What could cause the ABS cluster light to not turn on ( when key on, no diagnostic check), and the ABS module not to respond to the scan tool?

Here the scenario: when i had the rear axle swapped to the ford, air must have got into the ABS ( i am assuming) because they cannot get fluid to flow out of the port to the rear end.

Suspect is air in the BPMV. they ordered the funny tools but cant get the high pressure accumulators pin (side closest to engine) to depress. also the ABS part of the computer is acting like it doesn't exist, and so he can't run the diagnostic purge functions.

NOTES:
battery was unhooked/died for a few weeks.
ABS diagnostic check worked fine in the past.
they had to replace the brake switch as it was old and rickety.

Is there anything else besides the ABS that would stop flow to the rears? They have already put at least 3 quarts through the system!

Help!

 

If the EBCM were to have an internal fault, it could illuminate the ABS light and cause the problem with scanning the EBCM for codes.

 

By "internal fault" do you mean like the unit is damaged, or like a code has been thrown (possibly from the bleeding)?

The problem is not the light illuminating, its the light NOT illuminating, not even for its startup 2 sec diagnostic flash. And I could swear it always did before!

Could a defective break switch cause this? That is the only part they replaced on the brake system up front? I am at a loss and they are 4x4 mechs and mine is not a jeep or a toyota so they dont seem to understand when I tell them GM just made some tasks really sucky! ugh!

Any more insight would be appreciated. any ideas on why the accumulator pin would stick?

 

Sorry, I misread your post to mean that it comes on and stays on... The dash light should illuminate during the bulb test. If it doesn't, the bulb may be out in the cluster. Does the brake light come on when you first turn the key to the run position?

Are you sure that your scan tool is capable of accessing the ABS system on your truck? After looking at things a bit more in detail, I don't believe that the BPMV (brake pressure modulator valve - it doesn't have an EBCM) on the '96 & '97 trucks is a serial bus device like on the 98+ trucks.

The BPMV is powered through a fusible link. Test for power on the red wire at the BPMV (should be in a connector with a pink wire [pump motor feed from PCM] & a black wire [BPMV ground]).

 

The BPMV wires tested good! ugh.

Well the bulbs are leds so just the one shouldnt have burned out but I took them some extra bulbs just incase.
So if there is no serial component how do we trigger the ABS to fire off, to purge the BPMV?
Ill have em check the wires again?

UPDATE:
I replaced the VCM last year, so I took the old VCM down to them and they swapped it; now the ABS system is talking to the scanner.

HOWEVER: the VCM is reporting a fault with the ABS indicator, and neither hard stops, nor the tech tool can get the ABS to activate!

the most important questions though is what would cause the BPMV not to allow fluid to the rears, and why wont the accumulator pin push in?(port closest to engine)

So in the mean time I am going to try to find a working/cheap (haha; I know) BPMV to replace it with in.

Any experts on the 4WAL system out there?
Help!

 

Bought the BPMV off of the guy that I got the wheels from, so I know its a working unit.

We'll see if this gets fluid to the rears! ugh.

I am still interested in what would cause that valve to stick, at this point I am assuming it got clogged by junk in the fluid?