I just bought an 87 f250 with a 6.9 diesel and a c6 and now the rabs light and the brake light are both on all the time. I can stop and shut off the engine and when I start it the lights are off but as soon as I put my foot on the brake pedal both lights come on and stay on also the brake pedal is really hard like there is no power assist but the booster is working the truck does stop real good but its bothering me and I have no info as to how to retrieve the codes or what the codes mean! any help would be much appreciated. Thanks!

 

hmmm well how do u know your brake booster works ? and on theses simple brake systems u really dont need to know the exact codes cus there really are none. But things to check would be brake fluid level and condition of fluid because they can stick ur level sensor and make it think its low. And if those check out ok then u can check ur rear speed sensor on ur diff. Honestly if ur truck stops good then i wouldnt worry about it its a very simple set up

 

If both lights stay on you want to check the float and the plug on the side of the master cylinder before tearing into the system.
Mine was shorted/open wires at the plug.

 

your problem is most likely a dead vacuum pump.
diesels do not have intake vacuum like gas engines do, so they have a vacuum pump on the front of the engine to make vacuum for the brake booster to work. hard pedal is a sign of no vacuum to the booster.

 

To retrieve codes there should be a black/orange wire under the dash that can be grounded, and then the key cycled to flash codes out on the RABS light.

I don't know enough about the specifics of a diesel wiring harness to tell you exactly where this wire is.

 

you can not retrieve codes on a diesel with a C-6 or manual trans, because there is no computer to store them .
the only things that will activate the check engine light are low oil pressure or high coolant temps.

 

It's not a PCM, It's the RABS computer and entirely independent of any engine management.

It is the RABS light and the brake failure lights that are on...
It is the RABS light that flashes when you ground the wire and turn the key.

Do you think I have an engine computer in my DSII truck Tom?

I understand what you're saying about a problem vacuum pump causing a hard pedal.
Hydroboost is always an option.

 

I have fixed it! I already had a new vacuum pump and a new master cylinder already and I found the wire that is supposed to be grounded to check the codes but when I grounded it all the light would do is come on and stay on and go off only when the ground jumper was removed so I went a little further and checked the fluid pressure at the rear brakes and there was none and not much at the RABS valve so I borrowed the RABS valve off my 94 F350 and bled the brakes. now I have no abs light and the brakes work perfect so now I have to get a new one for the 94

 

no idea Jim, i don't know what a DSII truck is.
but i do know there is no way to pull codes from your 87 with a 460 either.

i can not see a 87 diesel having a ABS computer, because my 88 diesel does not have one. all it has is the valve in the frame that has been bypassed since the truck was a few months old.

 

mine has a little box about the size of a duraspark control module it is right above the cruise control module under the dash and below the radio it has 2 plugs with 4 wires each and is made of plastic. TJC, how did you bypass the abs valve and did you have to add any sort of proportioning or residual valve? Thanks!

 

unscrewed the lines going into and out of the valve, and put a short line between them.
my abs valve had one wire on it that went to the light. the light would come on if there was a loss of pressure in the rear brake circuit.
it was about as basic of a circuit as there was.

 

My truck is DuraSpark 2.

I CAN pull codes from my RABS computer Tom.
It's an entirely separate system from any engine management.
What do you think feeds a signal to the plug on the RABS valve located on the frame rail???
What do you think lights the bulb up on the cluster?

 

It sounds like you are talking about the Differential/Proportioning Metering valve and not a abs valve.

The abs valve has four wires going to it.

 

i dont know, it is the only thing in the rear brake circuit. about 5 inches tall, and 7-8 inches long.

 

That is not a RABS valve. It's a proportioning valve and the single wire going to it will activate the red brake warning light when you have an excessive pressure differential between the two sides of the valve.

You'll find that RABS, or ANY antiskid brake system for that matter, will always have an electronic controller as well as it's own warning light separate from the red brake warning light.