M923A1 Abs Light

[wsucougarx]

Anyway to verify the ECU toast without the Haldex diagnostic Computer thing? When turning on the power I do get the initial 3 clicks from the ECU but not the second set that normally causes the ABS light to turn off. I also found I have a bad battery. I think it may have a dead cell. I still get 24.4 volt reading with my voltmeter when the truck is off. Would a dead cell on the battery cause the ABS light to stay on? I guess I'll find out soon enough. I'm headed to NAPA to secure some 3 AMP fuses.

 

[nf6x]

I have the a Haldex diagnostic box that I bought on eBay, but I haven't tried using it yet. From your symptom of the light staying on, that sounds like exactly what the box is made for: reading error information out of the control unit when it's indicating a failure. I'd give that a try after replacing the fuses, assuming that they don't blow right away. If either of them blows, then I think that more detailed debugging is in order before trying to plug in the diagnostic box.

24.4V with the engine off sounds fine.

 

[Artisan]

On the edge of my seat waiting!

 

[wsucougarx]

Just replaced the fuse with the correct 3 amp one. Light stays on but NOW the 3 clicks are no longer there....hmmm. The mystery deapens. I've got a spare ECU that I'm going to just plug the electrical part into. I'll see if that works. If so then I'll hook up all the air lines etc.

 

[Heavysteven]

Interesting

 

[rrrr]

The unit operates from 21-32 v. There are two fuses, a 3 amp and a 15 amp fuse as you already know. Make sure you check the connector on the ecu and that its tight and clean. You are not getting a good self test on start up. How often do you drive the truck and did it sit for a while ? Is this your first issue with the abs or did it have this when you recovered your truck.

Your relay valves my not be moving during the test. Try putting some air tool oil in the emergency glad hand and service the truck air system with an external compressor to try and push some of the oil into the lines. Then do some brake applications, firm, to make sure none of the wedges are sticking. I know these truck have air dryers but can still have some issues due to moisture. I did this with my 932 because it didn't want to shift in/out of low easily. After doing this it made it much easier. I also have a haldex box to check the ecu codes. Nice tool to have and verified my abs issue with the codes I was able to get off the ecu. I also cleared all codes on my ecu while I had it connected.

 

[juanprado]

Mike,

A word of caution, the abs ecm can throw a code for under voltage as well as over voltage. You could end up frying your other ecm if you have a wiring issue, bad solenoid, sensor etc. Those ecms are downright expensive unless you have a source...(Csm Davis)...

I have a diagnostic infocenter. PM and we can work out a "loan" to get your system trouble codes and fixed.

Juan

 

[PETE BALLARD]

Here is som easy tips to start with, first do a visual inspection of wiring and sensors, if all look ok proceed to the abs ecu, unplug each sensor and using your ohm meter check the resistance of each sensor,acceptable range is 800 to 1200 ohms, if each sensor is in range continue checking each sensor for ac output by spinning each wheel while watching the meter, should produce 1+1/2 to 2 ac volts - if no voltage is present the air gap is too large or the exciter ring is dirty, usually there is a pilot lamp somewhere to tell you the active code. Don't get in a hurry because they usually tell you one code at a time and do not tellyou all active codes all at once. Also remember if you do get the codes and refer to a manual - you are only as good as the last person that worked on the vehicle. If the last guy did not plug the sensor in to the correct plug you might spend time on repairing the wrong sensor. Take your time and you will get good results. Abs systems look very intimadateing but are as easy to work on once you understand how they work

 

[PETE BALLARD]

One other item, if you are having trouble hearing or verifing your modulator (service valve)valve is working, with the key in the off position apply the service brakes and then turn the key to the on position, when the ecu cycles the modulator valve will exhaust air pressure that is audible verifing their operation

 

[wsucougarx]

OK here I am with tonights work.
Initial power up, the Yellow relay valve does the 3 clicks. However the Blue relay (ECU Body) does not do the 3 clicks. The ABS light is still on. I am not blowing the 15A nor the 3A fuses. When I am back at the ECU, I put my hand on it and do feel it click when power is on and it again clicks when the power is off. So there's something going on inside the ECU. I tried my spare ECU and it did absolutely nothing.
I do have a question in regard to the leads coming off the ABS Light itself. I notice the leads off the ABS Light are numbered. My question is what are those wires hooked to?
Here's how mine are hooked up:
1-Black wire
2-Purple wire (ABS Wiring Harness)
3- Black wire (ground?)
4- Black wire (3A fuse wire)

 

[wsucougarx]

The unit operates from 21-32 v. There are two fuses, a 3 amp and a 15 amp fuse as you already know. Make sure you check the connector on the ecu and that its tight and clean. You are not getting a good self test on start up. How often do you drive the truck and did it sit for a while ? Is this your first issue with the abs or did it have this when you recovered your truck.

