ABS and Parking Light On

[Iron Duke II]

These dash lights were staying off as they should until recently. I checked the ABS Sensor in the rear differential with an ohm meter. It seemed to be ok, but switched out any way with a new Dorman brand unit. Still have same issue. Wondering if the wiring to the ABS sensor is shorted. Has anyone replaced the wiring for the ABS Sensor?

 

[DavidC]

You could try unplugging the harness on the back of the trailer plug, cleaning it and see if that is an issue by leaving it unplugged. I had that once. Does the speedo work ok ?

 

[Road Dog]

The rear wheel ABS sensor is the usual suspect but there are other possible issues. You need a scan tool that can read the CAB (controller antilock brake) module. There's also a way to read the codes by grounding a pin on the CAB and counting flashes on the ABS lamp. It's poorly documented. From another site:

Short pin 13 (black wire) of the Datalink connector (above the driver's feet) to ground, and the ABS light should blink out the trouble codes.

Here is the list of the ABS-light blink codes for the 1994-1996 system:
1 Not used
2 Open Isolation Valve wiring or Bad Control Module
3 Open Dump Valve wiring or Bad Control Module
4 Closed RWAL Valve Switch
5 Over 16 dump pulses generated in 2WD vehicles
6 Erratic Speed Sensor reading while rolling
7 ECM fuse pellet open, isolation output missing, or valve wiring short to ground
8 Dump output missing or valve wiring short to ground
9 Speed Sensor Wiring (high resistance)
10 Speed Sensor Wirign (low resistance)
11 Brake Switch always on
12 not used
13 ECM PLL failure
14 ECM Program Check failure
15 ECM random-access memory failure

I've had the ABS light on when the brake fluid was low. I've also had an "erratic speed sensor" because the tone ring was chipped.

 

[Iron Duke II]

You could try unplugging the harness on the back of the trailer plug, cleaning it and see if that is an issue by leaving it unplugged. I had that once. Does the speedo work ok ?

Correction: Yes the speedometer does appear to operate correctly.

I have yet to unplug a harness from the trailer connection on the truck yet. There are two, both 7 ways. One mounted to the driver's side bed wall for a fifth wheel or in-bed camper and the other next to the rear trailer hitch. I didn't want to assume you may had another type receptacle in mind. The following is what I intend to perform. https://www.doityourselfrv.com/broken-7-pin-trailer-cord/

To further expand on the problem, today I pulled the ABS fuse #14 and Stop Light fuse #20, then reinstalled them. I then started the truck and the dash ABS and Parking lights went off. Then I drove it around town without them coming on or flashing intermittently as they have recently. I made multiple stops and parked several times, turning the truck off and restarting for about 45 minutes on this test drive, no dash ABS and Parking lights. However when I pulled into my driveway and placed the trans into park on level ground the ABS and Parking lights were back on. Interesting the driveway is on an incline on the approach from the road to the garage. Just before the garage the drive levels out. on the incline the lights stay off when trans in park, again on level and in park the lights come on.

 

[DavidC]

The plug I am talking of is on the back of the 7pin and is a factory plug. The one on the bed wall was added. I would check the wiring to those carefully. If the speedo works the problem is not in the differential as that signal does multiple things.

 

[DavidC]

Do you know if you have 2 wheel or 4 wheel ABS ?. If 4 wheel there are sensors on the front wheels also.

 

[Iron Duke II]

The rear wheel ABS sensor is the usual suspect but there are other possible issues. You need a scan tool that can read the CAB (controller antilock brake) module. There's also a way to read the codes by grounding a pin on the CAB and counting flashes on the ABS lamp. It's poorly documented. From another site:

Short pin 13 (black wire) of the Datalink connector (above the driver's feet) to ground, and the ABS light should blink out the trouble codes.

Here is the list of the ABS-light blink codes for the 1994-1996 system:
1 Not used
2 Open Isolation Valve wiring or Bad Control Module
3 Open Dump Valve wiring or Bad Control Module
4 Closed RWAL Valve Switch
5 Over 16 dump pulses generated in 2WD vehicles
6 Erratic Speed Sensor reading while rolling
7 ECM fuse pellet open, isolation output missing, or valve wiring short to ground
8 Dump output missing or valve wiring short to ground
9 Speed Sensor Wiring (high resistance)
10 Speed Sensor Wirign (low resistance)
11 Brake Switch always on
12 not used
13 ECM PLL failure
14 ECM Program Check failure
15 ECM random-access memory failure

I've had the ABS light on when the brake fluid was low. I've also had an "erratic speed sensor" because the tone ring was chipped.

