ABS questions

I have a 1996 K1500 Silverado pickup extended cab 5.7 liter, automatic trans, electric 4WD dash switch, with stock everything (as far as I know). I've got ABS error codes on my scan tool showing faulty left, right and rear wheel speed sensor problems and bpm valve error (sometimes on last two codes -- intermittent). I recently replaced the rear brake cylinders, and bled them the manual (pedal push and hold - 2 person) way. Could this have caused the bpm valve problem?

I've checked the connectors for the two front wheel sensors, and they seem immaculate inside. So,I've sent for new front wheel speed sensors, but am unsure about which sensor to get for the rear. I see lots of sensors advertised for the rear manual shifted 4WD transfer, but they don't mention the dash switched type. Are they the same speed sensor, or is it a different part?

And is the bpm valve problem likely to mean a bad control module or is it a matter of bleeding it properly? Thanks for your help. New to this stuff.

VTSilverado said:

1996 K1500... 5.7 liter, automatic trans, electric 4WD dash switch

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VTSilverado said:

I've got ABS error codes on my scan tool showing faulty left, right and rear wheel speed sensor problems and bpm valve error (sometimes on last two codes -- intermittent).

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Does the speedo/odometer work? Does the cruise control work? If so, it's unlikely that the rear wheel speed sensor is at fault.

LF and RF wheel speed sensor problems can be the sensor, the tone ring/hub assembly, or the harness. Sometimes it's the ABS unit itself. More diagnosis needed.

Use the scan tool to check the wheel speed sensors against each other--they ought to read the same speed, or very close.

As for the BPM Valve, use the scan tool to chatter the valves (auto-bleed ABS) and see what happens. Instructions for bleeding attached.

VTSilverado said:

I recently replaced the rear brake cylinders, and bled them the manual (pedal push and hold - 2 person) way. Could this have caused the bpm valve problem?

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Not likely. But I bet the ABS hasn't been bled since the truck was built, so it'd be a good idea to do that.

VTSilverado said:

am unsure about which sensor to get for the rear. I see lots of sensors advertised for the rear manual shifted 4WD transfer, but they don't mention the dash switched type. Are they the same speed sensor, or is it a different part?

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I'd expect the same sensor, but I've never done a non-manual-engaged transfer case.

Thanks very much Schurkey for your reply and all of the info. Much appreciated!

With regard to the speedometer and cruise control working, I don't know yet since the truck has been off the road, and it is presently up on jack stands with the box off. It has been off-road for three years now. I might get the box back on and roadable in the next day or so. Before it was mothballed, speedometer and cruise did work, and the ABS system had no errors.

I did get the truck registered and insured last week, and inspection appointment is for the 16th. I have a 14 day window to get it inspected by the 17th. Hope I make it. Vermont is tough, and the requirements are that the ABS light come on before start-up, and that it go out after startup and the ABS system be working properly if scanned. Thus no ABS "delete" possible here.

My scan tool (Topdon Artidiag 500) does not offer an ABS bleed tool for my truck. However I did yesterday try a manual (two person pedal pump) bleed of the rubber capped bleeder on the ABS unit. I have read your advice elsewhere that this does not really bleed the ABS unit, but just the front brakes. Nevertheless, the BPM valve error scan message has disappeared .. maybe just coincidence since it was intermittent anyway. But maybe just exercising the brakes that much (about ten times) freed something up. Fingers crossed it doesn't return. I could ask my mechanic who will do the inspection to bleed the ABS unit at that time.

Two wheel sensors are due to be delivered today, and the wheels are already off. I'll connect them before installing to see if the errors disappear. If they don't then I'll guess wiring might be the problem.

I did order a rear speed sensor which will be here Wednesday, though not absolutely sure it is the correct one -- it IS correct for the manual transfer type for my year truck. I should know once I compare the old with the new physically, and a temporary hookup should again remove the error code .... I hope.

FYI, present error codes are:

C0025 LF Speed Sensor or Circuit Open
C0021 RF Speed Sensor or Circuit Open
C0035 Rear Speed Sensor or Circuit Open
C0026 Missing LF Speed Signal

The "rubber capped bleeder on the ABS unit" is not a bleeder, and it's not on the ABS unit. It's the stem of the metering (Holdoff) valve; and it's on the combination valve bolted-to the ABS unit.

I tried getting fluid out of mine like the Youtube jocks were doing. I got NOTHIING--not air, not fluid.

If the truck is on jack stands with the wheels hanging free, I don't doubt you've got speed sensor errors. The wheels won't be turning the same speeds due to differential friction.

Clear the codes, see what comes back when all four wheels are on the ground, and the vehicle is driving around.

Thanks again Schurkey for replying, and correcting my poor description. Apologies -- it's a definite learning process for me.

To try to say it properly, the stem of the metering valve was depressed and when the brakes were pumped, fluid did come out for me.

re the truck on blocks: the wheels are off the hubs as well, so there have been no rotation tests. The initial symptom of any problem was that the dash ABS light stayed lit after turning the ignition on, which was not normal behavior. The scanner was used to read what faults were recorfded because the light was staying on.

I know now that the front wheel speed sensors were definitely dead. This morning I disconnected them and checked them with an ohmeter and they both were open circuit.

When the new sensors arrived this afternoon, I connected them without even installing, and when I turned on the ignition the ABS light on the dash went out.

Happy about that -- first time that has cleared. This also means the wiring to the wheel sensors is okay.

I'm not out of the woods because there could yet be ABS problems when running, as you point out. But at least the initial ABS trouble light startup problem has been solved. I now hope to be on the road by the weekend for testing.

I've run into further problems upon taking the brake calipers off. The dust plate on the driver's side is really deteriorated, and the bolts holding the plates on on both sides have really rusted heads. There isn't enough meat to back the bolts out with a socket wrench, the corners just flake off. These must be removed to access the old sensor locations. I'm thinking of welding a new bolt onto the old bolt heads to see if I can remove them that way.

Another new problem is, the dust plate (GM 5998039) is discontinued and I don't see any aftermarket versions. So I may have to fabricate that.

Meanwhile I've decided to replace rotors calipers and pads, and also the flexible lines. I'm really tired of old half-rusted parts, and these are the last of them on the truck. I redid the drums pads and cylinders on the rears last fall.

Thanks again for your advice and assistance.