Your relay valves my not be moving during the test. Try putting some air tool oil in the emergency glad hand and service the truck air system with an external compressor to try and push some of the oil into the lines. Then do some brake applications, firm, to make sure none of the wedges are sticking. I know these truck have air dryers but can still have some issues due to moisture. I did this with my 932 because it didn't want to shift in/out of low easily. After doing this it made it much easier. I also have a haldex box to check the ecu codes. Nice tool to have and verified my abs issue with the codes I was able to get off the ecu. I also cleared all codes on my ecu while I had it connected.

I just got this truck a couple days ago. Looks like the front relay valve isn't doing it's thing. The yellow (rear relay valve) is doing it's self check as it's suppose to.

Mike,

A word of caution, the abs ecm can throw a code for under voltage as well as over voltage. You could end up frying your other ecm if you have a wiring issue, bad solenoid, sensor etc. Those ecms are downright expensive unless you have a source...(Csm Davis)...

I have a diagnostic infocenter. PM and we can work out a "loan" to get your system trouble codes and fixed.

Juan

Just found out my non-start issue was due to a bad ground connection on one of my batteries. I disconnected the harness from the ECU and now the 3 clicks (Yellow/Rear Relay Valve) is self checking but the blue one isn't.

One other item, if you are having trouble hearing or verifing your modulator (service valve)valve is working, with the key in the off position apply the service brakes and then turn the key to the on position, when the ecu cycles the modulator valve will exhaust air pressure that is audible verifing their operation

I Pete, I just went out and did this test. I got two audible "Pssh Pssh" when I flipped on the power while holding the service brake down. Does this mean my ECU is OK?

 

[rrrr]

I recommend you do the air tool oil trick. You don't even need to start the truck. Keep air pressure on it from an external compressor. Work the brakes good just sitting still pumping and applying a firm boot to the pedal a couple of times. You might need to do the oil a couple of times. Only takes an ounce or two and work it into the air system. You don't want to put too much in at one time. It sounds like you have a sticking valve. Good luck keep us posted.

 

[wsucougarx]

Well after 8 hours of work, turns out I have a bad/fautly ABS Solenoid Valve unit. I replaced the unit and now she self checks and the light shut off. I did a write up in my M925A1 build thread.

 

[jonesal]

Mike,

What do you plan on doing with the bad unit? If it's going to the scap heap I would like to buy it just for the fun of disecting it to see what's inside...

Al
'84 M923

 

[nf6x]

If you dissect it, I'd like to see pictures of the innards.

 

[wsucougarx]

Mike,

What do you plan on doing with the bad unit? If it's going to the scap heap I would like to buy it just for the fun of disecting it to see what's inside...

Al
'84 M923

Hi Al, great minds think alike I'm going to tear into the unit to figure out why she failed. Either a sticky vavle or the electrical unit is bad. I'll post pics of my dissection. Sounds like I'm back in biology class

 

[Csm Davis]

Think I may have some bad ABS stuff call me and I will see about getting it to you to rip open.

 

[PETE BALLARD]

mike, when you hear the audible sound of air being released in sequence with the sounds coming out of the valve it is confirming the your service valve is alive and well. keep in mind these systems are very dependent on having the correct voltage and good ground to start. another place to look is the main powere plug that connects to the ECU. they are prone to corrosion. one last item,your truck might have sensor extension cables between the sensor and ecu - it is always a good idea to apply di-electric grease to these connections and if you have to replace any - inspect the male plug on the bad cord, if it is still serviceable cut it off - it makes a nice connector to keep handy with your ohm meter

 

[GHall]

I recommend you do the air tool oil trick. You don't even need to start the truck. Keep air pressure on it from an external compressor. Work the brakes good just sitting still pumping and applying a firm boot to the pedal a couple of times. You might need to do the oil a couple of times. Only takes an ounce or two and work it into the air system. You don't want to put too much in at one time. It sounds like you have a sticking valve. Good luck keep us posted.

What is the best way to hook up an external compressor to the air system and where would you introduce oil into the system?

Thanks!!

 

[nf6x]

What is the best way to hook up an external compressor to the air system and where would you introduce oil into the system?

I would apply air through the rear "emergency" glad hand on the passenger side. You will need a glad hand, which you can get inexpensively at a truck parts store. It doesn't hurt to get some extras to have on hand. Use some pipe fittings from an auto parts store, hardware store, etc. to connect your shop air compressor to the glad hand fitting, attach it to the rear passenger side glad hand on the truck, and open up the valve behind that fitting.