but would jump/shift its position. Causing all kinds of inaccuracies. Everyone was saying the control was no good! The boards are bad! The power from the electric Co. is bad! ETC. NOPE! A loose wire at the E-stop was all it was. That same machine still makes good parts from me tightening a screw 20 some odd years ago. Nutty huh?
We had a NC wire wrap machine in the factory that would trip out with a limit switch fault. Problem was, it was never near a limit switch when it tripped out. We ended up building a latch to capture the state of all the limit switches when it tripped to figure out which one was intermittent. The latch told us it hit all the limits at the same time. Loose terminal on the power feed to the limit switches.
Once again our seriously patient uber professional intrepid mechanic Wess takes on a brain twisting problem that's slightly agravating yet he still manages to fix the problem without multiple tossed tools and cussing that would make a 30 year minimum Chief Petty Officer blush. Another outstanding episode to entertain me on my way to the VA hospital for a pulmanary apointment. Thanks Wes.
He edits out the thrown tools and the cursing, let's be real. Conversely, we can tell Mustie1 has no such character aspects because he's always laughing and having a good time. At least we think so...LOL 😀
My kid had one of those truck , but with the 2.9 V6 . Called me for help one day . He had been off roading it and it quit on him . Behind that same kick panel was a switch that killed the electrics in case of a collision. Pushed the reset button really really hard and it reset . And I mean really hard . Good to go after that . I was a hero for a day lol
Wes you never fail to amaze me with your methodical electrical diagnosis! I have a buddy that I grew up with who is a professional Mechanic like you. I always call on him to fix the electrical gremlins that I run into. He in turn has me do all his welding projects. If you know, you know! LOL!
Wes I am glad you are part of a special group who looks to maintain these older vehicles and not give up on them. Granted the computer controlled vehicle is here to stay, there are still not many people who can and will take the time like you and the special class of mechanic you belong to.
Bad connector... I don't know how many times I've made a dead car start again by unplugging a connector and plugging it back in. Good thing you found it before ordering the ECU!
Obviously something was nagging at you to check again before firing the parts cannon, and that's what matters for a customer - making sure of the diagnostic where possible.
Don't doubt yourself Wes 🙂👍 You done a great job with the diagnosis and also the video. Glad to see you're doing well. Take care, be safe and happy wrenching 🔧
Dude hats off to you most mechanics would have given up half way through! I wish I had someone like you where I live. I have a 91 and a 94 and I love then both.
Wes, Thank you for putting out this video.I have been chasing my butt trying to figure out my issue with my 95 Ranger 3.0. I have the almost same identical issue. Only difference was I could crank and run it while it was completely cold. I threw the parts cannon at this turd. I replaced the fuel pump, filter,modulator, coil, crank shaft position sensor and the PCM. It wasn't till after I saw this video did I even try the coolant temperature sensor. Once I replaced, it crank and runs every time! Who woulda thunk it!!! Thanks for all the great content. Greatful, Jim Smith
Don't be hard on yourself. You told the story like it happened. You were on the right track from the get-go. This happens to doctors. Lawyers and such that they end up not knowing how or why the outcome was good , thats all that counts in the end. Sometimes, it's better to be lucky than good.
Yup. Ford's dual ignition system idea. It was my understanding that the idea of two plugs per cylinder was thought to provide a more complete fuel burn and, therefore, fewer emissions out the tailpipe. As a general rule of thumb Wes, if you find something on an internal combustion engine that is unnecessary, overly complicated, or just plain stupid, you can bet the shop our good ol' Uncle Sam was in some way involved when the dubious idea was hatched. Great job tracking down that particular nasty gremlin Wes.
I have had this exact problem on my 89 f150 on the ignition control module, boy oh boy it was a fun one to figure out and find... Wiring diagrams are gifts from God
Your hamster powered internet must be running hot Wes, It's so good to see you posting more often! Also can you buy an oil burning RAV4 cheap from down south and swap your rebuilt engine in?
The old Ranger was one of Fords best vesicles ever made in fact still many around (the small Chevy and Dodge trucks of that era not so much all rotted away or quit running.....new Rangers well remains to be seen.Wes is a wizard at repair of all kinds think how much he will know by time he gets gray hair and a few wrinkles.
