Humvee cluster acting up solved

[Eli77]

Hi all new to this forum hoping I am posting it in the right place
I have 1986 m998 that recently the cluster stopped working correctly. It seems like it is an electrical issue but I couldn’t find anything online similar to it .after starting the engine the gauges stay down and every 10 sec or so they spikes a little ,and then again they go dead. In addition there is some kind of noise. I am assuming it is a relay not 100% sure.
Any help or leads will be appreciate, I am attaching a link to my you tube video where you can see exactly what is going on

Humvee m998 gauges issue

youtube.com

 

[Mullaney]

Hi all new to this forum hoping I am posting it in the right place
I have 1986 m998 that recently the cluster stopped working correctly. It seems like it is an electrical issue but I couldn’t find anything online similar to it .after starting the engine the gauges stay down and every 10 sec or so they spikes a little ,and then again they go dead. In addition there is some kind of noise. I am assuming it is a relay not 100% sure.
Any help or leads will be appreciate, I am attaching a link to my you tube video where you can see exactly what is going on

Humvee m998 gauges issue

youtube.com

.
I am just fishing for ideas here but you mentioned "everything was just fine" and then it wasn't. Any particular thing that changed? Go for a spin around the block and it happened? OR maybe you went on a cross country trip, ran through a creek and now everything is "wonky" now?

Ground is always a possibility on anything electrical.

Just for the heck of it, get a piece of wire (maybe 14ga) and drag it from one of the ground connectors on the cluster and out to the to the negative battery post. Does that change anything?

 

[Eli77]

.
I am just fishing for ideas here but you mentioned "everything was just fine" and then it wasn't. Any particular thing that changed? Go for a spin around the block and it happened? OR maybe you went on a cross country trip, ran through a creek and now everything is "wonky" now?

Ground is always a possibility on anything electrical.

Just for the heck of it, get a piece of wire (maybe 14ga) and drag it from one of the ground connectors on the cluster and out to the to the negative battery post. Does that change anything?

.
I am just fishing for ideas here but you mentioned "everything was just fine" and then it wasn't. Any particular thing that changed? Go for a spin around the block and it happened? OR maybe you went on a cross country trip, ran through a creek and now everything is "wonky" now?

Ground is always a possibility on anything electrical.

Just for the heck of it, get a piece of wire (maybe 14ga) and drag it from one of the ground connectors on the cluster and out to the to the negative battery post. Does that change anything?

Hi. So actually yes I took it for a quick off-road drive and at some point it happened.
thanks for the reply I will try to ground it. As soon as I get home and keep you updated

 

[TOBASH]

Agree with Mulley. Ground is suspect but please specify issues.

I had this exact issue. I needed to clean the ground screw/bolt in front of and behind the firewall, as well as each and every internal contact. I actually used Copper wire to bypass some corrosion preventing a cluster light from powering up. (I also had several wires connected with backwards polarity that I was able to correct. )

 

[williamh]

I would say theirs a short someplace and the circuit breaker is tripping , comes on for 1 sec and Tripp’s off. You’ll find it when it catches fire. or you start looking for a shorted min feed wire

 

[TOBASH]

Am I

I would say theirs a short someplace and the circuit breaker is tripping , comes on for 1 sec and Tripp’s off. You’ll find it when it catches fire. or you start looking for a shorted min feed wire

Am I missing something??? Circuit breaker? IIRC, power goes straight through from the PCB/EESS.

 

[springer1981]

I think you are tripping a circuit breaker also. There are 2 behind the gauge cluster panel. One is directly off the RUN position. I would start looking for the problem before doing that again.

 

[Eli77]

I think you are tripping a circuit breaker also. There are 2 behind the gauge cluster panel. One is directly off the RUN position. I would start looking for the problem before doing that again.

I agree that’s what I am trying to do now what am I looking for besides possible short ?

 

[springer1981]

I would start at the circuit breaker. Remove the gauge panel and there are 2 circuit breakers. I attached a picture if you don't know what they look like.

CB-2 is the one that supplies power to everything. Find that one and pull one side of it and check for voltage to see what side is the feed. Then with it disconnected you can use and Ohm meter and check it to ground and see if it is shorted. I suspect it will be shorted. Leave the meter on it and systematically start disconnect circuits to try to isolate where the problem lies. I attached a manual that includes wiring diagrams at the end of it.

 

[TOBASH]

Wow. Had no idea those are in there! Thanks Springer

 

[papakb]

I think springer1981 has the right idea on this. The instrument cluster has a 5 lead "spider" harness that gets it's power directly from CB2 when the RUN switch is turned on. This is the power for all of the gauges. If it's tripping and then trying to reset it'll act this way. A key indicator here is the BRAKE lamp flashing. It gets it's power from the same breaker.

Find any wire with a #27 tag on it and measure the voltage there. It doesn't matter which one, they're all in parallel.

 

[86humv]

While the dash cluster is off...Look up to the right and check the ground...its is a stud thru the body.....and goes to all gages and more.

 

[Eli77]

Quick question. Could a bad glow box control cause a short? Just out of curiosity?

