What trailer brake controller do I need for a 69...

[bandit0067]

Guys , my 69 GMC is my work horse and I have only towed with a utility trailer, 4 pin hookup. I need to use a full size car trailer with a seven blade hook up. What controller do I need to purchase for my 69 to apply voltage to a trailer with brakes....thanks..never done this install.

[mongocanfly]

Any add on brake controller will work...the one I used is a centrifugal force controller....

[bandit0067]

Thanks, I did not know what to look for. I was assuming some type was mo better suited for our older trucks....will check that out..

[mongocanfly]

Mine is the curt discovery model 51120

[kipps]

The ultimate is a Kelsey-Hayes 81740. This actuates with brake pressure, and tees into the rear brake line at the master cylinder. It gives true variable braking, and is supposed to be much smoother than modern cheapo pendulum-actuated controllers.

They show up on ebay for 40-ish. You will want the variable resistor along with it. Some listings don't include that, but it's needed for lightweight trailers.

These controllers were sold from the 60's through the 90's. They didn't play so nice with modern ABS, so were dropped at that point. For the pre-ABS vehicles however, they are better than anything else. And they're period correct to boot.

If you google "Kelsey Hayes 81740" you'll get a bit of info on various forums about these. I'm installing one myself, but don't have it complete yet.

[SunSoaked]

Kipps-
I've got one of these in my 85 that came with the truck. I've never actually used it. What and where is this variable resister you mention?

[kipps]

It was evidently sold separately, but KH recommended it for nearly all cases. They say in their literature that it's to be mounted under the hood, on a firewall or fender somewhere. It could be possible someone mounted it out of sight below the dash, but it's supposed to be easily accessible.

Google "Kelsey Hayes resistor" to get a picture of what it looks like.

I have one on order, but it's not arrived yet, so I might be wrong in my description of how it works. The controller gets a battery hot at all times. The controller outputs on two wires. One is intended to operate the trailer brake lights. I'm ignoring that one, and wiring the trailer brake lights in with the vehicle brake light, like all modern vehicles. The other output wire goes directly to the trailer brakes.

This controller cannot differentiate between a light and heavy trailer. If you brake hard with a light trailer, it will lock up before the truck will, and be skidding across the road. To compensate, Kelsey-Hayes offered a variable resistor that mounts between the controller and the trailer brakes.

If you're pulling a light trailer, you'd pop the hood, reposition the "bridge" part of the resistor to an appropriate setting, then proceed like normal. If you were towing a very heavy trailer, you'd move the "bridge" to bypass the resistor, and give full braking voltage.

It's simply an adjustment you make when switching to a different weight trailer. It's not adjusted automatically or while on the go.

The manual that accompanied my new-in-box controller says the resistor can be skipped if you're always towing over a certain amount. It's a better bet to have that resistor though, so you can confidently plug up to an empty car trailer without worrying about over-applying the brakes.

I got a new-in-box 81740 controller for $50-ish shipped from ebay. I ordered a new-in-box resistor for $20 shipped. If I had watched my sales a little better, I probably could have found both sold together for cheaper. The biggest hassle will be getting the appropriate tee fitting for the brake line. This is not included with a NIB controller, since the factory didn't know what truck it was going to be installed on. I haven't pursued finding that yet.

[kipps]

Originally Posted by SunSoaked

Kipps-
I've got one of these in my 85 that came with the truck. I've never actually used it. What and where is this variable resister you mention?

Look for a new thread I started. Includes a copy of the 81740 manual.

http://67-72chevytrucks.com/vboard/showthread.php?t=782914

[bandit0067]

Hey thanks Kipps, I find all this very interesting as old as I am , I have never done much towing. I am learning that todays controllers are either time based or pendulum types. Meaning they look at an electrical signal from the brake light switch or the centrifugal movement by the braking of the . This K Hays appears to be converting brake fluid pressure to an set voltage output to the trailer brakes and thus the need of a resistor. Does this sound about right or what am I missing. My 71 GMC has a controller very similar to the KH pix you linked to . I will check that out as now I know what I m looking at....

[kipps]

Originally Posted by bandit0067

... This K Hays appears to be converting brake fluid pressure to an set voltage output to the trailer brakes and thus the need of a resistor. Does this sound about right or what am I missing. ...

I had a post typed up, then realized it wasn't quite correct per the manual. Look under "Selective resistor adjustment" in the manual for the "pro" recommendation.