M939 Stop Switch Questions...

[The HUlk]

The first question is, Is the stop switch a normally closed contacting type switch?


The reason I ask is...


I just purchased an M931A1 with inoperative stop lights. The signals and hazards work correctly along with running lights and headlights. With the light switch set to Ser. Drive all exterior lighting is illuminated. I removed the light switch, ran 24v into pin F and I get a 24v reading at pin A(cir. 75) which supply's the stop switch. When I measure for resistance in the wiring harness light switch connector pin A(cir75) to pin K(cir 75), both of which go to the stop switch, I get an open circuit. Reversing lead polarity still reads open. I put 24v on the wiring harness light switch connector pin A(cir 75) and measured from pin K(cir 75) to GND and do not read any voltage, even with the tanks at 90psi and stepping on the brake. No continuity and no power will go through the stop switch.


The second question is, If the wires to the stop switch are good is it safe to say the stop switch is defective?


Thank you you for any insight as I begin straightening out a few minor issues on this M931.

 

[juanprado]

When the stop light pressure switch gets pressure it closes completing the circuit. Bypass the switch and jump the wires. If you get lights, then you have a bad switch. A civilian one is easy to find but you would need to wire in as it does not have the military packard connectors but a literal fraction of the cost.

sold at Napa, made by Haldex:
http://www.haldex.com/en/North-Amer...Electrical/Electro-Pneumatic-Switches/BE13250

 

[Artisan]

Those pressure brake switches can be found pretty cheap on Ebay.
I offered one seller $16.00 EA for 2 of them just last week
and they accepted.

But say a prayer before pulling apart some of those old black
male / female rubber electrical connectors. I keep a mini torch, solder and
replacement Mil style electrical connectors in stock. When I reassemble
I use a lil dielectric grease so next time I do not have an issue, hopefully.

There are 2 kinds of guys who own these trucks, those that have
had an electrical connector break upon removal and those who
ARE going to have this happen to them. Some guys just use
a wire nut, I try to do it like my work will be inspected by someone
who knows what they are looking at...

Eriks sells the parts...

 

[The HUlk]

I figured that was how the switch functioned, thank you for confirming.

 

[The HUlk]

But say a prayer before pulling apart some of those old black
male / female rubber electrical connectors. I keep a mini torch, solder and
replacement Mil style electrical connectors in stock. When I reassemble
I use a lil dielectric grease so next time I do not have an issue, hopefully.

There are 2 kinds of guys who own these trucks, those that have
had an electrical connector break upon removal and those who
ARE going to have this happen to them. Some guys just use
a wire nut, I try to do it like my work will be inspected by someone
who knows what they are looking at...

Eriks sells the parts...

This had a rebuild in 2010 and all of the wiring I have seen is in like new condition so hopefully there will be no crumbling or grumbling during the replacement. And for the record I would solder the correct ends on if any damage occurred. I'm in the "do it right the first time so you're not revisiting it in the near future" crowd.

 

[Artisan]

I solder all my connectors too. A Harbor Freight mini torch and a roll
of flux filled solder works well. I got a replacement switch on Ebay
for $16 and thru it in and all is well.

 

[Coffey1]

On my tan M923 the wires were good and there was no problem with the stop light pressure switch.
It turned out to be the service light switch.
The headlights and everything would work except the brake lights.
Changed it out and all was good.

 

[The HUlk]

On my tan M923 the wires were good and there was no problem with the stop light pressure switch.
It turned out to be the service light switch.
The headlights and everything would work except the brake lights.
Changed it out and all was good.

Thank you for this post as I thought this could be my problem as well. It may be the problem in someone's else's case but not mine.

Through troubleshooting with the manual, and answers from the good folks here, I was able to identify the problem without throwing parts at it.

No love from the stop switch, jumper the wires that go to it and the brake lights come on.

 

[The HUlk]

I solder all my connectors too. A Harbor Freight mini torch and a roll
of flux filled solder works well. I got a replacement switch on Ebay
for $16 and thru it in and all is well.

I'm an electrical engineer and it's been said a soldering iron is an extension of my hand. I use a Weller adjustable temp iron but fortunately did not need to use it.

The rubber was not brittle and with a little rotating pull the wires came right off. One of the pins on the bad stop switch busted off so I will remember to go easy on connecting the wires to the new switch and use dielectric grease to decrease friction.

 

[The HUlk]

Replaced the stop switch today and that solved the problem. For those who have difficulty locating it, my 1981 AMG M931A1's is on the inner left frame rail between the transfer case and the rear corner of the fuel tank.

 

[HASSON1911]

Old thread but instead of starting a new one, what would cause my truck to have turn, brake, hazard and marker lights but no running lights?

Also when I hook up to a trailer all lights work including running lights.

Sent from my XT1254 using Tapatalk

 

[simp5782]

Old thread but instead of starting a new one, what would cause my truck to have turn, brake, hazard and marker lights but no running lights?

Also when I hook up to a trailer all lights work including running lights.

Sent from my XT1254 using Tapatalk

You have a bad ground.

Sent from my SM-G860P using Tapatalk

 

[HASSON1911]

You have a bad ground.

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Wouldn't a bad ground also make my blinkers and brake lights inoperable

It's both front and both rear signal lights.

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[simp5782]

Not necessarily. When you plug the trailer in the ground for the truck plug is grounding the trailer lights and your truck lights hooked to it.

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[HASSON1911]

Not necessarily. When you plug the trailer in the ground for the truck plug is grounding the trailer lights and your truck lights hooked to it.

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Hmm...
So check the ground for all 4 turn signals on the semi then.

See I still have blinkers hazard and brake lights even without a trailer on the semi

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[Artisan]

Look at BOTH BULBS too if not LED.

I had an issue, DAM, BOTH BULBS were bad!

 

[The HUlk]

The goofy problems with these trucks never ceases to amaze me!

According to the wiring diagram the tail/marker lights are fed from the light switch. Try disconnecting the light switch and jumping a wire from pin F(+24V) to pin H(tail/marker lights) on the harness connector. When the battery switch is on the tail/marker lights should be on with the jumper in place. If this is true and the tail/marker lights are not on with the light switch connected and in the SER DRIVE position then I would suspect the light switch is not functioning properly.

 

[HASSON1911]

The goofy problems with these trucks never ceases to amaze me!

According to the wiring diagram the tail/marker lights are fed from the light switch. Try disconnecting the light switch and jumping a wire from pin F(+24V) to pin H(tail/marker lights) on the harness connector. When the battery switch is on the tail/marker lights should be on with the jumper in place. If this is true and the tail/marker lights are not on with the light switch connected and in the SER DRIVE position then I would suspect the light switch is not functioning properly.

Im gonna pull one off another truck this weekend and check it out! The thing that dumb founded me was that my trailers running lights worked.

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[The HUlk]

The thing that dumb founded me was that my trailers running lights worked.

According to the wiring diagram the trailer tail/marker lights are fed from the same pin and wiring as the tail/marker lights on the truck so I am dumbfounded as well. Perhaps there has been some re-wiring somewhere in the truck? Or trailer???. I suggest separating the trailer from the truck and troubleshooting the truck by itself. After it is working properly alone then add the trailer into the mix.

 

[HASSON1911]

my hypothesis, assuming wiring is unmolested, is that this problem is in the fuse panel. But again that should mess with the trailer lights. There is of course the rare oddity that all 4 lights have the running bulb burnt out or bad grounds, but should that not also affect the blinkers brake and turn? And like we discussed earlier should it be the 3 lever then why are my trailer lights working. All should be answered tomorrow on my day off!