M923A1 Abs Light

[jonesal]

Please take a look at the documents again and then get under your truck! Whoever said there are four sensors on two axles didn't do that. This is a two sensor, two modulator value system and it is clearly outlined in BOTH the P2P and Haldex Info Center manual. Sensor 2A is on the drivers side on axle 3 and sensor 2B is on the passenger side on axle three.

That being said, once you have a fault code stored in the ECM, the Haldex Info Center can clear it for you. Is there any other way? I don't know, does any one else know? Rumor has it, and its just rumor, that a sequence of the off-run switch can do that. Until we can verify with facts, you're going to need the Info Center to clear the fault code AND diagnose what's wrong.

 

[martinwcox]

If the 15A fuse blows immediately you almost certainly have a blown ECU.

See my thread here: http://www.steelsoldiers.com/5-ton-up/90856-blown-abs-ecu-how-not-do-etc.html

I found an ECU and valve assembly on ebay, $500.00

 

[rrrr]

Please take a look at the documents again and then get under your truck! Whoever said there are four sensors on two axles didn't do that. This is a two sensor, two modulator value system and it is clearly outlined in BOTH the P2P and Haldex Info Center manual. Sensor 2A is on the drivers side on axle 3 and sensor 2B is on the passenger side on axle three.

That being said, once you have a fault code stored in the ECM, the Haldex Info Center can clear it for you. Is there any other way? I don't know, does any one else know? Rumor has it, and its just rumor, that a sequence of the off-run switch can do that. Until we can verify with facts, you're going to need the Info Center to clear the fault code AND diagnose what's wrong.

If you find anything about clearing it other than with the unit please let us know. I will also be looking. I will probably pull the two rear tires today and do some work on it. I will take pics to post and will check my fuses. Since there is a good self test and my light doesn't come on until I start driving it I believe my fuses are fine.

I read through the test unit literature and since mine doesn't come on until the truck starts moving it says to check the exciter and sender.

 

[nf6x]

I haven't gone searching for the ECU yet in my trucks, so I don't know what it looks like. Does it appear to be potted? If it can be disassembled, then component-level repair of blown ECUs may be practical.

The same question applies to the PCB. If it can be opened up without too much trouble, then it may be practical to replace blown protection diodes.

 

[rrrr]

Ok, I pulled both rear wheel assemblies to get at the transducer and exciter. Both sides didn't look bad. I cleaned them up anyway, the left side being the worse of the two. I also tapped on the side of the transducer to get it to protrude from the bracket more in case it was out of tolerance. Don't push the unit in, it will stay in. I had to tap it with a hammer to get it to work back out. Its too tight for pliers and even using a wire to grab the back of it didn't work.

Once together I started the truck and the abs light came back. I moved the truck forward a few feet just in case the transducer was touching the exciter and causing the issue. I then shut down again waited a minute and restarted. Took the truck for a test drive and everything seems to be working great

Attached are the pictures of the exciter and transducer.

Hopes this helps others that have the same problem.

Edit to add a picture

 

[martinwcox]

I have my old one apart, will take some pic's.

It's not potted but the board does have a thick layer of epoxy over the components. Lots of surface mount and IC's along with drivers for the valves and I assume voltage regulators for the logic chips.

No fixing yourself, I asked Haldex, when its dead its dead!

I haven't gone searching for the ECU yet in my trucks, so I don't know what it looks like. Does it appear to be potted? If it can be disassembled, then component-level repair of blown ECUs may be practical.

The same question applies to the PCB. If it can be opened up without too much trouble, then it may be practical to replace blown protection diodes.

 

[nf6x]

Of course Haldex says that. Most of their customers aren't electrical engineers who design circuit boards and rework surface mount stuff every day.

Edited to add: Does anybody have a dead ECU that they'd like to give me to see if I can do anything with it?

 

[martinwcox]

PM me your address, it's all yours!

 

[nf6x]

PM me your address, it's all yours!

Will do! No guarantees that I can do anything to it, but I'll at least autopsy it and share my findings. I don't think I'm the only electronics geek on the forum.

 

[juanprado]

correction

I got under the truck today and checked the middle axle. My apologies as it has been noted on the other posts there is only 2 sensors on the rear wheels and none in the middle.

 

[nf6x]

Upon further examination, my new M936A1 doesn't light up the ABS light briefly at turn-on time, and at least one of the fuses is pulled out. Hmm... it may turn out that I need to use my diagnostic box after all. Off to the TMs to learn some more before I start working on it!

 

[diverman555]

I have a M923A1 and a M923 both of them have the abs light stuck on, but it still works.
I have found the biggest problem with the A1 with super sigles. When you get on the brakes with any force the truck starts to bounce and then slide sideways with the rear coming around to the left. If it starts to do that get off them and right back on a little easier. don;t let start bouncing or it will come over on you.
(this was done by a professional driver on a closed course, do not try this yourself) had to add that.

 

[nf6x]

If the ABS light is on and you need to pump the brakes to keep the truck from swapping ends, that sounds to me like the ABS is not working.

 

[diverman555]

question for you please. What would cause the rear brakes to grab before the center wheels do?

 

[DerangedMadman]

Subscribed

 

[rrrr]

question for you please. What would cause the rear brakes to grab before the center wheels do?

The rear axle is the lightest loaded axle under braking with an empty truck.

 

[diverman555]

great, thank you much.

 

[govt]

When I picked up my M931A1 last year the ABS light was off. I noticed some bucking last weekend and sure enough the ABS light was on. After researching I found that I have been doing an improper shutdown all along. Hoping I haven't popped the ECU. Now to troubleshoot. Good post here! Thanks for all the info!

 

[jonesal]

TM 9-2320-272-23-2 should get you where you need to go. I has both methods for troubleshooting: with and without the Haldex Info Centre (spelled correctly). It's tedious but follow the procedure. I did and found my problem was a shorted wire. Troubleshooting took two hours and the fix took 2 minutes.

Good luck.

Al Jones
'84 M923

 

[goldneagle]

TM 9-2320-272-23-2 should get you where you need to go. I has both methods for troubleshooting: with and without the Haldex Info Centre (spelled correctly). It's tedious but follow the procedure. I did and found my problem was a shorted wire. Troubleshooting took two hours and the fix took 2 minutes.

Good luck.

Al Jones
'84 M923

Are you sure about the TM number? i cannot find a TM 9-2320-272-23-2 only 24-2