m998 3 amp battery drain question/help

[Schuster]

I have a 90 m998 that was running and starting great with no issues, I replaced the rear,front and side marker lights with LEDs and all of a sudden i have a 3 amp parasitic draw, I unplugged the alt. (60 amp) from system and draw goes away and the protective control box the draw goes away.
I don't want to through parts at it, my question is could the box be my problem or the alt. and how do you isolate them. my alt. was charging 27.5 volts and amperage verified full load 35-40 amps (seems low to me ) no glow plug issues.
where does the charge wire from alt. go? anyone have any ideas for me? thanks for your time.
Robert
St. Pete, Florida

 

[Action]

Did you put the standard lights back in and see what happens? I would first eliminate the thing you changed that caused the problem. You may need a resistor for the LEDs.

 

[Schuster]

The old light were in pretty bad shape I can't reinstall them

 

[NDT]

See if voltage is present at the alternator excite wire with ignition off. If so, the draw is from the alternator, but is the normal draw. The protective control box may be sending the power when it is not supposed to. I really don't think installing the LEDs is related. With the master light switch off, the vehicle has no idea if there are LEDs or not.

 

[Schuster]

When I unplug the excite wire the small one with the boot the draw goes away there is battery voltage at both excite wires and the starter wire

 

[NDT]

Others here need to chime in before you replace anything, but I believe the protective control box is to blame. The alt is ok.

 

[Schuster]

I am also leaning that way. Just was hoping it was something simple and cheap. Thanks for your help

 

[Action]

Look at your dash at night. Are the dash lights on?

 

[Schuster]

No lights on and switch unplugged does not drop the draw.

 

[Action]

Are you unplugging the switch, PCB, alternator, etc with the batteries still hooked up?

 

[Schuster]

Never.

 

[papakb]

Go out (or online) and buy yourself a DC clamp on amp meter. They start around $25 but make sure the one you buy measures DC current. Put it on your + battery lead and look at what causes the current draw. If it starts when you turn the switch to run then start disconnecting things until it goes away. That'll point you down the trail to what's causing the current draw.

If your old marker lights are toast then just unplug the LEDs and see what happens. The lighting circuits are completely separate from the start - run circuits but you need to figure out what path is drawing current before proceeding and the clamp on meter will do that for you.

 

[Schuster]

I am a diesel technician I have a high and low amp probe I did as you stated and when you unplug the box the amp draw goes away also when you unplug the alternator the amp draw goes away I don't have schematics to see how those two work together to isolate which one it is and I don't want to throw Parts on it that it does not need the alternator Exciter wire has constant voltage which leads me to believe it is the Box stuck on causing the drain

 

[someoldmoose]

If unplugging the control box makes the draw go away then it is the prime suspect. The alternator is the "dumb" load in the circuit (which I believe you already know) like the light bulb that stays on when the switch is definitely ​off (because the switch is bad inside). It's where the draw is occurring but not why.

 

[Schuster]

Sometimes a little affirmation is good thanks for the help I got to control box, I'll be curious to take that one apart and see what's inside

 

[papakb]

The schematics can be found for free by downloading the 280-20-3 TM. They're at the back of the manual. In vehicles with the 60 amp alternator the output of the alternator goes thru the control box so it's not surprising that the current goes away when you disconnect it. BTW, as mentioned you always want to disconnect a battery lead when making or breaking electrical connections in the HMMWV.

 

[Schuster]

Great info thank you. Thinking about it when I strip the truck down for paint I didn't disconnect the batteries and I pulled all the lights should have did my research first thanks everyone for the help

 

[Action]

Lots of electrical stuff in the HMMWV runs through the PCB. Thus unhooking that, could disconnect anything that is really the problem.

 

[AOR]

What alternator do you have? Does it have a dual voltage regulator? Are there any wires connected to the regulator 14 volt stud? There is a reason I am asking and I will explain if you do

 

[papakb]

AOR, the opening post says the truck has a 60 amp alternator in it.

Schuster, the fact that the current draw goes away when the control box is disconnected does not necessarily mean that it is suspect since so many other circuits draw power through it. Don't discount it as the source of your problem but don't let yourself get totally focused on it either. If you want to troubleshoot with the batteries disconnected you can use your ohm meter for that. The resistances for most lighting circuits will be low but it's a good way to pick up dead shorts. You can thank Englishman George Lucas, the inventor of the short circuit for that!

When I first got my HMMWV I took the schematic to the local copy shop and had it copied double size and then laminated. That way I could draw on it with a grease pencil to trace out the wiring and could then wipe it clean for the next time. There are plenty of lines that run parallel to each other and it's easy to cross over and waste time troubleshooting the wrong circuit.

Also be careful that you get the schematic for your truck. While 98% of them are the same the wrong one can have you wasting hours chasing your tail. Check the title block in the lower right corner and make sure it matches the model of your vehicle.