barrman : Brake lights

Brake lights
Post by barrman on Sept 6, 2006, 3:10pm

OK, I am planning to switch my truck over from the hack wiring job the fire department (DSPO) did. I still need to drive it 2-3 days a week and don't want to be driving a vehicle that isn't legal.

The DSPO cut off the rear wiring harness above the axle. They just unplugged the front b/o lights. I think my truck last ran with the large blinker box because of the letters on the cut off wires under the hood. But, a small solid state blinker box was bolted to the firewall with the proper pig tail screwed in and about 3 inches of wire. They added an aftermarket blinker set up and just ran all the blinker wires next to the stock wiring.

I got my Packard connectors in the mail today and want to do the swap RIGHT NOW. I already proved with my bench wiring set up that the blinkers and such work. I still have to find the dash end of the 4 blinker wires, but that should be easier once I cut out the DSPO stuff. The only question I have is the brake lights.

http://www.m715zone.com/manuals/wiring/lighting.pdf

I don't know how to copy/paste a pdf page, sorry about that. Anyway, the above drawing shows that wires 75 and 75A are the leads to the brake switch. One is power and the other power back to the 3 lever switch once the brakes are used. I have tried 3 different 3 lever switches and with power in to the "F" lead and the top lever to "Brake light or Service Light." I never get any power to either 75 or 75A. I am doing this without the blinker box hooked up, but that shouldn't matter for this. I think I am missing something simple. Anybody know what it is?

Thanks.

Re: Brake lights
Post by brute4c on Sept 6, 2006, 4:02pm

You cant put a pdf page in a post...so dont feel bad there....

Not sure where to start on the brake light problem....maybe like this:

Try checking the pins A and K and see if either actually has power going out of the 3 lever light switch when you have it in the right position...as you mentioned above....

If power is going out of the switch and not getting to the other end, then the wire must not have continuity for whatever reason....there isnt anything else in that path....I took a whole harness apart some time back and know this to be true...

Could it be that a ground is needed....the switch grounds through the brake line and then the frame....maybe it isnt getting there for some reason?

Re: Brake lights
Post by k8icu on Sept 6, 2006, 8:39pm

The switch shouldn't be grounding. It is just a switch it is either open or closed. If closed then power should run through it. If open then no power. It should only close when your push the peddle and fluid pushes againts it. It sounds to me that you may not have continuity either through the three lever or in the power feed.

If you check the TM-20 it should have a section on electrical trouble shooting.

My M151 manual (should be the same as it's mil standard) says to check the voltage at pin F on the conector to the light switch. If you have 24V there (12 in your case?) then you should jumper a wire to the different circuts like A etc and see if the lights light. If they do then the three lever switch is bad. If they don't then you have a wiring issue.

Hope that helps.

As my wife always says.....RTFM (read the friggen manual)...lol

Re: Brake lights
Post by barrman on Sept 7, 2006, 12:00pm

Jumping power from F (Power to the 3 lever switch) to K and pushing the brake pedal gives me voltage at A. I just finished permanently wiring the two brake leads to the brake switch. The truck is about 4 feet from me as I type.

We have the front b/o lights working along with the head lights. We also stripped all the civilian lighting wires out. I now have to run wire from the rear shock area to each tail light, find the firewall end of all 4 blinker wires, hook the blinker switch and blinker box into the system. I really should do all of this before I try to drive home tonight too.

I am really regretting going through the mud hole that is my avatar today. I am covered head to toe in Tennessee dirt right now.

Re: Brake lights
Post by brute4c on Sept 7, 2006, 12:05pm

So you have 3 different 3 lever switches and none of them will route power to K to go out to the brake light switch without being manually jumped from F....weird....

Maybe I am reading to much into this though....

Re: Brake lights
Post by barrman on Sept 7, 2006, 12:38pm

Jon,
I had 3 switches. One I already knew was bad and confirmed that by taking it apart. Parts went flying and it is never going together again. The other two didn't seem to work with me holding a test light on a bare wire end under the truck. Just before lunch and this Zone session, I connected two male Packard fittings to the brake wires under the truck and connected them to the brake light switch. (By the way, this is the NAPA switch that I got for around $10.00) I don't know if the factory switch took male or female connectors, mine took a male, so that is what I have. With the switch off the truck. I stuck a jumper wire into the F position on the plug. The other end of the wire was in the K spot. I put a test light in the A position and pushed the brake pedal. The test light worked with the pedal. Once I get caught up on the Zone, eat lunch and get my students back, I will start wiring up the rest of the stuff and see if it works.

Re: Brake lights
Post by brute4c on Sept 7, 2006, 12:42pm

I see...yep...I jumped ahead in my conclusion...sorry about that Chief!!

Re: Brake lights
Post by barrman on Sept 8, 2006, 8:59pm

It was the 3 lever switch. I finished wiring everything up today and still had no brake lights. I put in my 3rd and final 3 lever switch and everything worked like it was supposed to. Including the b/o stop lights since the M35 rear buckets have that option. Well, the dash lights don't work on "Dim." I think that is more a matter of my system being 12 Volt because none of the other switches made it work either.

By the way, I ordered 20 feet of mil spec wiring thinking it would be enough to do the blinker harness and then most of the tail light stuff. Nope. 8 wires need to be connected between the master cylinder area of the firewall and the blinker switch on the steering column. I got 6 of them done before that 20 feet was used up. Figure on at least 30 feet or buy the harness pre made for $60.00.