Hey guys, as you know from an earlier thread, I have been having trouble with my brake lights coming on the dash on my 1989 Ford F250. After talking to several people including the dealership, I believe it is probably the vehicle speed sensor. From what I have read, it is attached to the top of the differential. So my question is, how easy is this job? Is it just like taking out a spark plug or is it more involved? Is there a significant chance of breaking it off inside the differential? Is it safe to use PB blaster or a similar chemical to loosen it up? Do any of you have any diagrams or anything that might possibly be of help?

Thank you all for any answers you can provide.

 

It's very simple. Get under there and take a look, and you'll think "Oh, piece of cake". Just clean around it (obviously) before taking it out.

 

First off your vehicle speed sensor is not on the rear end. It is located either in the tail shaft housing of the transmission or if it is a 4X4 in the transfer case.
The sensor on the rear end is the anti-lock brake sensor and has nothing to do with vehicle speed and it signal only goes to the anti-lock brake module.

What do you mean by "been having trouble with my brake lights coming on the dash"?
If this is the ABS lamp then yes the RABS sensor on the rear end may be bad.

 

Yes, that must be it then. Both brake lights are on and have not gone off in several days. Master cylinder and brake fluid have been replaced and I can't think of anything else it could be. I could have swore that the dealership said it was called the speed sensor lol.

 

When both lights are on you need to check the brake fluid.

 

Yup, I have, and the brake fluid is perfect. Has been flushed and refilled by a reputable mechanic twice.

 

Take a screwdriver and reach into the master cylinder and bob the float on that sensor up and down just to make sure it isn't stuck on the bottom.

 

Alright, will do.

 

The rear end sensor is not one of the items that will turn on both lamps.
Items that will turn them both on:
LOW BRAKE FLUID
MASTER CYLINDER FLOAT
DIODE/RESISTOR ELEMENT
DIODE/RESISTOR ELEMENT PARKING BRAKE
ANTI-LOCK VALVE SWITCH
MASTER CYLINDER SWITCH
BRAKE LIGHT WIRING

 

Um, my 94's speed sensor is in the rear end, there is no speed sensor in my tranny at all. Unless I am missing something...

As for the OP... the connector (sensor?) on the front of the brake master cylinder could be bad, or like previously mentioned float could be stuck. There is also a non-replaceable fuse(fusible link?) in the wiring harness going to that connector on the BMC, at least that's what it says on my 94, if bad it says to replace the whole wiring/plug/fuse assembly. Check that out too. Just throwing ideas out there. Hope I was helpful!

 

Well the brake fluid is fine as of now. The other problems you listed, how hard are they to fix or expensive for someone else to fix in general?

 

My mechanic said that the connector you are mentioning was clean but could be the cause of the problem, but he couldn't replace it because his supplier didn't carry it.

 

OP's truck is an 89. Ford changed a few things regarding the speed sensors, not sure which year was the changeover though

The switch on the front of the MC is for cruise control deactivation. The fusible link harness is installed as part of the recall. If the pressure switch leaks, the fuse will *hopefully* snap preventing the truck from burning to the ground. That harness carries B+ voltage at all times, even with the ignition off and key removed. Prior to the recall, if the switch would leak, brake fluid would get inside the connector, and the result would be a raging inferno, sometimes taking your garage along with it.

 

Wow, kinda makes ya nervous about owning that thing lol.

 

Nothing in the above post is right for a 1989 truck except the "float could be stuck".