Elusive Electrical Gremlin

[diesel_dave]

Looking for some words of wisdom from a HMMWV expert for an extremely intermittent electrical gremlin that has only occurred maybe 5 times in probably 30 drives now. Basically what happens is, while driving or stopped, the tach drops to zero, voltage drops to battery voltage (no alternator current) and what I think is the most important piece of the puzzle, the switched accessories such as the A/C shut off. It will either come back after a few seconds by itself or if it doesn't, it will come back by turning the engine off and restarting it.

Initially I thought it was probably an issue with the alternator or the regulator but now that I know the A/C compressor and fan (same on the heater setting) shuts off when its in the condition, I am not so confident that its a bad alternator or regulator. I have a suspicion it is the PCB flaking out but I struggle to find any TM that goes in depth what role the PCB plays in the charging/tach system.

I plan to drive around with a multimeter in the truck. Any ideas on what I can check when it does this next time to try and narrow things down? I will definitely check for voltage at the regulator IGN lug to see if the PCB is supplying power to the regulator but I am all ears for anything else that I should look for next time.

It isn't super frequent and it's only a minor inconvenience at this point, but my worry is that I'll be more than a few miles from my house and it will crap the bed for good and I'll be left with a truck that isn't charging the battery. I am not sure how long it will drive with the accessories off and the alternator not charging but I don't really want to find out.

Thanks for any help.

 

[Mullaney]

Looking for some words of wisdom from a HMMWV expert for an extremely intermittent electrical gremlin that has only occurred maybe 5 times in probably 30 drives now. Basically what happens is, while driving or stopped, the tach drops to zero, voltage drops to battery voltage (no alternator current) and what I think is the most important piece of the puzzle, the switched accessories such as the A/C shut off. It will either come back after a few seconds by itself or if it doesn't, it will come back by turning the engine off and restarting it.

Initially I thought it was probably an issue with the alternator or the regulator but now that I know the A/C compressor and fan (same on the heater setting) shuts off when its in the condition, I am not so confident that its a bad alternator or regulator. I have a suspicion it is the PCB flaking out but I struggle to find any TM that goes in depth what role the PCB plays in the charging/tach system.

I plan to drive around with a multimeter in the truck. Any ideas on what I can check when it does this next time to try and narrow things down? I will definitely check for voltage at the regulator IGN lug to see if the PCB is supplying power to the regulator but I am all ears for anything else that I should look for next time.

It isn't super frequent and it's only a minor inconvenience at this point, but my worry is that I'll be more than a few miles from my house and it will crap the bed for good and I'll be left with a truck that isn't charging the battery. I am not sure how long it will drive with the accessories off and the alternator not charging but I don't really want to find out.

Thanks for any help.

.
For what it's worth, if the electrical system shuts down or flat out quits AND the motor is still running, and if your mechanical IP is pulling fuel forward - you are driving a diesel so it aughta keep running. Your brake lights will be gone too so think about it before you have to stop quickly...

Wish I had more or better to offer.

 

[diesel_dave]

.
For what it's worth, if the electrical system shuts down or flat out quits AND the motor is still running, and if your mechanical IP is pulling fuel forward - you are driving a diesel so it aughta keep running. Your brake lights will be gone too so think about it before you have to stop quickly...

Wish I had more or better to offer.

Reminds me when the driver got the truck started and off his trailer while I was moving my cars around the driveway to make room. After he left, I realized the truck was running and the front battery wasn't even connected. The lead was just flopping there haha.

As long as the fuel cut-off solenoid can stay energized, the truck should run. Well and the transmission controller gets power so you can have more than 2nd gear limp mode. I just don't know how long it can run like that on battery power alone. Probably a decent amount of time because I can't see there being much current draw when operating just a few solenoids and such.

 

[springer1981]

Based on your description, I'm going to focus on the tach. I'm not sure about your truck specifically but here is the basic operation of the tach. There is a sensor on the back of the motor with 2 wires. The sensor creates a pulse signal and it goes to the tach. The only thing the tach needs from the truck electrical system is power (24v and gnd). If the tach is shutting off it is losing power or ground. I'd focus on troubleshooting the tach because you can trace that power pretty easy. Since other stuff shuts off also it could be the self resetting circuit breaker behind the dash. Do you lose the dash gauges as well?

