M998 lights pulsate when turn signal is active at speed.

[unknrbl]

Vehicle: 1986 AM General HMMWV M998 w/ 6.2l Detroit Diesel
Electrical Mods: H4 Bulb adapter, 24v to 12v conversion with no accessories installed.

Hello Steel Soldiers,

About a week ago, I was driving home when my lights started to have power spikes which ended up shortening out both the LED headlights of 2 years while driving. I then found out that my battery terminal cables came loose and was flopping around while in motion. I've secured those down and continue to drive home without actual headlights (zip ties and a flashlight). While driving, I've noticed that while in motion with the lights on service drive and the turn signal active, all of the lights (interior/exterior) will pulsate (brightening/dimming) with the turn signal and the lighting appears to go faster than normal. I parked the truck and tested the head lights (military and H4) with a multi meter with success. I also double checked the wiring and tested the headlight with both incandescent and LED H4 bulbs. Incandescent burnt out immediately where LED lasted a long time. I then ordered a new set of 12k LED H4 bulbs because why not.

Once the bulbs were installed, I took the truck out for a test drive. Approximately 20 minutes into the test drive, I've noticed that the pulsating lights issue reappeared when the turn signal was engaged. This time, the drivers side headlight burnt out while the passenger stayed on. I then took it back home, parked it, removed the headlights, recheck the wiring and could not find anything out of the ordinary. I then did a voltage drop test of the generator to make sure everything was functioning, which it was. I then checked all of the wiring harnesses and connections while following the wiring schematic in the TM to make sure nothing was loose or damaged. Other then looking aged, the connections and harnesses appeared to be fine. I then did another road test to isolate the symptoms

When parked and no lights were on: No issues.
When parked and stop light/service drive active: No issues.
When parked and stop light/service drive active w/ interior lights: No issues.
When parked and stop light/service drive active w/ turn signal (L/R/4): No Issues
When parked and stop light/service drive active w/ turn signal (L/R/4) and interior lights: No Issues

Vehicle starts normally. No unusual sounds/movements.

When driving and no lights were on: No issues.
When driving and stop light/service drive active: No issues.
When driving and stop light/service drive active w/ interior lights: No issues.
When driving and stop light/service drive active w/ turn signal (L/R/4): All lights pulsate
When driving and stop light/service drive active w/ turn signal (L/R/4) and interior lights: All lights pulsate

At approximately 5-10mph is when the issues appear. I've also notice that going faster equals more intense frequency, but I'm not 100% sure. Can't go fast in a mobile home community to test this theory.

TL;DR: My interior/exterior lights pulsate when the turn signal is active at speed which has shorted out 3 headlight bulbs. Once the vehicle is stopped, all the lights function normally again with the turn signal active. All tests and inspections appear to suggest that everything is fine, but it isn't.

Any suggestions? Recommendations? Advice?

Thank you in advance for your replies.

P.S. I'll try to post videos of the issues later on at night as its easier to see what is going.

 

[Mogman]

I would put a volt meter on this thing!!!!!!!!!!, you are blowing lights, that is a bad sign.
Off hand I would say you have a 60A alt that has gone wonky

 

[unknrbl]

I would put a volt meter on this thing!!!!!!!!!!, you are blowing lights, that is a bad sign.
Off hand I would say you have a 60A alt that has gone wonky

I've put a multi meter on it in any spot I can think of (batteries, headlights, the 60a alt), but the readings are looking normal though.

My guess was also that the alternator might be going, but I'm not 100% sure currently.

 

[Mogman]

I've put a multi meter on it in any spot I can think of (batteries, headlights, the 60a alt), but the readings are looking normal though.

My guess was also that the alternator might be going, but I'm not 100% sure currently.

Have you monitored it while driving?

 

[unknrbl]

Have you monitored it while driving?

I have not monitored directly it while driving yet. I can rig up the multi-meter since I got probe extenders. I take it it's the main positive battery terminal to the negative alternator bolt like with the voltage drop?

 

[Mogman]

No I would monitor the 24V output of the alt to ground or the voltage from the battery 24V post to ground.
Have you ever measured the alt output voltage? if so what was it while the engine is running?
If you are monitoring the battery voltage and it is steady when the lights are acting up you may have a ground issue

 

[unknrbl]

No I would monitor the 24V output of the alt to ground or the voltage from the battery 24V post to ground.
Have you ever measured the alt output voltage? if so what was it while the engine is running?
If you are monitoring the battery voltage and it is steady when the lights are acting up you may have a ground issue

I'm not 100% sure if I did or not. The only test I did was the main positive terminal on the battery to the negative bolt on the alt with the engine/lights on. It held steady at 30.4v and the batteries were at 15.6v and 14.8v when I tested them individually with the engine/lights on.

Is there a better spot to test it with the multimeter? I do apologize in advance that I'm rusty when it comes to diagnosing electrical.

 

[Mogman]

I'm not 100% sure if I did or not. The only test I did was the main positive terminal on the battery to the negative bolt on the alt with the engine/lights on. It held steady at 30.4v and the batteries were at 15.6v and 14.8v when I tested them individually with the engine/lights on.

Is there a better spot to test it with the multimeter? I do apologize in advance that I'm rusty when it comes to diagnosing electrical.

That is a little too high, 28.5 MAX would be much better, sounds like the reg in that 60A is going south, the regulation usually get worse the hotter the alt gets, like on the road so you are likely spiking much higher at times.
This is a common issue with the 60A alt.

 

[unknrbl]

That is a little too high, 28.5 or so would be much better, sounds like the reg in that 60A is going south, the regulation usually get worse the hotter the alt gets, like on the road so you are likely spiking much higher at times.
This is a common issue with the 60A alt.

Sounds like I need to look for a new 60a alt, rebuild the existing, or consider upgrading to the 200a alt since why not, lol.

 

[Mogman]

Sounds like I need to look for a new 60a alt, rebuild the existing, or consider upgrading to the 200a alt since why not, lol.

The 200A is a much better alt and you balance the voltage on the batteries, If you can find a good alt, complete install kits are common and reasonably cheap.

 

[unknrbl]

The 200A is a much better alt and you balance the voltage on the batteries, If you can find a good alt, complete install kits are common and reasonably cheap.

I'll go down that route then since it's the better option overall.

Thank you for your time! I'll keep the forum posted of any updates.