Wes, Great job on that one! I had a 90 Ranger of the same Flavor. Your Connector delete is the ticket. Had the same issue. You could crank on that thing all day long but wouldn't start. I went through 2 computers before we started looking elsewhere. My buddy just happened to pull that connector apart and found all the green crusties and burn marks. We deleted and that truck never had that problem again after that. I sold it and it went until around 2006 or 2007 when that old man finally junked it because the frame rotted out.
Oh yes, Ford's great idea for dual ignition systems that have 8 plugs on a 4 cylinder. And one would think the addition of the second ignition system would yield extra power...not in this universe. You seriously have to understand how to pull out in traffic if you want to survive such a blend into traffic move without a hospital visit or worst! Love your diagnostic efforts every time. I say you have the absolute best vehicle problem-solving channel on UT... Thumps Up!
More spark doesn't equate to more power, I mean what are you going to do ignite the A/F even harder or twice? I think it's an emissions thing but I know nothing about that
Ford wants to build more EV's which I think is a terrible idea becuase every Ford that has been in my family since the 1960's has had some kind of electrical problems straigjht from the factory!
@@Hjerte_Verke Don't know how it works but Nissan built a 2ltr 4cyl twin spark fuel injected in the eighties & they were fantastic would do 200kph easy & reguarly.(I had one).
I give it 3 months and that Ranger will be back ... you've made friends with it now and it will miss you. The perils of not using hammers unnecessarily ... Thanks for sharing 🇨🇦
Yep, when I talk about replacing 8 spark plugs on my four cylinder truck, people look at me like I'm nuts. I loved that little truck. I still wish I'd kept it. Good save on a 32 year old truck. I almost bought one just like that. It probably sounds stupid, but that red dashboard was so close to the same as that on my '82 Escort, and that car had been such a piece of junk, that I chose the blue one because I couldn't stand the reminder. That Escort forced me to learn mechanic skills. I couldn't afford to pay for all the work it continually needed.
I appreciate Wes' transparency and thoughtfulness, both with the repairs and the videos. It does make me wonder however, how many PCMs have been replaced over the years just due to faulty connectors, where the problem was actually fixed just by virtue of removing and reinstalling things. Of course integrated circuits and printed boards are not infallible, but I think too many folks assume they are at fault just because they don't take the time to understand them.
Great job! It's very frustrating when it just starts working when you barely touch it. I love the line "don't thing I have enough editing skills to make me look like I know what I'm doing" 🤣
Wes, your vijayos have given me the confidence to, to, to, well, drive my unrusted snot bucket to upper Illinois from Washington state and see what you can do with it. 1980 f-100 Ranger with 5.0 Mustang mill. Cheers, K
drove one of these 4 cyl. 8 plug machines back when I was a kid... in the mountains of New Mexico, at that altitude the 5th gear was only useful when going down hill
Enough editing skills to make it look like you know what you’re doing? Wes, this is textbook diagnostic work. You are a awesome technician!!! This is the kind of fix that makes a man legendary. I’ve seen too many “technicians” just aim and fire the old parts cannon.
Great video Wess. A man that can miss diagnose something then is man enough to ad mit it. That's just good ole honesty. That why I love watching you chanel . Keep coming. .
Sometimes we get off track...ya still fixed it which is what is important. My OCD would not have let me get rid of the connector. I'd have put a weatherpack/metripak in it just to annoy the Ford guys...and I have a ready stash in the shop. 🤣
Sometimes you get those what the heck and still get it fixed... Dont ask, just appreciate it got fixed. 50+ years, yeah its all good Wes. You did good.
Beautiful truck I love the roll up windows. Wes your good bud and you get better all the time. I notice your tools are always clean and your never challenged. Plus your funny.
I've also run into some confusing wiring problems on older Fords. On a friend of mine's truck I had no fire at the plugs. Each individual component seemed to test good. Finally I just took apart and cleaned every connection in the system. That did it and he said it had never run better. A couple of other times I had crank no start and tested it with an extra ground cable from battery to engine block. That fixed both of them and I don't really remember what pointed me in that direction. Maybe the cable was getting hot while cranking. Those were a Mustang and a van.
