I Made a STUPID Mistake
Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
I Made a STUPID Mistake
Made a lot of electrical upgrades this winter which involved removing the front lamp harness. I wound up with an "extra" ground wire in that harness. Instead of referring to my photos of the harness still in its loom, I just attached it to a front-end framing member.
It was the ALTERNATOR ground!
While I never drove the car without the alternator ground I did run it a bit when doing testing.
Now that I found the problem and have everything back together my gauges in the gauge pack are wacky--oil pressure too low, voltage too high (just before its red line) and temperature is non-functional.
Is it possible that running without the ground connected to the alternator has damaged the gauges and/or the alternator?
As far as I can tell everything (including my added electrical "goodies") are working perfectly. *
*I installed a 3rd (high) brake light inside at the top of the rear window. It's LED and I'm pretty sure I tapped into a black wire in the rear harness for its ground. When I step on the brakes the fuel level gauge goes up! It would seem I tapped into the dedicated ground for the fuel level sensor.
It was the ALTERNATOR ground!
While I never drove the car without the alternator ground I did run it a bit when doing testing.
Now that I found the problem and have everything back together my gauges in the gauge pack are wacky--oil pressure too low, voltage too high (just before its red line) and temperature is non-functional.
Is it possible that running without the ground connected to the alternator has damaged the gauges and/or the alternator?
As far as I can tell everything (including my added electrical "goodies") are working perfectly. *
*I installed a 3rd (high) brake light inside at the top of the rear window. It's LED and I'm pretty sure I tapped into a black wire in the rear harness for its ground. When I step on the brakes the fuel level gauge goes up! It would seem I tapped into the dedicated ground for the fuel level sensor.
Safety Car
Joined: May 2004
Posts: 4,363
Likes: 1,587
From: los altos hills california
Are you trolling for stupider mistakes? I had my carb off, distributor cap off and was checking compression with my son cranking the motor. Sort of forgot about the fuel pump line aimed at the distributor.
Well once the fire was out* I hung the scorched ignition shielding support bracket over my workbench as a reminder.
As to your alternator concern, I think all you had was an open circuit.
_______________
*once the 911 call is made, the firemen come out no matter what. I managed to throw a wet towel over the flames but had to explain to a couple of hunky firemen in their suits how stupid I was. Very embarassing.
Well once the fire was out* I hung the scorched ignition shielding support bracket over my workbench as a reminder.
As to your alternator concern, I think all you had was an open circuit.
_______________
*once the 911 call is made, the firemen come out no matter what. I managed to throw a wet towel over the flames but had to explain to a couple of hunky firemen in their suits how stupid I was. Very embarassing.
Last edited by ignatz; Mar 16, 2017 at 05:10 PM.
Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
That's what I thought as well but I got some bizarre voltage readings when troubleshooting my new compass/temp/garage opener/map light mirror as it was draining the battery. I even returned the mirror at the sellers suggestion for him to find no problem and when received today it works properly. The only change I made was to properly locate the ground to the alternator.
Le Mans Master
Joined: Aug 2006
Posts: 7,426
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
I installed a 3rd (high) brake light inside at the top of the rear window. It's LED and I'm pretty sure I tapped into a black wire in the rear harness for its ground. When I step on the brakes the fuel level gauge goes up! It would seem I tapped into the dedicated ground for the fuel level sensor.
Use the bird cage as a new 3rd light ground and see if the problem goes away.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
Make sure that fuel gage ground is good/not corroded. The brake light is trying to find a path to ground, it might be using the fuel sender if the sender ground is bad. As already mentioned, probably a good idea to give the third brake light it's own ground.
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Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
The mirror body is somewhat larger than the original but no problems with visibility.Given the "aircraft inspired" interior of C3s I do believe that heading and temperature indicators would have been included had there been room in the dash.
Le Mans Master
Joined: May 2003
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From: Fernandina Beach FL
2023 Restomod of the Year finalist
2020 C3 of the Year Winner - Modified
Hey Mike-
It does sound like you have hit the fuel sender wire to ground the LED brake light. Remember the LED will not act like an incandescent bulb. As far as seeing a ground though the LED light- it functions as a diode and there is no filament wire to act as a resistor. Plus the LED's pulls so much less current it's very possible the sensor wire is acting as a ground.
Richard
It does sound like you have hit the fuel sender wire to ground the LED brake light. Remember the LED will not act like an incandescent bulb. As far as seeing a ground though the LED light- it functions as a diode and there is no filament wire to act as a resistor. Plus the LED's pulls so much less current it's very possible the sensor wire is acting as a ground.
