Brake Lights

[NCarolinadawg]

A little help. 1989 M998 6.2 3 speed. While sitting in garage in neutral, brake lights work. So all this time I think I am good during my pre-drive check LOL. But while driving down the road the brake lights do not work? ALL other lights work, except blackout which I do not have any bulbs, but don't think that would matter. Any ideas why brake lights work sitting in garage in Neutral, but not going down the road?

 

[DREDnot]

I had the same issue. It was cured with a new brake light switch.

I opened up the original switch and the point of contact had years of wear and burnt metal from arcing across the break in contact. Tried cleaning it and reassembled it but no better. You couldnt get a consistent pedal travel setting that stayed. The sweet spot was right at the erosion area of the contacts.

I ponied up for the NOS switch and it was able to be adjusted to a reliable and consistent position

 

[papakb]

There's a basic flaw in the design of the brake switch system in that there's no free play adjustment in the linkage so you have the brakes coming on before the switch contacts for the lights makes and this gets aggrivated by years of the switch arcing internally and needing to move even further to make the light switch. The mounting adjustment of the switch is fairly coarse and sometimes it gets tricky to setup. I finally gave up on mine and installed a normally closed switch on the brake lever that turns the lights on as soon as the pedal leaves the stop.

 

[TOBASH]

I had the same issue. It was cured with a new brake light switch.

I opened up the original switch and the point of contact had years of wear and burnt metal from arcing across the break in contact. Tried cleaning it and reassembled it but no better. You couldnt get a consistent pedal travel setting that stayed. The sweet spot was right at the erosion area of the contacts.

I ponied up for the NOS switch and it was able to be adjusted to a reliable and consistent position

View attachment 804697

To fix something like this you can try to TIG weld and then file down vs. high temp silver based solder if metal is compatible vs. braze on brass if metal compatible vs. place a rivet in place of the worn contact vs. filing the contacts down to smooth and getting a few more years out of the unit.

Sometimes it is cheaper to replace. Other times it is cheaper to fix.

 

[juanprado]

Just seems nuts to me as the civilian world has figured out how to make reliable stop light switches for a while. This crazy contraption is right out of the automotive stone ages...
A simple switch screws on to a bracket that contacts the pedal would be all that it takes and no I am not an electrical/mechanical engineer but stayed at a holiday inn express many moons before covid...

 

[papakb]

When you understand the history of the HMMWV you'll understand why some of it looks like your grandfather (great) designed it. AM General has been building military trucks forever and they simply used the same POS brake switch they put into the M151 in the '60's and God only knows how many other trucks. There was nothing leading edge when these things were designed in the early '80's so in an original M998 your looking at 35+ year old or older design concepts.

 

[TOBASH]

Just seems nuts to me as the civilian world has figured out how to make reliable stop light switches for a while. This crazy contraption is right out of the automotive stone ages...
A simple switch screws on to a bracket that contacts the pedal would be all that it takes and no I am not an electrical/mechanical engineer but stayed at a holiday inn express many moons before covid...

So the OP's vehicle is 31 years old and the brake switch wore out and as such is considered unreliable. LOL

My GM cars all needed new brake switches after 15 years, so 31 is OK in my book.

These former military vehicles are old, tired, and retired. They were rode hard and put to bet wet. If they were new, Uncle Sam would not have auctioned them off. We all bought used vehicles and might need to change a light bulb occasionally. LOL

Love you Juan, but the OP vehicle is 31 for Heaven's sake.

 

[NCarolinadawg]

There's a basic flaw in the design of the brake switch system in that there's no free play adjustment in the linkage so you have the brakes coming on before the switch contacts for the lights makes and this gets aggrivated by years of the switch arcing internally and needing to move even further to make the light switch. The mounting adjustment of the switch is fairly coarse and sometimes it gets tricky to setup. I finally gave up on mine and installed a normally closed switch on the brake lever that turns the lights on as soon as the pedal leaves the stop.

View attachment 804717

Can you send a link of what you purchased to fix please?