Always good to see the problem resolved! But it will keep you awake at night waiting for the truck to come back because the intermittent ground fault will sneak back and bit you in the ....:)
WES, kudos to U ! Myself, worked in electronics field 37 years. Disliked it working on intermittent problems. But, my company frowned on "to much time on diagnostics!", it was just plug in a new mod to get it up and running! Thing was, it seemed they always found time for you to FIX it in the worst weather conditions in the winter!
i remember those three chromed screws holding the fake 4 wd switch image cap on those aluminum wheel rangers. they came with the plastic inserts my ranger never left me stranded
Simple Ohm's Law. Fairly high resistance across the connector on the +12v side that's supplying the module power. With your small test light you're only drawing a few hundred milliamps so the voltage drop is relatively low. With the headlamp drawing 3-4 amps the IR drop increases dramatically dropping the voltage below what can light the filament.
I don’t think there’s a more realistic, practical, and down to earth repair channel on CZcams. Thanks for sharing, Wes, we love you!
Seriously
3 days in a row. with all the crap going on in the world, there is a glimmer of hope. Thanks Wes. another good one. I'm getting used to this.
Yeah, plenty of crap.
Wes, you are getting too good at solving these things, I miss the 1 hour videos 😉
nah his editing just got better. :)
@@crisprtalk6963 That maybe means more videoes, more often… 😀
Also miss the old ones... Longer and with less eletric problems... More mechanical things...
I can't make everyone happy.
@@WatchWesWork keep up the the good work.
Don't feel bad. I saw a similar problem on a medium-sized CNC mill. One loose power connection screw (there were six) caused all sorts of faults in the machine.
It took a very experienced electrician all day to trace the problem. A fix is a win. 👍
Yup sh!t like this happens all the time. Mysterious are the ways of the electron.
Had an E-stop button switch do that to a CNC years ago. All the panicked hand-smashing of the E-stop all those years loosened the connections just enough that the machine would not alarm out but would jump/shift its position. Causing all kinds of inaccuracies. Everyone was saying the control was no good! The boards are bad! The power from the electric Co. is bad! ETC. NOPE! A loose wire at the E-stop was all it was. That same machine still makes good parts from me tightening a screw 20 some odd years ago. Nutty huh?
We had a NC wire wrap machine in the factory that would trip out with a limit switch fault. Problem was, it was never near a limit switch when it tripped out. We ended up building a latch to capture the state of all the limit switches when it tripped to figure out which one was intermittent. The latch told us it hit all the limits at the same time. Loose terminal on the power feed to the limit switches.
3 videos this past week is so cool. A real treat. Those older vehicles can sure have a lot of gremlins as they age don’t they.
Once again our seriously patient uber professional intrepid mechanic Wess takes on a brain twisting problem that's slightly agravating yet he still manages to fix the problem without multiple tossed tools and cussing that would make a 30 year minimum Chief Petty Officer blush.
Another outstanding episode to entertain me on my way to the VA hospital for a pulmanary apointment.
Thanks Wes.
Editing does wonders don't it. You did see him attack the back wall with a car not too long ago right?
That was the next option.
It helps to be self employed sometimes. lol
@@WatchWesWork ironically and even funnier Mortske just made the same reference about it in his video that dropped this am as well LOL
He edits out the thrown tools and the cursing, let's be real. Conversely, we can tell Mustie1 has no such character aspects because he's always laughing and having a good time. At least we think so...LOL 😀
Don't be too hard on yourself Wes! You found something most of us would never even know how to look for. I'm still amazed at your diagnosing skills.
"We towed it in, we drove it out!" That's what counts and we count on Wes for the best, most logical repairs. Amen.
My kid had one of those truck , but with the 2.9 V6 . Called me for help one day . He had been off roading it and it quit on him . Behind that same kick panel was a switch that killed the electrics in case of a collision. Pushed the reset button really really hard and it reset . And I mean really hard . Good to go after that . I was a hero for a day lol
Honesty is always the bedrock of integrity that's why we keep coming back. Keep doing what you're doing please.
Yupp
When I used to work for a Ford Dealer I had a rash of (NEW) vehicles with bad crimps from the factory causing many different problems.
Towed it in, drove it out, but let's not forget that you saved the customer a chunk of money!
That doesn’t make you look bad, v skilled diagnosis and the truck worked in the end. 10/10 video.
