99 Blazer 4x4 VSS issues with a twist
 

Ok, One night, a few months ago, we had a lightning storm in our area. The day of the storm, all systems on the blazer worked fine. The day after the storm, the speedometer became inop, and the transmission went into limp mode. I read previous posts on the forum (thank you) about the VSS sensor being the possible culprit for the issues. I had a used transfer case in the basement, from a transfer case swap a few years ago. I pulled the VSS from that case and installed it into my current case. Still no luck. I found info about troubleshooting, and test procedures, and performed the procedures. These are the results:
VSS circuit, pin 29-30, ohm: 1400 ohms, intermittent (had to mess with harnesses to get a reading. Original thought was wiring issue)
VSS circuit, pin 30-ground resistance: 030 ohms/13 ohms
VSS circuit continuity at VSS connector, key on: 11.64 volts


I still have to test AC voltage while manually spinning a drive wheel. Waiting for an extra hand to assist.
My original thought was, the lightning storm fried a computer..However, when I first checked VSS circuit resistance, it was infinite. When I jiggle the harness above the distributor, I get a reading. It has disappeared once again while testing, and another jiggle brought it back..Also, resistance fluctuates slightly while jiggling. So, that lead me to think there's a short to ground..However, with the VSS circuit voltage being out of spec at the VSS connector, I'm not so sure about that..
For now, is there anyone that can make sense of the data I have, and guide me in the right direction? Thank you very much

Rather odd that the ABS light didn't come on. The EBCM monitors WSS data, and the PCM monitors VSS data. If/when another module requests the data, it is shared through class II serial data. Find and fix the crappy connections and go from there.

Thank you for the reminder. ABS light had been on for a couple of months before the "VSS" issues. I chocked it up to bad WSS. The hubs aren't very old, but they're ebay cheapies.. Thank you for your reply. Thinking about running new wiring from the VCM, to the VSS. Anything specific I may need for repair, or otherwise replacing? I read a few things about the VSS wiring being shielded twisted pair, but the few places in the harness that I have it exposed do not seem to be that way. It appears as any other wiring in the harness..

Might want to check the EBCM for DTC's. The EBCM stores DTC's like all of the other modules, but it is the only one that runs a static and dynamic self test at each ignition cycle. You'll need a scan tool capable of accessing the ABS/EBCM, but it might just give you a clue where to look for the problem.

Unfortunately, that's where I hit the proverbial brick wall. I don't have a way to scan specific modules..unless I take the vehicle to a pro shop.
I beat myself every time I need to scan a module, and can't. Sort of hunting in the dark on that aspect. My Cadillac, on the other hand, has module scanning capabilities built into the HUD. Why can't they all be that user friendly? Lol

There's an Advance Auto parts store not far from you, and they probably stock the Actron ABS capable scanner. It's entry level, but it at least retrieves the DTC's. They might even have it on their tool loaner program.

Thanks, I'll look into that. I don't know if I've noticed a scan tool on the loaner list before, but then again, I don't shop at advance very often. I'll let you know what I find.

No luck at Advance, or any local part stores. I luckily have a friend who has a scanner capable of reading ABS codes and such. So I borrowed it. So far, the only ABS code I have is C0225- Left fnt WSS CKT Open, or short. I don't feel like that has any play in my VSS issue. I was able to get some parameter readings from the trans etc., haven't started the engine and driven while reading.. But, standing still with key on, engine off, I have these readouts specifically:
Vehicle speed:0 mph
Trans ISS:0 RPM
Trans OSS:31.88 RPM
That's all I've looked at so far, standing still. Seems odd that the ISS doesn't match the OSS. Any input on that, or where/what I should look at next?
Again, thanks for your help.

Ok, Connected the scanner and read the data while driving.. The only WSS with data that changes is the right front. The left front, and rear values stay the same. Also, the trans ISS value doesn't change with vehicle movement. The value remains at 0 RPM.. The trans OSS value changed to: 0.38 RPM when the engine was running. No change with movement.

Engine running, vehicle stationary
LF WSS:2.99 MPH
RF WSS: 2.99 MPH
Rear WSS: 2.99 MPH
Trans ISS: 0.00 RPM'
Trans OSS: 0.38 RPM

Engine running, vehicle moving at varying speeds:
LF WSS: 2.99 MPH
RF WSS: Variable MPH
Rear WSS: 2.99 MPH
Trans ISS: 0.00 RPM
Trans OSS: 0.38 RPM

Well there ya go, nice work! Now you know where to look for a problem. With the intermittent stuff earlier, and now this, it sure sounds like a loose connection, or chaffed wire harness.

Ok, thanks. I appreciate the help. I'll look into it. Could this be a ground issue? Also, Out of curiosity, if I were to replace the entire circuit, is there anything special about it? I remember reading that it is shielded, twisted pair. But, when I exposed different sections in the harness, it appeared like standard wiring..

CORRECTION IN POST #2: The EBCM monitors WSS data, and the PCM monitors VSS data.... don't ask, no clue on the reasoning there ;)


Neither of the (front) WSS harnesses are shielded. The VSS harness is shielded, and the shield grounds to the "left rear of the engine". My guess is the rear of the left cylinder head;) On the schematic, it shows the two VSS wires running up to a connector at "the top rear of the automatic transmission". There are 4 wires in the connector, and they continue to the PCM. Another interesting thing about the shield, is that the schematic does NOT show the shield going the entire distance from the sensor, to the connector. They may have done that for the ease of presentation on the schematic, or to isolate the VSS signal from other sensitive circuits, (or vice versa) such as CKP and CMP sensors, again, who knows ;)


Start with a good visual inspection of the harnesses from each sensor to its respective module.

That harness is a nightmare. Thank you for that info. In order to thoroughly inspect that harness, I'd need to pull the cylinder heads, the transmission, or both. It's very possible that the connector behind the cylinder head has come loose, but it doesn't seem possible to reconnect it without removing the head. I did find out that the wires in question are indeed shielded twisted pair. So, I ordered some from an online supplier, and will be replacing that circuit. I'll update when that is done. Again, thank you for your help.

I received my cable today. I installed it on the truck without any issues. I test drove the truck after installation, and I'm happy to report a successful repair. I'd like to sincerely thank you again for all of your help. I really do appreciate it. This is the cable I used : 2-Wire (Twisted Single-Pair) Tray Cable #16 AWG Shielded . It has the drain wire, to ground at one end, just like the original. It's in a PVC jacket, which is sunlight resistant, and oil resistant. 12 Ft of it cost me $14.01 with shipping. I'd post a link to it, but I don't know if that's allowed. Speedometer functions again, and transmission shifts as it should Again, thank you.

Glad you got it taken care of.
In post #8:


WSS & VSS default to ~3MPH when the vehicle is not moving.
The absence of data from the LF WSS can be checked at the sensor connector using the VAC range on a DVOM. Rotating the wheel by hand should show ~350mVAC output from the sensor. With zero output, the ABS light should be coming on. If it's not coming on, I would suspect a software glitch in the scan tool. It is acknowledging the DTC in memory, but not seeing LF WSS data.
As for the absence of Trans ISS, (Input Shaft Speed) and OSS, (Output Shaft Speed) again, could be a glitch in the scan tool. The two signals are compared in the PCM for determining transmission slippage.