Thread: Learned something about Rostra Cruise Control

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Last year someone warned me that the brake lights on my 46 coupe were hard to see. Now I've been running blue dot lenses ever since I built the car and I knew that they could make the lights a little dimmer. I checked them out and sure enough they were pretty dim. I decided to replace them with LED lights and picked up some super bright lights at the NSRA Nats in Knoxville.

I finally got around to putting them in this weekend. Man those things are bright! They look to be 4 times as bright as the old lights, but I now had another problem. Every time the key was turned on, the brake lights came on.

I first thought that the signal light switch may have failed somehow because it has been acting a little wonky lately, but finally traced it to a back feed from the Rostra Cruise Control. There is a wire on the CC that connects to the brake light side of the brake switch that the CC uses to kill the cruise when the brakes are applied.

I thought that the CC unit was looking for 12 volts when the brake lights were on, but it turns out that it's actually looking for a ground path through the brake lights. After a little poking around on the net, I found a link to a Rostra trouble shooting page and discovered that the Rostra CC is not directly compatible with LED lights. There is too much resistance in the LED's to allow the unit to find the ground it wants. It sends a low current 12 volt signal through the brake lights. It wasn't enough to light the old incandescent bulbs, but the LED's don't take much current to light up.

You have to wire in a 5 pole relay with a ground on the normally closed 87A terminal. All this relay does is supply a ground to the CC. When the brakes are applied, the relay picks up and drops out the ground signal. The lose of ground drops out the cruise.

Rostra has a pdf with the wiring diagram. Here's a link if anyone needs it:

www.rostra.com/manuals/250-4382_Form5260.pdf

Live and learn as they say. I certainly learned that the CC doesn't work the way I thought it did. Oh, while I was at it, I changed out the signal switch, too. Of course the new switch had a different connector than the old one, but what else would you expect.

thanks for the info as I am looking into putting cruse on the 53, although I don't have led lights. I noticed my check engine light always has a dim glow. Like you said, it's an led and it has just a bit of voltage through it.

That's good info HR46, thanks for sharing it.

I wouldn't have figured it out!!!! LOL..

Your welcome. I hope this info will help someone.