2008 - Tow package brake sense line problem
 

(Newbie, first post)

I just bought this truck and wired in my Prodigy brake controller from the other truck. It kept actuating the brake unexpectedly. My wife took it to where we have our trailers inspected and they said my wiring and the brake controller was fine, it was the brake switch. So she took it to the used car place (3mo/3kmiles) and they wouldn't believe her because "The brake lights always go off" and that was, to them, that.

So I had to (bleeping) check it out myself -- I did the following with and without the controller connected, same results.

Engine Off - voltmeter reads 0 volts.
Engine On, foot off pedal - voltmeter reads .4 volts
Engine On, foot on pedal - voltmeter reads 14+ volts. Brake lights on.
Remove foot from pedal - voltmeter drops to 0 briefly, then returns to .4 volts, brake lights off.

And that's seems to me to be all good (don't understand the 0.4V) and is what happens most of the time. However -

Jab the pedal a few times and, more often than not, the voltage sticks at 13.5V!! And will stay there apparently forever unless you hit the pedal again!! However, the brake lights are properly off.

WTH???

Sound's like your switch may be starting to fail. You should not be reading ANY volts if the switch is in the open position. (disclaimer, I am not sure for your specific application, but most switches are usually an all or nothing application.)

Either one of two things are going on. You have a short to ground somewhere and it is back feeding the line somewhere. Or, the switch is starting to fail and allowing a small amount of current to bypass the switch. This would explain the small amount of voltage you are measuring. Additionally, this would explain the drop to 0 volts then the slight increase. Additionally that small amount of current may also be setting off your brake controller.

Of course the short else where is also a possibility, but i usually start with the simple things first (switch).

Thanks.

So it's not some "virtual" switch made of silicone and software as I had feared? It seems like it must be a double pole switch, as the brake lights like I said do not reflect the problem. Where the heck is it? I looked what I thought was all the way up the brake pedal and didn't see a switch, I opened the hood and couldn't identify anything either?

Anyway:

On Monday it was working perfectly but then as of today it's stuck at 14.4V, I took it and my voltmeter down to the (small used car) dealership and argued with the service guy and he finally gave in and said to bring it back this afternoon. He had no concept of the Chevy tow package, he thought I put the harness in myself! And therefore how could he be expected to deal with it?

Good grief. Finally he said he would look at it but criticized me for "having an attitude". :rolleyes: I pointed out that I didn't arrive with one, and maybe he shouldn't be so quick to challenge a guy with an MSEE over diagnosing the functionality of 4 stupid wires. Not to mention this is the third time I've installed this exact controller...

Interested to see how it works out. keep the updates coming

A-ha!! Apparently fixed... I only say apparently because I haven't had a chance to hook up the voltmeter yet, 99% sure.

So my wife takes it down. Service guy says it will probably take all afternoon. She tells the service guy she has "nothing better to do but wait for it since I can't take my horses anywhere" and plants herself at the dealership. Not exactly what he wanted, apparently. So this inspires him to get messing around, he mutters and hems and haws and finally admits "your husband was talking over my head" and claims again he can't see anything wrong with the truck. BUT: he decides "it's just a $15 part so I'll put it in for you even though I don't see how it's going to work".

She takes it home, hooks up the trailer and goes out for a spin... perfect!! Loads up the horses, takes them for essentially a drive around the block (the horses must have wondered WTF that was all about) and it pulled and stopped beautifully.

She brought the switch home, and it was as expected - it has 4 connections, so I'm sure that one set is for the brake lights and the other is for the tow package wiring. Thus the brake lights can work properly but the trailer brake can still be screwed up.

I'll post the voltmeter readings when I get to it, I'm sure they will be an unsurprising 0 and 14.4V.

Humm, without looking at the wiring schematics, I am at a lost at what exactly all four poles do. Usually have one for ground, one for hot in, one for hot out. But, i'm sure these new fangled do dad computer controlled cars, anything is possible :) I'm glad it worked out for you and I'm glad the horses got to go for a joy ride. :)