Wes you never fail to amaze me with your methodical electrical diagnosis! I have a buddy that I grew up with who is a professional Mechanic like you. I always call on him to fix the electrical gremlins that I run into. He in turn has me do all his welding projects. If you know, you know! LOL!
Been watching for years now. Look forward to each one. Wes is a great mechanic and honest, too.
Wes I am glad you are part of a special group who looks to maintain these older vehicles and not give up on them. Granted the computer controlled vehicle is here to stay, there are still not many people who can and will take the time like you and the special class of mechanic you belong to.
Bad connector... I don't know how many times I've made a dead car start again by unplugging a connector and plugging it back in. Good thing you found it before ordering the ECU!
Good morning, Wes! Three in a weekend? Good work! Hope you're enjoying the lovely warm weather after the rounds of storms we saw.
Obviously something was nagging at you to check again before firing the parts cannon, and that's what matters for a customer - making sure of the diagnostic where possible.
Your electrical troubleshooting skills are phenomenal!
Don't doubt yourself Wes 🙂👍 You done a great job with the diagnosis and also the video. Glad to see you're doing well. Take care, be safe and happy wrenching 🔧
Dude hats off to you most mechanics would have given up half way through! I wish I had someone like you where I live. I have a 91 and a 94 and I love then both.
Man I don’t miss these. Fords fix for incomplete combustion, second plug.
Someone once told me it’s a Hemi…uh, No.
Nice work!
I really appreciate you talking out loud as you do your analysis of these problems.
You stepped away instead of firing the parts cannon. That is something to be proud of. Not many do that.
Yea West like you the first guy that has gone down a rabbit hole before, I stop counting years ago. 😁
I wish I was closer to you to take my vehicles to you, you are one in a million! Bravo Wes!
Wes, Thank you for putting out this video.I have been chasing my butt trying to figure out my issue with my 95 Ranger 3.0. I have the almost same identical issue. Only difference was I could crank and run it while it was completely cold. I threw the parts cannon at this turd. I replaced the fuel pump, filter,modulator, coil, crank shaft position sensor and the PCM. It wasn't till after I saw this video did I even try the coolant temperature sensor. Once I replaced, it crank and runs every time! Who woulda thunk it!!! Thanks for all the great content.
Greatful,
Jim Smith
No repair channel I watch is as easy to understand and fun to watch as this guy
Don't be hard on yourself. You told the story like it happened. You were on the right track
from the get-go. This happens to doctors. Lawyers and such that they end up not knowing how or why the outcome was good , thats all that counts in the end. Sometimes, it's better to be lucky than good.
your diagnostic skills are amazing as well as the repair work/ thanks for the video
My little brother and me put a 302 in a little first generation ford ranger back in the day and it was awesome.
Well done Wes! Always a pleasure watching you wrenching on the old stuff.
Take the win mate, you’ve earned it twenty-fold.
For it being early 90s stuff, you still did great!
Yup. Ford's dual ignition system idea. It was my understanding that the idea of two plugs per cylinder was thought to provide a more complete fuel burn and, therefore, fewer emissions out the tailpipe. As a general rule of thumb Wes, if you find something on an internal combustion engine that is unnecessary, overly complicated, or just plain stupid, you can bet the shop our good ol' Uncle Sam was in some way involved when the dubious idea was hatched. Great job tracking down that particular nasty gremlin Wes.
I have had this exact problem on my 89 f150 on the ignition control module, boy oh boy it was a fun one to figure out and find... Wiring diagrams are gifts from God
I remember this truck. Good to see it's not in hospice care yet.
Videos on back to back to back days! Thanks for making my breaks in between working on my taxes much better Wes
You bet!
This man’s vast knowledge and experience is impressive and educational. I even find it applicable in my everyday life! (I am an auto mechanic)
Your hamster powered internet must be running hot Wes, It's so good to see you posting more often!
Also can you buy an oil burning RAV4 cheap from down south and swap your rebuilt engine in?
Wes you’re a good man. This Ranger had you all over the map 🗺️.
3 videos in 3 days!! Is that dome kind of Easter gift or something? Love it!
Burning through the backlog.
Great old truck. The stick shift is why it is still on the road.