 

[diesel_dave]

Based on your description, I'm going to focus on the tach. I'm not sure about your truck specifically but here is the basic operation of the tach. There is a sensor on the back of the motor with 2 wires. The sensor creates a pulse signal and it goes to the tach. The only thing the tach needs from the truck electrical system is power (24v and gnd). If the tach is shutting off it is losing power or ground. I'd focus on troubleshooting the tach because you can trace that power pretty easy. Since other stuff shuts off also it could be the self resetting circuit breaker behind the dash. Do you lose the dash gauges as well?

I am probably wrong but I have a vague recollection of reading somewhere the turbo trucks get the tach signal via the alternator/regulator. The dash gauges stay operating but the voltmeter does dip down due to the alternator not supplying current. You can hear the reduced engine load when this happens too.

 

[springer1981]

I am probably wrong but I have a vague recollection of reading somewhere the turbo trucks get the tach signal via the alternator/regulator. The dash gauges stay operating but the voltmeter does dip down due to the alternator not supplying current. You can hear the reduced engine load when this happens too.

I wasn't sure what truck you have ... yes I think you are correct. If I could find a wiring diagram for your truck it would be helpful. I will make a guess though, it may have the same type of signal generator and the chances are you are losing power to the tach and not the signal. If I were doing it, I would pick 1 thing to troubleshoot and I think the tach is the easiest choice. Put your meter on the power to it and see if it goes off. Then follow until you get to where it doesn't go off. Whatever was the last item in the circuit was that lost power is likely the issue.

 

[springer1981]

Ground your meter to the battery ground and if you don't lose 24v to the tach power swap to check the ground. Put the positive meter lead on the battery +24 and put the neg meter lead on the ground at the tach. If you lose power that way, you have a ground issue.

 

[diesel_dave]

Ground your meter to the battery ground and if you don't lose 24v to the tach power swap to check the ground. Put the positive meter lead on the battery +24 and put the neg meter lead on the ground at the tach. If you lose power that way, you have a ground issue.

Good idea. I wish I could easily recreate it in my driveway so I could check a bunch of things but of course it only seems to happen when I am out driving around.

 

[springer1981]

I did find a tach installation diagram and it looks like it taps the power off the positive feed to the fuel gauge. It also grounds to the gauge panel. When it goes off do you lose your fuel gauge? I'll guess that you do and assume it is similar to the earlier wiring diagrams and gets that power from CB2. You probably have 2 circuit breakers behind your dash panel, one of them is CB2. If you lose all your gauges I'd bet you are tripping CB2 (Circuit Breaker 2).

 

[diesel_dave]

I did find a tach installation diagram and it looks like it taps the power off the positive feed to the fuel gauge. It also grounds to the gauge panel. When it goes off do you lose your fuel gauge? I'll guess that you do and assume it is similar to the earlier wiring diagrams and gets that power from CB2. You probably have 2 circuit breakers behind your dash panel, one of them is CB2. If you lose all your gauges I'd bet you are tripping CB2 (Circuit Breaker 2).

Very interesting find! Now that you mention it, I am not sure that I ever specifically stared at the fuel gauge when it happens. I just know temp gauge and definately the voltmeter are still functioning. I'll try to get some video next time it happens to document the behavior for a closer look afterwards.

The only weird thing I am not sure that would explain is the blower fan and condenser fan in the back also shutting off and the alternator stops generating but it might be electrically interconnected some way that I don't see.

 

[springer1981]

The only weird thing I am not sure that would explain is the blower fan and condenser fan in the back also shutting off and the alternator stops generating but it might be electrically interconnected some way that I don't see.

From just the little research I've done since reading your post and no experience with your model at all, I think the AC connects in the battery box. Also if the alternator stops functioning it may be losing it's field voltage (not an expert on HMMWV alternators). It could be something in the battery area like a loose power wire on the shunt or something.

If you are not losing the Volt and Temp gauge it is not CB2. You would lose all your gauges. So if your tach is dropping out and your other gauges are working then your tach is losing it's signal wire from the alternator. The tach must run off the AC (alternating current) output of the alternator somehow. Losing the AC (air conditioner) is probably happening in the battery box. Start tracing the air conditioner power back to the battery box.

 

[springer1981]

Here's the AC wiring, at the bottom it shows the battery box and has nothing else in common with the rest of the electrical system. That is what leads me to believe it is in the battery box.

 

[springer1981]

Good idea. I wish I could easily recreate it in my driveway so I could check a bunch of things but of course it only seems to happen when I am out driving around.

