What is it?

[Brad B]

First I'll try to explain what I'm looking at because I don't have my digital camera handy. Following the vacuum line from the pump it leads to a distribution block of sorts. From that, there is a short line that runs to a disc shaped object that is fastened to the driver side wheel well. This object has what looks like 3 wires coming out of it that lead to a larger bundle of wires then into the fuse box under the hood.

I noticed that a nearby wire bundle has rubbed a hole in the insulation of the wires coming out of that disc shaped object. But I don't know what that little disc shaped deal does! Everything is running as usual but I'm wondering if I should dig into trying to replace the disc deal and it's wire or just tape over the hole and see what happens on down the road.

Apart from the usually reported sinking brake pedal problem associated with these trucks, my vacuum system seems to be fine. But it would be nice to find a diagram of that system to see what this little disc gadget does!!

Any ideas?

 

[Patrick Feeley]

It's the low vacuum warning switch. Cheers!

 

[Brad B]

OK, what does it do and how does it work? When my vacuum pump went out a few years ago there wasn't a warning light or anything so does that mean that the switch wasn't working?

 

[Patrick Feeley]

It's a normally open solid-state switch that closes when manifold vacuum stays below 10 inches for more than 7 seconds. When it closes it applies a ground to the Brake Indicator causing it to illuminate. When my vacuum pump was going south, the brake indicator would come on for a bit and then go out (and keep doing it as you used the brakes). Cheers!

 

[Brad B]

Thanks for the info. Now I understand why my brake light comes on and gradually fades out if I've been sitting with the brakes on but engine off. I guess I'll just dab some liquid electrical tape on the bare wires and add some chafe protection.

Great guys on the board, thanks for all the help.