Won't shut down with headlights on

[Sierra_Grande]

Hey folks, hoping for some insight into this. Fist the disclaimer - This truck has been modded quite a bit, but not much to the lighting circuits.

It's a 1973 3/4 ton GMC Suburban. 6.5 turbo diesel swapped from a 1993 Chevy K3500. 1993 was the only year the 6.5TD had a mechanical injection pump, so no computers installed.

Fun video of my issue:

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[Hunter79764]

Does it die when you turn the dimmer down?

My guess is the dimmer circuit is spliced into the ignition circuit somehow, possibly in the radio wiring. Other place I'd look is any relays you have that are powered with ignition source and do something with the headlights. I had a relay rigged up to turn on a warning light when my high speed electric fans came on, and as a result, the car would backfeed power to the ignition through the high speed fan circuit when it was engaged and not let the car turn off. Your situation reminds me of that. I also had an issue on my first stereo install in high school where the running lights came on any time the radio was on, I miswired my subwoofer remote signal into the dimmer circuit. Which is why I'm thinking radio.

 

[Sierra_Grande]

So you can see the ignition is energized when the headlight switch is turned on. I could almost live with that, if it weren't for other weird things.

Symptoms I noticed:
1. Ignition energized whenever headlights are turned on
2. Dash lights come on whenever you turn the key on
3. Dash lights cannot be dimmed with key on
4. Running lights are on whenever key is on
5. With headlight switch unplugged, truck turns off normally (no surprise), no running lights, but dash lights still come on with ignition

I found out the hard way that the shop who did the motor swap ran the signal wire for the alternator through the ammeter circuit. Last year I decided I wanted a volt meter instead of a long defunct ammeter, and swapped in a new volt gauge I bought from LMC. Afterward I noticed my voltage was always low and started chasing that issue.
Charging troubleshooting post:

https://www.dieselplace.com/threads/low-alternator-voltage.997705/?post_id=10058313#post-10058313

Gauge post:

Mutt gauges?

That is what I did, swapped out the ammeter and wired it to the temp gauge! Here's a good write up for doing that swap. http://www.73-87chevytrucks.com/techinfo/Electrical/73C10AmmeterToVoltGaugeSwapR1-0.pdf

www.gmsquarebody.com

New volt meter is wired to the coolant temp gauge.

I traced all the wires and found the wire for the "L" pin on the alternator was run like this. I don't understand how it worked to begin with. With the wiring the way it is now the alternator is charging again.

Here is a diagram of what I did:

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I don't understand how altering this circuit has caused all this. The 3 and 4 pins only go to the old ammeter. I pulled all the wiring schematics and it seems to confirm they were only used for this. Is there some ground downstream that I missed?

Appreciate any help.
-Ian

 

[Sierra_Grande]

@Hunter79764 Thanks these are good suggestions. I did work on the stereo some a little while ago because my left speaker was disonnected. I'm going to disconnect it and see.

You were quick! I was busy putting other details in the next post. I don't like super long posts and thought I'd put the video mostly in #1 lol

 

[Hunter79764]

@Hunter79764 Thanks these are good suggestions.

You were quick! I was busy putting other details in the next post. I don't like super long posts and thought I'd put the video mostly in #1 lol

Don't tell my boss I'm watching threads this close! jk, just good timing I suppose.

Based on that, I'd definitely look into the dash wiring, Might be way off base but if you have 12v feeding the lights circuit, the "ground" from the lights as your "12V" to the gauges, then an actual ground? Or something like that, where a circuit energized by the dimmer is feeding the ignition, and the dash lights (should be the dimmer) is fed by ignition?

I hate shops that do that kind of stuff. When it's me doing this to my own stuff, that's one thing, but I expect folks to do better on other's stuff...

 

[Sierra_Grande]

I definitely wasn't posting this elaborate explanation during work hours

 

[H2OnSnow]

Why is your voltmeter connected to your temp gauge? What terminals of each are connected? Are you sure that those are the terminals that are connected together? Are you really sure?

 

[Sierra_Grande]

@H2OnSnow It's the easiest and closest place to grab +/- for a voltage reading.

I decided I'd go ahead and pull the dash and start tracing wires, and low and behold, I disconnected the fuel pressure gauge and everything went back to normal. I'll post up if I figure out what was crossed.

You must be registered for see images attach

You must be registered for see images attach

 

[H2OnSnow]

The only gauges that should have 12v and gnd on them would be digital gauges. I can't see in your pic what you have there. Your volt meter CAN be connected to any 12v point, but it would be best to connect it to some particular 12v point and some particular gnd, as voltages vary around the vehicle under different conditions. You might prefer to directly monitor some point over some other point (alt out, bat, ign, etc.), or you might just want a general health check where you could legitimately use any old point. Bottom line, not all +12vs and gnds are created equal, choose purposefully. If you know all this already, please excuse the oversplain.

 

[awar]

Make it a power with key on so the gage doesn’t stay hot with the key off. Draws are hard on the battery.