You might be able to by probing around in the battery box looking for a lose connection. Use a wood dowel or stick to probe around so you don't short anything out.

 

[diesel_dave]

You know what is also interesting is the PCB seems to somehow control the A/C fan power because the blower runs for about 2 seconds after flipping the ignition to OFF. I am thinking its that "+24VDC from control box" signal to the speed switch. I need to see if the tach signal passes through the PCB at all to see if the PCB is the common thing linking all of these seemingly unrelated issues. If I can find that the PCB tells the alternator when to generate power, that would be pretty convincing. There is some kind of delay that keeps the alternator from generating current for a few seconds after engine start but I don't know if that is a PCB function or a regulator function.

 

[springer1981]

Since you say that, look at the AC wiring diagram hi/low switch and there is a 24v feed from the control box. However that's only for the blower motor circuit.

 

[diesel_dave]

Since you say that, look at the AC wiring diagram hi/low switch and there is a 24v feed from the control box. However that's only for the blower motor circuit.

Sure is. I need to follow that back to see where the source of this issue is. Maybe the PCB is cutting power to the tach, accessories and alternator field current but somehow not the gauges and IP fuel cutoff solenoid. I'll have to study the diagrams more closely to see if that's even possible.

 

[TOBASH]

Please remind us. What year truck and what amperage alternator?

In older 60 amp trucks, all the current runs through the PCB. In trucks with a 200 amp and above alternator, there is a current bypass. This could be as simple as a bad PCB connection or a PCB going bad on a 60 amp system.

That one failure could be the “choke point” causing all your symptoms.

With a newer alternator, maybe it’s a regulator or grounding issue?

Best of luck!

 

[diesel_dave]

Please remind us. What year truck and what amperage alternator?

In older 60 amp trucks, all the current runs through the PCB. In trucks with a 200 amp and above alternator, there is a current bypass. This could be as simple as a bad PCB connection or a PCB going bad on a 60 amp system.

That one failure could be the “choke point” causing all your symptoms.

With a newer alternator, maybe it’s a regulator or grounding issue?

Best of luck!

It's that 2006 M1152 in my signature. It has the 200 amp alternator. I suspect the PCB is cutting the output for tach, alternator field current and accessories for some reason or perhaps the PCB is losing the AC rpm signal from the alternator and shutting those things down? I'm not exactly sure of the PCBs logic. Next time it happens I will see if there is 24v present at the IGN terminal on the regulator. That will tell me if the PCB stopped telling the alternator to generate current.

 

[Action]

I am probably wrong but I have a vague recollection of reading somewhere the turbo trucks get the tach signal via the alternator/regulator. The dash gauges stay operating but the voltmeter does dip down due to the alternator not supplying current. You can hear the reduced engine load when this happens too.

I doubt the reduced engine load is from the alternator not charging. It is still spinning. The reduced load is probably from the a/c clutch disengaging.

 

[blutow]

Looking for some words of wisdom from a HMMWV expert for an extremely intermittent electrical gremlin that has only occurred maybe 5 times in probably 30 drives now.

I'm not sure I can help with the wisdom or hmmwv expert thing, but intermittent electrical stuff is often caused by a poor connection. If you have not already done so, it's probably worth going through all the ground points and also the major positive connections at the bus bar, starter, generator, etc.

When I got my truck a couple months ago, I had an intermittent issue with the wait light blinking instead of staying on. It would happen maybe 1 out of 5 tries. I thought maybe it was battery voltage or unbalanced batteries. Maybe a problem with the start box. I checked all my glow plugs and they were good. My truck was overhauled in 2009 and had no usage since then, so I figured all the electrical connections should be good since there is no corrosion and everything looked new. I ended up going through all the ground locations and found some issues. The main negative cable going from the battery box to the back of the block was loose at the connection point under the truck. They also didn't do a good job of cleaning off the paint at the gound points on the back of the motor (both locations). Finally, one of the crimps/lugs on my battery cables was terrible and loose and some of the negative connections inside the battery box were also sloppy. I spent several hours disassembling, cleaning/polishing, and greasing every "non-plug" connection I could find. I have not seen the blinking wait light since. Even if it doesn't solve the problem you are seeing, it's probably worth the time to make sure all connections are solid. I considered doing the aftermarket ground harness that some folks seem to swear by, but I honestly didn't understand the point of it when it was primarily duplicating the factory grounds. If your factory grounds are good, I don't see the point of the harness and there are a couple situations where I think it could be a safety hazard.