 

[TOBASH]

Short answer… unlikely but anything’s possible I guess

 

[springer1981]

First I don't believe it's a ground issue. A bad ground wouldn't trip the breaker like that (assuming it is a tripping breaker we hear).

I have attached a basic wiring diagram, M997 in this case since I couldn't find a M998. They should be close enough for this discussion. I have also attached the full wiring diagram in PDF form.

In the picture I color coded some of the wires for explanation purposes.
Note, The Yellow circle is the Brake light that flashes on and off.
Note, the upper left corner is the PCB (Protective Control Box).

The RED wire coming from the PCB goes to the Master Start Switch (Ignition switch). I believe the power to the RED wire basically passes through the PCB.
The BLUE wire coming from the Master Start Switch is the RUN position on the switch and goes to CB-2. I suspect what you hear clicking is CB-2. If you find CB-2 and touch it while it is tripping you will feel it turning on and off easily. I would do this FIRST to confirm my guess.
Also coming from the RUN is the GREEN wire that tells the PCB it is in RUN Positions. NOTE it does NOT go through CB-2 and therefore unlikely to be the problem.
Continue following the BLUE wire from CB-2 and it takes you to a junction that sends power to 4 different location.
1 The wiper motor.
2 The Wait Light
3 The Brake Light
4 All the gauges.

The BROWN wire comes from the PCB and goes to CB-1 and then to the Heater Fan Switch. This circuit is unlikely to have anything to do with the issue and I only point it out because it is the other Circuit Breaker, CB-1.

Lets assume you felt CB-2 turning off and on and confirmed the issue. Now you can narrow down what is causing the circuit breaker to trip.
1 Unplug the Wiper Motor.
2 Unplug the power side of all the gauges.
3 Do not unplug the brake light.

Now turn the power on to RUN and see if the brake light stays on. If it does, then plug in the gauges one at a time and test after each one to see if any of them are the problem. Last plug the Wiper Motor back in and see if that causes the issue.

 

[Action]

What if power wire is shorted between the circuit breaker and wiper motor, brake lights (or switch), or dash? Then, everything could be unplugged and still trip the breaker. I think that would be a worst case scenario though.

 

[springer1981]

What if power wire is shorted between the circuit breaker and wiper motor, brake lights (or switch), or dash? Then, everything could be unplugged and still trip the breaker. I think that would be a worst case scenario though.

I'm going to guess it is not shorted at a wire because if it was the brake light would not come on. The way it is acting, it seems as if something has a high current draw rather than a dead short. Maybe the wiper motor is no good and the switch is on for example. Or maybe there is a bad gauge. A dead short would go direct to ground and no power would go to the light or the gauges.

I have a good electronics background and have done lots of wiring and troubleshooting however this is all guess work since all I have to go by is a short video and the fact that I have never worked on any HMMWV's other than the one I own.

 

[williamh]

A dead short “would” allow for the breaker to reset for a second before re tripping , the resistance from the length to the short would allow it to reset , the lower voltage takes a second to heat up unlike the house voltage that trips instantly , higher voltage heats wire faster.

 

[springer1981]

A dead short “would” allow for the breaker to reset for a second before re tripping , the resistance from the length to the short would allow it to reset , the lower voltage takes a second to heat up unlike the house voltage that trips instantly , higher voltage heats wire faster.

No, sorry. Please see attached picture and explain to me at what point in that circuit would the light come on? Hint, the answer is never. If any of the positive (BLUE wires in the diagram I posted) were shorted to ground anywhere it would be the same as the circuit in the picture below. Electricity will ALWAYS take the path of least resistance. Doesn't matter the voltage, doesn't matter if it is AC or DC.

 

[Milcommoguy]

I'm going to guess it is not shorted at a wire because if it was the brake light would not come on. The way it is acting, it seems as if something has a high current draw rather than a dead short. Maybe the wiper motor is no good and the switch is on for example. Or maybe there is a bad gauge. A dead short would go direct to ground and no power would go to the light or the gauges.

I have a good electronics background and have done lots of wiring and troubleshooting however this is all guess work since all I have to go by is a short video and the fact that I have never worked on any HMMWV's other than the one I own.

"I have a good electronics background and have done lots of wiring and troubleshooting "

YES, you do. Good analytical thinking and pulled the prints. BIG POINTS to Springer... Beyond the call of duty.

I note too... the volt meter swing to low "yellow" guessing about 19 +/- volts, figuring in the slow ballistics of the needle movement to get a "snapshot". Not a solid "DEAD" short... but a unwanted very low resistance or there is a wire getting very hot, very fast. LOL.

As noted, Pull one of the CB-2 leads should get one going in the right direction. Then follow those wire numbers out to all the BLUE connection, one at a time. Clear the "blue" circuits one at a time (print is the truth table) and see where it lead one. Keep in mind the breaker could be wonkie too.

PAY attention to everything he as stated.

The applied theory is spot on... Process of guess elimination... as nothing is text book with HMMWV. LOL

As a double check... does the truck start and run with CB-2 disconnected?

That's my guess for the year, CAMO