Brake Lights Work Under Brake, Not Under Service

[Brad4321]

I have a 71 m35a2 with the newer electonic switch. My brake lights and turn signals work fine when the "brake lights" are activated. When service lights are turned on, the brake lights do not light up (the rear running lights are on, however), but everything else works as it should.

I was told that this was by design as the rear lights just have a single filiment, but that doesn't seem too likely, so I wanted to check here as well. If this isn't right, any likely problem? The brake switch and all wiring must be working for the brake lights to work under the brake setting...

 

[AceHigh]

I bet it is a grounding issue somewhere. They can be fun to find.

 

[Brad4321]

Although I do not know anything about the wiring on a deuce, that just doesn't seem likely to me. The lights are grounded, since they work under brake. The brake switch works, since they work under brake. It seems like it would be in the electronic keypad, which is the only other component in the system that I am aware of. I don't know what to look for, but I haven't pulled it apart yet either. I was surprised that this truck had the keypad and not the switches, which makes me wonder why it was changed out.

 

[Oilleaker1]

I would borrow a old 3 lever switch and try it. Just a thought to eliminate the push-type switch. John

 

[Barrman]

I agree, it is probably your switch.

 

[glcaines]

I agree, it is probably your switch.

Ditto

 

[cessnatwin]

I am having the same problem only the service lights do not work yet the brakes due, and it is not the switch, I tried that!

 

[Barrman]

The 3 lever switch does a lot of things. I went through about 9 or 10 used ones until I found a unit that worked on every function when I was building the M715. I have had a pretty good run lately on the other trucks.

Here is the best wiring diagram I have ever seen about what pin does what on the switch:
http://www.m715zone.com/vb/pages/manuals/wiring/lighting.pdf

You have to be a member of the Zone to see it though. If you don't want to go there. Look on the back of the switch. Each pin has very faint and hard to see letters next to it. Here is a short and simple list of what each pin does:


A= power to the brake light switch
B= power out to the dash lights
C= goes to terminal D on the blinker switch, wire 22, it is the power out to the brake lights when A and K are connected.
D=power out to the B/O drive light
E= power out to B/O marker lights
F= Power into the switch from the battery
H= power out to service tail lights
J= Goes to pin G on the turn signal switch.
K= stop light switch return to the 3 lever. Completes the circuit started by pin A with the stop light switch in the middle.
L= parking light power
M= head light power out to dimmer switch
N= Power out to B/O stop light

Here is a list of what wire is supposed to do what:
M-SERIES VEHICLES CIRCUIT NUMBERS

 

[Chieftawny]

I remember reading somewhere that the push button switches have to be grounded at the dash. When you buy them new, they come with a ground lead that is supposed to go to the firewall. Maybe thats missing?

 

[popacom]

Crazy as it seems,also make sure all the bulbs are the correct type and number.
I got into this same issue and it was the bulbs had been replaced with the wrong ones.
Also remember your turn signal control can give these same problem,as the wiring for the stop lights are routed thru it and switched in and out as the turn signals are applied. I would substitute a new
or known good turn signal switch.and see what that does. These are just some notes from my
experiences,and I am by no means a Guru on the subject LOL

popacom / BILL in Ky.