ABS issues on a 94 RRC

signalMTB

Well-known member
quick summary, I have a 1994 RRC LWB. The ABS is kicking in randomly, even without touching the brakes sometimes. I have replaced the front calipers, rotors, and did a brake flush/bleed. It worked fine without issue for about a week. Problem has come back in full force, when depressing the pedal the ABS instantly comes on much of the time. The only way to get real braking is to tap it then press the brakes and pray.
Where to start? Accumulator? Sensors? Pump? What's your best assessment of how to troubleshoot?

Tried the jumper technique to read codes, couldnt get it to work. The ABS light doesnt always come on either, just sometimes. Then goes off.

Which scan tool might I buy? Or can I simply bypass or disable the ABS all together simply?
 

4RF RDS

Well-known member
My 93 had the same issue and it turned out to be the accumulator. The black ball high pressure thingy in the inner wing.

Good luck 👍
 

signalMTB

Well-known member
My 93 had the same issue and it turned out to be the accumulator. The black ball high pressure thingy in the inner wing.

Good luck 👍

I'm to the point where I may just replace it all and hope it fixes it. Accum and sensors.

what'st the symptoms of a bad pump?
 

signalMTB

Well-known member
Amazed its lasted this long......

yeah, that's certainly true ... just hope the pump isnt going out as well. My D110 build can't start soon enough, between the RRC issues and my L322 being a weekly money pit, what I really need is another 25 year old Defender to take my mind off it all.
 

nas90tdi

Well-known member
Did you go through the entire 9000 step process on bleeding the brakes from the service manual? That brake system requires a fairly complex sequence of events to actually be bled completely. If you don't follow it fairly religiously after working on the brakes it will do some seriously weird things.
 

signalMTB

Well-known member
Did you go through the entire 9000 step process on bleeding the brakes from the service manual? That brake system requires a fairly complex sequence of events to actually be bled completely. If you don't follow it fairly religiously after working on the brakes it will do some seriously weird things.

My shop did the work and he?s good but i can?t 100% say for sure if he followed the Wabco approach. He thinks it may be the plug going into the pump. The abs was tripping/going off just sitting parked.
 

nas90tdi

Well-known member
That sound exactly like the brakes aren't 100% bled. The pump is supposed to run for less than 45 seconds at start up ,if I remember the manual right, then hold that pressure. If you have a bit of air in the system that allows that pressure to release, it will make the pump kick in over and over.

Just a theory however. Those brakes on the RRC can behave fairly strangely sometimes.
 

signalMTB

Well-known member
That sound exactly like the brakes aren't 100% bled. The pump is supposed to run for less than 45 seconds at start up ,if I remember the manual right, then hold that pressure. If you have a bit of air in the system that allows that pressure to release, it will make the pump kick in over and over.

Just a theory however. Those brakes on the RRC can behave fairly strangely sometimes.

hmmm, let me dig up that 18000 step procedure and see about that. Currently it's driveable as I pulled the fuse.
 

luckyjoe

Well-known member
Callsign: KD2PXL
If I understand you correctly, you have actual ABS pulse activation, not just a constantly running pump?

As far as I am aware, ABS faults are only going to result from the wheel speed sensors - air gap, broken/damaged wires/connectors - resulting in the system thinking one wheel is not rolling normally like the other three. You could try reseating all four sensors and makes sure you do not have a failed wheel bearing introducing extra slop.

Only other suggest is to go back to the factory bleed procedure, and make sure you run a significant volume of fluid through ABS/ETC valve block.
 

Vtrover

Member
I agree that sounds like symptoms of a bad accumulator, the pump was running excessively because the diaphragm in the accumulator isn't holding pressure anymore. I would try to re-bleed the system.
 

pmatusov

Technical Excellence Contributor
Callsign: AK6PM
New accumulator did not fix the issue.
My WAG is that one of the ABS sensors (more likely, one of the front ones) has an intermittent fault. It has been the case with ALL of my trucks of that vintage - the ABS sensor cables coming out of the sensors have zero stress-relief, and when the insulation dries off and becomes brittle, one of the wires loses connectivity right at the entrance to the sensor body. You can diagnose this by connecting the ohmmeter to the sensor connector terminals, and by bending the cable every which way near the entrance to the sensor body. The normal resistance is about 1kOhm, and you'll see the resistance go up to infinity when connectivity breaks.
This condition trips the brake controller into thinking that everything is okay (after all, if there's connectivity and signal is okay - what else can be wrong?), and then things go haywire in a turn or on uneven pavement.

I can make my last 95 LWB and my first 96 D1 ABS light to go dark by taking off straight forward. The ABS light will remain ON if I back out of a parking spot or make a turn around the speed when ABS activates.

There are ridiculously cheap ABS sensors on eBay; on two occasions on 95 Classics, replacement of the front sensors immediately cleared ABS light and pedal behavior similar to what you described. Yet, on one of them it returned - so either I should have replaced ALL sensors or cheap sensors are crappy.
 

azarmadillo

Member
You can replace the RRC's accumulator and pump with one from a P38. The P38 ones never go bad. It's just some creative plumbing and a new banjo fitting and a few other fittings. Never worry about it again.
 

Mr_Bodgers

New member
Until you pull the fault codes you are shooting in the dark. Go to a repair center of your choice and have them pull the codes, record them and then clear. Drive the vehicle until the symptom reoccurs and look at the current fault(s). Likely the trouble lies with a faulty wheel speed sensor and possibly the brake pedal switch. That's just my 2 bits worth of 27 years of working on these things.
 

O2batsea

Well-known member
Has it been in deep water or mud? If you have a solid pedal; that is you can easily stop it with mere big toe pressure, then it is probably not the accumulator or air in the line but rather a sensor that needs to be reseated. Try the zero dollar fix first then move up the troubleshooting tree.
I had a 93 that would go into ABS mode all the time. It was gunky reluctor on the rear wheel.
 

RDavisinVA

Technical Excellence Contributor
quick summary, I have a 1994 RRC LWB. The ABS is kicking in randomly...

We had the same problem in a 1993 LWB and here's how we fixed it.

Get yourself a half dozen or so of the small plastic DOT 4 brake fluid containers.
Bring a pot of water to boil and then turn the heat off.
Place one or more of the DOT 4 brake fluid containers into the pot of hot water.
Wait 30 minutes.
Remove the brake fluid container and bleed the brakes with the hot brake fluid.
Cycle the brake fluid containers with the hot water in order to be able to have a steady flow of the heated brake fluid until all of it has been flushed through the system.

This cured our braking problems.
 
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