'74 won't turn off - help?

[John76]

15 hours ago, Ducky982 said:

What is that relay?

That is the voltage regulator as @Son of Marty mentioned above.

Easy to test:  Measure the battery voltage with the engine OFF.

Then start the engine and measure the battery voltage again. Compare both OFF and ON voltages. 

The voltage regulator is working correctly if the running voltage at the battery is appox. 13.6 - 14.2V.

The diodes that SoM is referring to are inside the alternator. Take it to you local auto parts store for a complimentary alternator test to determine the condition of the diodes.

This still does not explain why your coil is still getting voltage when the ignition is OFF after a start.

Did you check the continuity of the ignition switch?

 

[Mark92131]

1 hour ago, John76 said:

Did you check the continuity of the ignition switch?

 

John, doesn't "P" on the ignition switch power the parking light feature on early cars?

 

Mark92131

[John76]

17 minutes ago, Mark92131 said:

doesn't "P" on the ignition switch power the parking light feature on early cars?

Yes ... it goes to the early turn signal switch with a "P" terminal.

Happy New Year!

John

[Mark92131]

3 minutes ago, John76 said:

Happy New Year!

 

Happy New Year to you!  Thanks for all your wonderful electrical drawings.  I store them in a folder and reference them all the time.

 

Mark

[Ducky982]

Ok, another update… I tested the alternator and it showed bad diodes. Swapped for the AL41X as that seemed like a sensible upgrade that I’d probably want even if it didn’t solve my problem. Naturally it did not solve the problem. 
 

Disconnecting the D+ wire allows the engine to start and stop normally. 
 

I began to unwind the wiring harness somewhat and nothing looks pinched crimped or chewed upon. 
 

I removed the gauges and the engine starts and stops normally. A PO had installed led gauge lights. On a whim I removed them all, plugged the gauges back in, and the car starts and stops normally. Then I reinstalled the lights until things stopped working. Turns out the light corresponding with the “L” indicator is the culprit. With just that light removed the engine will turn off normally. 
 

Careful inspection of the gauge pod reveals two oddities: one appears to be a non-original resistor, and the other is a burned conductor on the PCB. 
 

I somehow suspect that removing the L bulb is just indicative of an issue somewhere else. 
 

Any insights based on this new info? I’d hate to put a new pod in and instantly burn it up because I didn’t fix the problem yet. 

[jimk]

Are you certain what you believe to be a resistor is not a diode.  Resistors are usually color marked for the resistance value.  Diodes are painted with the symbol for a diode, I believe.

[dlacey]

Yes thats a Diode...  an LED is also a diode BUT many LEDs have protection from electrostatic-discharge built-in... that protection means they don't behave as a diode..and can even appear 'dead-short' in reverse bias (that could burn the track on your PCB before the LED burns out).  So that diode soldered on the board is possibly a PO attempt to provide protection for reverse-biased LEDs (the bulb fitting can be fitted two ways.. OK for a filament lamp, not good for LEDs)

[Ducky982]

So do we think the LEDs porked the PCB, and that’s the whole root of the problem?

[Son of Marty]

First thing is no led's for the "L" light for the alternator to charge properly, the PO probably installed the diode to get the system to charge and it's providing a path for the alternator/battery to feed 12v to the ignition system so the engine won't shut off. I'd remove the diode and replace the "L" light with a incandescent bulb and you should be good to go or maybe stop would describe it better.

[John76]

If the incandescent light bulb in the "L" (alternator) socket was changed to a LED bulb, then that could be a problem.

The filament in the bulb provides excitation voltage to the alternator's electromagnet. LED bulbs do not. However, this might explain the diode. Looks like a few things have been "hacked" on your dash pod.???

 

 

[jimk]

The burned trace might be reparable with a drop of solder.

[Ducky982]

Well that did it!!! Thank you all very very much for the help along the way. No way I'd have guessed that a simple light bulb was causing the problem.

 

I swapped all the bulbs in the panel out for regular incandescents, and repaired the PCB trace with some solder. Everything seems to be working as it should. I also noticed that the "L" light actually comes on like you'd expect (key on, engine not running) and goes out once running, which I hadn't seen before with the LED in place. After chasing my tail on this for a couple months, I gotta say I'm shocked that it was a 5 minute $0.70 fix. 

 

I'm also kinda shocked by a PO who had a running car, "upgraded" it and made it not work, and didn't immediately suspect that the last thing he just touched might have been the culprit for why it wasn't working.

 

Thanks, gang, for all the help on this. Gotta love car guy communities. Almost driving season up here in Vermont, can't wait to rip her around...

[tzei]

Upgrades can be counted as success if less than 20% of orginal functions dissapears.