Richard
Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
Hey Mike-
It does sound like you have hit the fuel sender wire to ground the LED brake light. Remember the LED will not act like an incandescent bulb. As far as seeing a ground though the LED light- it functions as a diode and there is no filament wire to act as a resistor. Plus the LED's pulls so much less current it's very possible the sensor wire is acting as a ground.
Richard
It does sound like you have hit the fuel sender wire to ground the LED brake light. Remember the LED will not act like an incandescent bulb. As far as seeing a ground though the LED light- it functions as a diode and there is no filament wire to act as a resistor. Plus the LED's pulls so much less current it's very possible the sensor wire is acting as a ground.
Richard
Race Director
Joined: Apr 2007
Posts: 11,152
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From: South Western Ontario
The ground wire at the alternator is to ground other electrical equipment, not to ground the alternator.
The alternator is grounded to the engine block by the brackets. If your main cables are good, the engine block then from the engine block it has a nice solid ground to the frame and then to the battery.
So, leaving that wire off means some things were not grounded. It's possible these things found a ground source through your gauges to the engine block.
As for the 3rd brake light. I find it hard to believe that the ground could be weak enough to still work with the light off yet not work when the current from this LED light flows into it. I bet you used the tan wire which is the sensor wire.
The alternator is grounded to the engine block by the brackets. If your main cables are good, the engine block then from the engine block it has a nice solid ground to the frame and then to the battery.
So, leaving that wire off means some things were not grounded. It's possible these things found a ground source through your gauges to the engine block.
As for the 3rd brake light. I find it hard to believe that the ground could be weak enough to still work with the light off yet not work when the current from this LED light flows into it. I bet you used the tan wire which is the sensor wire.
Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
The ground wire at the alternator is to ground other electrical equipment, not to ground the alternator.
The alternator is grounded to the engine block by the brackets. If your main cables are good, the engine block then from the engine block it has a nice solid ground to the frame and then to the battery.
So, leaving that wire off means some things were not grounded. It's possible these things found a ground source through your gauges to the engine block.
As for the 3rd brake light. I find it hard to believe that the ground could be weak enough to still work with the light off yet not work when the current from this LED light flows into it. I bet you used the tan wire which is the sensor wire.
The alternator is grounded to the engine block by the brackets. If your main cables are good, the engine block then from the engine block it has a nice solid ground to the frame and then to the battery.
So, leaving that wire off means some things were not grounded. It's possible these things found a ground source through your gauges to the engine block.
As for the 3rd brake light. I find it hard to believe that the ground could be weak enough to still work with the light off yet not work when the current from this LED light flows into it. I bet you used the tan wire which is the sensor wire.
In '79 the fuel level sensor wires are black (frame ground to sensor) and pink (sensor to the gauge). I 100% connected to that black wire and presume that when on the light is on the resistance seen by the gauge is reduced--thus the increasing reading.
I still haven't tracked down the problem(s) with the other gauges. I'm betting that at least one of the gauge terminals is shorted against the metal retainer.
Race Director
Joined: Apr 2007
Posts: 11,152
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From: South Western Ontario
Yes, I think all the C3's have a similar grounding scheme. It's to ensure the rad support and other components that the wiring grounds has a good path back to the battery and alternator. The alternator itself is grounded to the engine block, just like most other GM's on the road.
The ground wire for the sensor is a piece of about 18 gauge wire connected to ground point. A LED light doesn't require much current, certainly not enough to effect the circuit IF the ground wire is solid and properly connected to ground.
The ground wire for the sensor is a piece of about 18 gauge wire connected to ground point. A LED light doesn't require much current, certainly not enough to effect the circuit IF the ground wire is solid and properly connected to ground.
Thread Starter
Melting Slicks
Joined: May 2015
Posts: 2,365
Likes: 428
From: Cape Girardeau Missouri
Yes, I think all the C3's have a similar grounding scheme. It's to ensure the rad support and other components that the wiring grounds has a good path back to the battery and alternator. The alternator itself is grounded to the engine block, just like most other GM's on the road.
The ground wire for the sensor is a piece of about 18 gauge wire connected to ground point. A LED light doesn't require much current, certainly not enough to effect the circuit IF the ground wire is solid and properly connected to ground.
The ground wire for the sensor is a piece of about 18 gauge wire connected to ground point. A LED light doesn't require much current, certainly not enough to effect the circuit IF the ground wire is solid and properly connected to ground.
During this work I found more than one connection and connectors that once disconnected (likely for the first time ever) would not conduct properly when reconnected. I've become very adept at removing terminals from Packard 56 connectors