Brake fluid level is at the proper level.

Researching the way to read the codes by grounding a pin on the CAB and counting flashes on the ABS lamp. Still looking.

 

[GAmes]

If the speedo works the problem is not in the differential as that signal does multiple things.

For your edification 2nd gens up to '97 get their speed signals from a sensor on the transmission (2WD) or the transfer case. The sensor in the differential is for ABS only.

 

[GAmes]

Wondering if the wiring to the ABS sensor is shorted. Has anyone replaced the wiring for the ABS Sensor?

The wire diagrams for rear wheel ABS start on page 8W-34-1, all wheel on page 8W-35-1 in the '96 FSM. When I was troubleshooting my ABS light I found that disconnecting the trailer brake controller the light would go out. I assumed it was the controller failing so I bought a new one and plugged it in. The ABS light came back on. Using the wiring diagrams I traced the circuits and found that a the hot wire for the trailer brakes and the ground wire to the ABS sensor in the differential were in the same connector. I pulled it apart and about a teaspoon of water poured out. After drying it and applying dielectric grease the problem was solved. The moral of the story is there isn't a one thing fixes all in the ABS system. You need a manual as often as not.

 

[DavidC]

For your edification 2nd gens up to '97 get their speed signals from a sensor on the transmission (2WD) or the transfer case. The sensor in the differential is for ABS only.

Good to know, Thanks

 

[Iron Duke II]

The wire diagrams for rear wheel ABS start on page 8W-34-1, all wheel on page 8W-35-1 in the '96 FSM. When I was troubleshooting my ABS light I found that disconnecting the trailer brake controller the light would go out. I assumed it was the controller failing so I bought a new one and plugged it in. The ABS light came back on. Using the wiring diagrams I traced the circuits and found that a the hot wire for the trailer brakes and the ground wire to the ABS sensor in the differential were in the same connector. I pulled it apart and about a teaspoon of water poured out. After drying it and applying dielectric grease the problem was solved. The moral of the story is there isn't a one thing fixes all in the ABS system. You need a manual as often as not.

The truck hasn't seen rain since December 2018. I downloaded the manual. Thanks

 

[DavidC]

It doesn't necessarily have to be a liquid. Plug connections can build up resistance with a poor connection, a chafed wire somewhere, bad ground etc.

 

[Iron Duke II]

Do you know if you have 2 wheel or 4 wheel ABS ?. If 4 wheel there are sensors on the front wheels also.

My understanding has been it is 2 wheel ABS. When you look at the brake lines that go to each front caliper, there is no wiring.

 

[Iron Duke II]

It doesn't necessarily have to be a liquid. Plug connections can build up resistance with a poor connection, a chafed wire somewhere, bad ground etc.

Concur. Likely oxidation and or dirt.

 

[DavidC]

You may want to carefully inspect the wiring from the frame to the rear axle as that is a known problem area also given the movement in that area.

 

[Iron Duke II]

Didn't isolate an issue with the truck trailer receptacles or brake controller wiring that helped correct the ABS and parking brake light anomaly. There are a couple small cracks in the ABS harness about 7 to 9 inches from the ABS differential sensor, that do not go through to the bare wire. The rest of the harness is intact. I have attempted to locate the harness online, if it turns out to be faulty.

The last thing I did was go back to the fuse panel and thoroughly clean the #14 and #20 fuses and their panel contacts. Since doing this the ABS and parking brake lights have functioned normally. Tomorrow is another day.

I would like to move on to the fuel heater delete and fuel pump linkage brass bushing replacement.

 

[Ozymandias]

I replaced the wiring between frame and sensor on mine, it was internally broken.
Since then, no ABS light anymore.

 

[JMcCoy]

Lots of good information here. I've been fighting the same problem, except my airbag light is also on. Any luck on finding a harness? I'll be replacing my bed soon, so that would be a good time for me to do it.

 

[Iron Duke II]

Lots of good information here. I've been fighting the same problem, except my airbag light is also on. Any luck on finding a harness? I'll be replacing my bed soon, so that would be a good time for me to do it.

Nope.

 

[Iron Duke II]

Knock on wood, the lights have been operating as they should since my last posting on this traffic.