The old Ranger was one of Fords best vesicles ever made in fact still many around (the small Chevy and Dodge trucks of that era not so much all rotted away or quit running.....new Rangers well remains to be seen.Wes is a wizard at repair of all kinds think how much he will know by time he gets gray hair and a few wrinkles.
Sometimes it's better to be lucky than good, this is a prime example.
Wes,
Great job on that one! I had a 90 Ranger of the same Flavor. Your Connector delete is the ticket. Had the same issue. You could crank on that thing all day long but wouldn't start. I went through 2 computers before we started looking elsewhere. My buddy just happened to pull that connector apart and found all the green crusties and burn marks. We deleted and that truck never had that problem again after that. I sold it and it went until around 2006 or 2007 when that old man finally junked it because the frame rotted out.
Oh yes, Ford's great idea for dual ignition systems that have 8 plugs on a 4 cylinder. And one would think the addition of the second ignition system would yield extra power...not in this universe. You seriously have to understand how to pull out in traffic if you want to survive such a blend into traffic move without a hospital visit or worst! Love your diagnostic efforts every time. I say you have the absolute best vehicle problem-solving channel on UT... Thumps Up!
Aircraft run dual spark plugs, and have done so for decades. For light aircraft, it is a FAA requirement
More spark doesn't equate to more power, I mean what are you going to do ignite the A/F even harder or twice? I think it's an emissions thing but I know nothing about that
Ford wants to build more EV's which I think is a terrible idea becuase every Ford that has been in my family since the 1960's has had some kind of electrical problems straigjht from the factory!
@@Hjerte_Verke Don't know how it works but Nissan built a 2ltr 4cyl twin spark fuel injected in the eighties & they were fantastic would do 200kph easy & reguarly.(I had one).
it wasnt for more power it was to burn the fuel left over in the exhaust for emissions standards.
you're just spoiling us with 3 videos in a weekend. another detective Wes on the wiring trail to enlightenment . Thanks looking forward to next one.
Wes, your strive and attitude on these old machines to me is what makes these videos unique. Do not give up on them.
It's always the Ground----- except when it's not. Damn Ford ignition module with its special bolts.
Got to have the tool.
Wow, 3 videos in 3 days, what an awesome way to start the week
That's a nice little truck. I'm glad you don't have to eat a PCM.
I give it 3 months and that Ranger will be back ... you've made friends with it now and it will miss you. The perils of not using hammers unnecessarily ...
Thanks for sharing 🇨🇦
Wes videos 3 days in a row. Life is good
A win is a win. Nice job!
Yep, when I talk about replacing 8 spark plugs on my four cylinder truck, people look at me like I'm nuts. I loved that little truck. I still wish I'd kept it. Good save on a 32 year old truck. I almost bought one just like that. It probably sounds stupid, but that red dashboard was so close to the same as that on my '82 Escort, and that car had been such a piece of junk, that I chose the blue one because I couldn't stand the reminder. That Escort forced me to learn mechanic skills. I couldn't afford to pay for all the work it continually needed.
I really love these kind of videos on repair projects etc
The point is, you found the problem and made a successful repair. A Win is a Win.
Having a couple of months to work on that truck and double-checking your original diagnosis worked out very well for you! I just love happy endings!
I appreciate Wes' transparency and thoughtfulness, both with the repairs and the videos.
It does make me wonder however, how many PCMs have been replaced over the years just due to faulty connectors, where the problem was actually fixed just by virtue of removing and reinstalling things. Of course integrated circuits and printed boards are not infallible, but I think too many folks assume they are at fault just because they don't take the time to understand them.
Good for you to Double check your self!!!!! Keeping the old Stuff going is good economic wise!!
Keep up with the Great work.
All the best from Canada
Simply difficult, the most aggravating problems.
Great job Wes, another happy customer blissfully unaware of the headache.
Thanks for sharing
Love when you do the Oliver Hardy face. Great video 👍 👍
I had that same truck in a 97'. After I sold it, the spring shackles performed a delete tactic.
They will do that!
Great job! It's very frustrating when it just starts working when you barely touch it. I love the line "don't thing I have enough editing skills to make me look like I know what I'm doing" 🤣
Reminds me of fixing my 93 f350 by getting pissed and flopping some wires around on the side of the road. Fixed it.
