Mysterious C0236 - Missing Rear Speed Sensor

[LoCascio_Inc]

I've been struggling to sort out the ABS in my 1998 K2500 since I bought it. At this point I'm stumped. Current situation is as follows:

• Speedo works fine.
• ABS light and C0236 code are triggered as soon as brakes are applied after moving (<20mph).
• ABS does not function and rear wheels lock up under "panic stop" braking.
• Reluctor wheel is visually present inside of transfer case (unsure of tooth count).
• 1A Auto speed sensor is brand new and assumed to be functional.
• Both wires show continuity from speed sensor connector to PCM connector.
• Clearing codes and leaving battery disconnected for 20min seems to do nothing.

My current guesses:

• Could the PCM be malfunctioning? Previous owner had Black Bear re-flash it for a cam swap.
• Are both the old and the new speed sensors defective?
• Could the sensors be too far from the reluctor ring somehow and not getting a proper reading?
• Is there another component that could be defective and trigger a code C0236?

 

[honkon]

Have you checked for signal voltage on either of the wires or at the pins at the pcm? One of them should have signal voltage, assuming that's how the sensor operates.
Have you checked the resistance between the wires with both ends disconnected?
Do you have an oscilloscope you could verify the operation of the sensor with? N.E.W. Never Ever Works.
Just thinking aloud here. I'll look in the service manual when I get a chance.

 

[LoCascio_Inc]

Have you checked for signal voltage on either of the wires or at the pins at the pcm? One of them should have signal voltage, assuming that's how the sensor operates.
Have you checked the resistance between the wires with both ends disconnected?
Do you have an oscilloscope you could verify the operation of the sensor with? N.E.W. Never Ever Works.
Just thinking aloud here. I'll look in the service manual when I get a chance.

I haven’t conducted any other electrical tests beyond continuity. I suppose that’s the next step, but I’m unsure of what correct voltage or resistance should be.

I don’t have an oscilloscope. How would that be used to verify speed sensor operation?

 

[Supercharged111]

Do you have access to Alldata or something of the like? They spell out all the conditions that must be met before the light is set.

 

[pcmpower]

you may also ...after testing want to suspect the EBCM

 

[honkon]

It seems that this is an issue of the EBCM not receiving the speed signal from the VCM, not an issue of the VCM not getting the signal from the sensor. A single yellow/black stripe wire carries the signal from the VCM to the EBCM.
To verify the operation of the sensor with an oscilloscope you'd probe what I guess is the signal return wire pin with the connector plugged in, and then hook the other probe to a ground I guess. Then you'd see some kind of activity on the scope as the tone ring passes the sensor. Not exactly sure of the technicalities of it. But that is irrelevant at this point.
I'll post the service manual entry when I get a chance.

 

[east302]

Here’s the diagnostic from the 98 service manual. PDFs can be downloaded at the link below, but I think that someone has posted them on this forum as well.


http://67-72chevytrucks.com/vboard/showthread.php?t=700011

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EBCM connector C1:

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VCM connector C1:

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Edit: not sure why, but tapatalk won’t upload a clear (readable) scan from the manual. It keeps downsizing the resolution. Here is the entry from alldatadiy.com, it looks to be the same:

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[LoCascio_Inc]

Wow guys, this is super helpful.

 

[LoCascio_Inc]

After following steps 2 and 4 above, I've determined The EBCM is getting more than 10v from the VCM/PCM via the 1827 CKT. After clearing all DTCs and driving the truck, the ABS light is triggered as usual. According to the service manual, all of this indicates that the EBCM needs to be replaced. I assume this is not a serviceable item?

Can a replacement EBCM be purchased by itself? This seems odd, but I can only find used EBCMs complete with valve body online. The valve body under my EBCM has 12765501 cast into it. Searching this part # shows several options on eBay.

 

[east302]

Hopefully this link will work. This parts catalog shows that the EBCM was available separate from the modulator valve.

If you don’t want to go the salvage yard route (and it does show it to be a different part number for vacuum vs hydraulic brakes) then I would ask a dealer for the part number.

https://nemigaparts.com/cat_spares/epc/chevrolet/54c-k/04/tc04-002/

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At a glance, it looks like 12474925 for vacuum and 12474923 for hydraulic brakes. A dealer could confirm. Gasket (#3) was 12548260 per the link above.





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