hey guys and gals:

Currently having an issue with my ABS/Brake light popping on then staying on- doesnt happen 100% of the time but a majority of driving time they are both on!

I have replaced the rear axle speed sensor as well as the transmission sensor. The speedo needle will bounce very erratically and the brake pedal will sometime fall while sitting at a stop.

 

I am thinking i have a booster problem or a proportioning valve issue. the left front tire will sometimes lock up hard under medium brake application. All the brakes lock up fairly easily on wet roads though. 

 

I have done full brake jobs with new wheel cylinders in the rear and new calipers on the front along with new flex lines on the front as well.

 

Any ideas or thoughts or "I've had that happen and this was the fix"

 

Thanks for your time on this.

 

The speedometer signal goes from the rear sensor to the ABS controller to the PCM to the speedometer.  The transmission sensor does not factor into this.   This could be an ABS controller problem. 

 

A falling pedal can be a brake master cylinder, weak brake hoses but you replaced those, or a leak but you did not mention a fluid loss. 

 

A booster would affect all brakes with lack of assist rather than a falling brake pedal.  The proportioning valve affects the rear brakes only, there are none for the front brakes.

 

Does the truck have front brake ABS sensors?  Pleas fill out your signature, so we know what year and model truck it is.

IBMobile:

Got sig finally! Just rear ABS, thought about the brake pedal issue, I probably rolled the seal cups over rough internal when we did the replacement on the brake parts!

Thinking wiring or the ABS module- can get a junk yard one for 125$ locally with a 1 month warranty on it. Got to chase the wires and make sure they are good and no high resistance in them. I am assuming they are a twisted pair using an AC signal from the sensor??

 

I mentioned the hyd booster as a possible based on the FSM mentioning the spool valve. A master cyl is much cheaper that a booster!!

Check the connector between the cab and frame on the left side of the truck.  The signal from the rear speed sensor is

very low AC voltage.

 

   

Above is the voltage reading at 45 MPH.  The voltage reading never went above this value even at 75MPH.

 

I have seen old brake master cylinders go bad after bleeding the brake system using the brake pedal method.

15 hours ago, Stubilly said:

I mentioned the hyd booster as a possible based on the FSM mentioning the spool valve. A master cyl is much cheaper that a booster!!

No a great deal in the booster and easy to rebuild, there are 2 kits, one is just seals, the other is the full deal with everything, I put up with mine leaking for months before looking for kits, biggest problem I found was most kits are made in china, Pirate Jack (I think) has both kits made in the USA I ordered both as being in the UK and my truck is a work vehicle I like to cover all bases.

I did use the full kit in the end, everything was good quality but I found the kit covered more than just my booster so there were parts over, I'd methodically laid it all out as I stripped it so was easy enough to ID what I needed and what I didn't.

The kit does not address any problems with the accumulator (blue canister)  the FSM has a test for this of so many brake applications with engine off before pedal goes hard IIRC, mine passed anyway

Wil440:

 

Thanks for the reply. hopefully I dont have to rebuild my hyd booster and its a master cylinder and wiring issue!

 

 

Update!!

 

I did my AC noise check. last time I did it i got a reading of 0.025Mvac, however now it's up over 10.2Mvac so looking at a rebuild or a new one-- Nations looks good but over 500$ for one powder coated and a 2 gage wire is a bit pricy right now:( More troubleshooting after that gets fixed!

also make sure you check the junction plug behind the drivers front wheel well cover.  It can get covered in road debris and lead to a bad connection of all wiring going to the back of the truck.  

 

Since the speedo runs through that junction you might find poor continuity as a result.

1 minute ago, Stubilly said:

Wil440:

 

Thanks for the reply. hopefully I dont have to rebuild my hyd booster and its a master cylinder and wiring issue!

 

 

@Stubilly yes agree mastercylinder is easier as bolt on new but booster isn't anything to be bothered about, the key is lay it all out as it comes apart, there are seals that fit over the spool and they were pretty tight to get back on without them being turned if that makes sense