Wes, your vijayos have given me the confidence to, to, to, well, drive my unrusted snot bucket to upper Illinois from Washington state and see what you can do with it. 1980 f-100 Ranger with 5.0 Mustang mill. Cheers, K
XLT in Ranger, stands for xtra long tracing. At least it will run now. Good fix Wes.
3 videos in a week????? Holy cow!! Thank you!!
Said it before, love it when Wess gets out a single line.
drove one of these 4 cyl. 8 plug machines back when I was a kid... in the mountains of New Mexico, at that altitude the 5th gear was only useful when going down hill
Hey Wes👋, good video on the ranger. Just had fun with that issue on 96 f250. Thanks for sharing 👍 Have a great week ahead
Enough editing skills to make it look like you know what you’re doing? Wes, this is textbook diagnostic work. You are a awesome technician!!! This is the kind of fix that makes a man legendary. I’ve seen too many “technicians” just aim and fire the old parts cannon.
Great video Wess. A man that can miss diagnose something then is man enough to ad mit it. That's just good ole honesty. That why I love watching you chanel . Keep coming. .
Sometimes we get off track...ya still fixed it which is what is important. My OCD would not have let me get rid of the connector. I'd have put a weatherpack/metripak in it just to annoy the Ford guys...and I have a ready stash in the shop. 🤣
Well at least ya got a good end result brol, maybe a few more great hairs too. Safe travels
like a gift that keeps giving
Sometimes you get those what the heck and still get it fixed... Dont ask, just appreciate it got fixed. 50+ years, yeah its all good Wes. You did good.
Beautiful truck I love the roll up windows. Wes your good bud and you get better all the time. I notice your tools are always clean and your never challenged. Plus your funny.
Sometimes you have to walk away and come back later. Fresh ideas does it every time.
Here's what happened. You got it running. Have a nice day.
5:02
The parts are made in Malaysia. Proud to see it there.
Loose pin tension is always a fun one to hunt down. Thankfully, it didn't lead to a full on fire inside that connector.
Lol I truly enjoy watching you work Wes 👍
Great video, we just need more videos. Thanks for all you do
Neat Job Wes. Have a great week.
Very good documentary Wes. It had a red herring and everything. Listen to that instinct :D
Thanks Wes, i really like your test lights, I have made a few of my own and shoegoo'd them to magnets, whatever works.
Shoegoo is my secret weapon.
Moses sandals, 3 days in a row for uploads! A 90's Ford with the Porno red interior is such a timeless classic!
The old Ford ... Still kicking
I've also run into some confusing wiring problems on older Fords. On a friend of mine's truck I had no fire at the plugs. Each individual component seemed to test good. Finally I just took apart and cleaned every connection in the system. That did it and he said it had never run better. A couple of other times I had crank no start and tested it with an extra ground cable from battery to engine block. That fixed both of them and I don't really remember what pointed me in that direction. Maybe the cable was getting hot while cranking. Those were a Mustang and a van.
Always good to see the problem resolved! But it will keep you awake at night waiting for the truck to come back because the intermittent ground fault will sneak back and bit you in the ....:)
Bypassing those connectors for 40+ years.
I had a truck just like that, absolutely hated it! Couldn't get out of it's own way.
WES, kudos to U ! Myself, worked in electronics field 37 years. Disliked it working on intermittent problems. But, my company frowned on "to much time on diagnostics!", it was just plug in a new mod to get it up and running! Thing was, it seemed they always found time for you to FIX it in the worst weather conditions in the winter!
Hey drove my '91 Geo Metro today and our lake dried up too! Greetings from Waterdown, Ontario, Canada!
What the heck😂😂😂. My 2000 is not starting also. You just gave me another idea to check for this before I buy a PCM. 😂😂
i remember those three chromed screws holding the fake 4 wd switch image cap on those aluminum wheel rangers. they came with the plastic inserts my ranger never left me stranded
This has been the best 3 days in a while, back to back wes tearing up the crusties!
Simple Ohm's Law. Fairly high resistance across the connector on the +12v side that's supplying the module power. With your small test light you're only drawing a few hundred milliamps so the voltage drop is relatively low. With the headlamp drawing 3-4 amps the IR drop increases dramatically dropping the voltage below what can light the filament.