Help needed ! (51 M1 with 12V conversion)

I've been having a battery drain problem that I've traced to my pressure actuated brake light switch. Stock wiring, OEM style switch, replaced the switch once thinking it was defective, new one does the same thing.

These are fed power through the green wire and "make" the circuit when around 90 psi of hydraulic pressure is applied.

Problem seems to be that under no pressure, one of the 2 bullet type pins still has high resistance but not infinite, this seems to be allowing a trickle of current to ground? Acts the same whether the taillights are connected or not.

Has anyone ever seen this? I can't see any way to isolate it.

 

I saw something similar with my 6 volt truck and that particular wire about 2 months ago. As you've already figured out, that wire is always hot (even when the ignition is off). Mine had an open spot up near the firewall and another wire beside it was frayed somewhat. ONE single strand from that other wire touched that constantly hot wire and gave me fits until I focused my eyes and noticed it the next day. Look "upstream" and see if you can find a naked spot or another wire hitting the end of that hot wire, etc.

 

I doubt that you can, …….as it has to be isolated internally where the contacts are and the pressure overrides the spring to make a circuit. I’m kind of old school and not much for meters so if there was enough drain to kill the battery (how long) I would think that you should get a mini spark when you touch the hot lead if there was a short to ground. I would disconnect the hot lead over night (week?) and see if the battery stays charged. Just like medical science, a process of elimination

 

Doc, my post was generic and strictly coincidental even if it doesn’t look it, I had no idea you where posting just before me. I have auditory dyslexia (self diagnosed learning disability) and must write everything on a word processor so it sometimes takes a while to get a message composed and other posts get posted. Sorry.

 

Oh I didn't even think twice about that or even really read anything into it at all. Plus you talked about a different possibility and how to diagnose it anyway. I've been sitting here thinking about that. Considering he's tried 2 different switches, it has to be some sort of problem with a ground somewhere close. But how it's draining a 12 volt battery and not obviously activating something that should be "off" is a mystery.

 

You say stock wiring and 12VDC. I don't have a 51 stock diagram but have a 48-49. If yours is the same, there are two other wires and the green wire attached together at the C/B panel. You will need to remove the green wire to trouble-shoot the stop light circuit, other wise one of the other two wires may have the high resistance short/drain. Disconnect both ends of the green wire (C/B panel and stop switch). Place a meter on one end of the green wire and the other lead to ground. High resistance or not??? Check wire attach points on the stop light swirch to case to see if a high resistance shows. If both ok one of the other two wires on the C/B could be the problem. On the 48-49 one of the other wires is the main light switch and we know when they are old they can have shorts and some cause fires. Just guessing, chuck

 

Thanks for the responses, I'll add a few comments for clarity.
-stock wiring harness, but a brand new reproduction when the truck was restored 3 years back. 12VDC negative ground.
-I originally had a modern style plastic (normally open, electrical only) brake switch mounted on a bracket under the floor, when the pedal moved away the switch made contact and activated the lights. It wasn't weather tight and eventually the contacts failed. It was isolated (plastic) and I never had the battery drain.
So last winter I decided to add the hydraulic OE style back in and that's when the problem began.

If I disconnect the negative battery cable and put my test light between the post and cable, I can see the test light filament just glow orange. (flow)

If I disconnect the green wire at the firewall it goes off immediately. (that was my old school method, unplug until you find the offender)
If I unplug the hot green at the switch it goes away. (hot has 12.6V there)
I would think if the wire itself were the culprit I'd have drain regardless of the switch being connected.

The one pin on the switch has infinite resistance to ground, the other has something like 1.3 mega ohms but the meter never really steadies on one number. I actually tested the new switch before putting it in and it did the same thing, but at one point both poles were infinite.

Could it be they are sensitive to mounting position? I have it mounted horizontal an don't recall the OEM orientation. Maybe it should be vertical?
Grasping at straws...

Thanks again guys!

 

Forgot to mention, If I drive the truck on the weekend, by Tuesday night cruise night it is drained to the point where I'll get one slow crank before the sound of the solenoid clicking (laughing) at me.

If I disconnect the negative cable, no problems, just a nuisance.

 

isn't a 51 master under truck on frame? (i work mostly with 40's, could be wrong). are taillites staying on? almost sounds like residual pressure in master, activating switch. (air in master or one set of shoes dragging just enough to cause heat, producing pressure.) are there small clips that lock wires on switch? are they touching? is there enough free play in master rod? what shape is main light switch in? might be the culprit.

 

Hope I'm reading this correctly. Two pins, no wires, one has high resistance? If so, put output wires to pin with high resistance, power feed to the good pin.

 

How about adding a short extension to the green wire and tying it to the "On" terminal of the ignition switch so it's only under power when the key is on? Yeah, I know. It's not stock, but if it fixes the problem and you don't have to always disconnect the battery....

 

GFW - I thought about that too, but it made no difference.
Petey- Brake lights aren't on all the time, master is under floor, no clips, light switch was new 3years ago with the harness.

So to add to the mystery, today there's no draw. Both switch pins have infinite resistance and it's like the problem is gone.
I haven't driven it to warm anything up, but I hadn't yesterday either so I can't assume it's temperature or pressure.
(when I put the new switch in yesterday, it had draw immediately and I hadn't pushed the pedal or done anything to create pressure)
The odd thing today is the old switch still had 1.3 Mega Ohms on one pin but even when I plug the harness into it and ground the switch, no draw.

So now the problem is intermittent and I don't know when to trust it.

On the diagram the green wire in question is called 14G, it is connected at the circuit breaker to 16B-Bl and 15YR. 16B-Bl goes to the courtesy lights and 15YR goes to the headlight switch. From there I don't see 15YR anywhere else so it must be a feed line for switched headlights and parking lights.

Hopefully?? the draw comes back so I can try a few more of these ideas and nail it down.

Thanks all!

 

Have you replaced the kick switch for the bright headlights? It's a normal culprit because all your power goes thru that switch. It frequently fails and is the first thing I'd swap out.

Available at NAPA, make sure you get one for the right connections you have. They carry both screw down and push/post connections.

 

MT, not sure I follow, there shouldn't be any power in the 12RY line unless the headlight switch is on 1st?
This switch was also new 3 years ago, it wouldn't take much to isolate it but if I follow that plan of action I'd have to replace everything again and to clarify, all the electrical switches and wiring was new when the truck was restored.

Like I said when the 14G wire was disconnected originally the drain immediately went away. That feeds the brakelights, nothing else.

If I get the drain to re-appear I'll try disconnecting it, just to rule that out.

Thanks!

 

is it possible water dripped on lite switch and damaged unit? sounds like something somewhere may be slightly rusted and intermittingly grounging out. i would pull out switch and inspect/test it. check all wire crimp joints, also.