ABS-VI System Questions - Technical

Blue Bowtie · 10-04-2006, 11:48 AM

And DON'T tell me to grab my TECH-I/II.... (That'll be about enough of that) This goes beyond what a scanner will diagnose, since all it will display is "Zero Signal" or "High Signal" messages. Where is the cut-off?

What is the minimum wheel speed sensor output voltage at MPH? GM isn't letting on, so I wondered if anyone has found the threshold through diagnostic experiences? I've seen a few numbers thrown around, like "350mV" and "600mV" but nothing definitive.

Any ideas?

Blue Bowtie · 10-04-2006, 03:11 PM

I can read a minimum of 0.24V P-P (.07V RMS) with ANY hub movement, and voltage increases to about 100mV P-P at 1 RPS (60 RPM) of the hub (0.29V RMS). I can spin the hub at about 4 RPS (240 RPM) and make well over 850 mV RMS (2.6V P-P).
Pulse counts are consistent, so the prox seems to be reading every tooth of the reluctor.

Need any other information?

EDIT: Corrected "RPM" to "revolutions per second" for clarity.

BNaylor · 10-05-2006, 01:15 AM

Are you having fun Jeff?

For ABS VI, I recall a minimum output voltage of 100mV (threshold) out of the wheel sensor just using a digital multimeter set on AC volts while spinning the respective wheel by hand. If below that then the wheel sensor is questionable. Plus an ABS DTC should set like C12XX.

Blue Bowtie · 10-05-2006, 11:43 AM

You have a twisted, perverse idea of "fun."

I'm not having any luck finding a specification. I've used a "plain" DVM (8022), a peak-holding DVM (8024B), and a true-RMS DVM (8026B). I ran out of Flukes, so I broke out the Tektronics to look at the actual wave and P-P output. The EBCM recognizes inputs from each of the wheel sensors since it cycles the (new) modulator every time I spin an individual wheel (vehicle is on four stands).

Here's the symptom - The ECBM runs its diagnostic on ignition power up. Warning lamp is ON for 3-4 seconds, then turns off as it should. I start the engine (cycle GAGE circuit power) and the ABS-VI power relay therefore cycles, and the EBCM again diagnoses and turns the warning lamp on then off. With all wheels suspended, I can spin any individual wheel at about 3-5 MPH and the modulator cycles. With the vehicle on the ground, the same occurs,but once road speed is at about 5 MPH the warning lamp illuminates.

Did I need to manually "set" or Synchronize" the modulator unit? It has been pressure bled according to factory procedure - Cycling the modulator, bleeding the master cylinder/modulator (rear first), and them bleeding each wheel. The pedal is firm and travel is minimal. Fluid level is correct, and the level switch is closed.

BNaylor · 10-06-2006, 11:14 AM

Quote:

Originally Posted by Blue Bowtie

Did I need to manually "set" or Synchronize" the modulator unit? It has been pressure bled according to factory procedure - Cycling the modulator, bleeding the master cylinder/modulator (rear first), and them bleeding each wheel. The pedal is firm and travel is minimal. Fluid level is correct, and the level switch is closed.

I am not aware of any synch procedure for the ABS modulator. Couldn't find anything in a GM service manual circa 1997 about it. What GM car are you working on?

Blue Bowtie · 10-06-2006, 07:41 PM

'95 N-Body. Through some trial and error, I found that the 70mV signal at the rear wheel sensors is more than adequate. The scan I did showed a LF sensor intermittent reading on the MT-2500. And here I was worried about the signal threshold from the relatively weak rear sensor and 20 feet of twisted pair to the EBCM, while the 850mV+ signals up front weren't getting there. Of course, the unit doesn't display actual parameters, just what the EBCM is telling it, not al all unlike the Tech-II's habit of "lying" to tech (More on that later).

Back on task, the intermittent sensor was from a previous "technician" attempting to solve a connector problem by replacing original connector and twisting/taping the connections. Really professional. The sensors were all fine, but the harness was not. The intermittent was apparently introduced when the suspension bounced or steering turned, and the harness was subsequently flexed. Sure enoug, about six inches up into the convoluted tubing was a wad of tape and wire. (People who cannot solder/shrink should not touch copper.) Ten minutes of repair later, she was out the door.

Back to the "marvelous" diagnostic devices, has anyone else caught on to the fact that Tech-IIs can be a great paperweight at times? Ditto the Snap-On (OTC?) MT2500. I've found that one thing in particular is the oxygen sensor data on both OBD and OBD-II system, and MAF data on OBD systems, and the CTS, TPS and IAT on either system. The scanner doesn't look at the real-world data, but only what the PCM THINKS it is seeing. It gets really bad with the O²s, in that O² sensors are shown "good" (with adequate crosscounts and mV variation) when it is in fact an internal PCM subroutine using backup parameters (like an O² simulator) for the dead sensor, strobing that "data" to teh PCM processor bus to satisfy the DTC tables, and then referencing the post-cat sensor for some minor fuel trim. You GM techs need to ask your "home peeps" about that clever little deception - That's why I use a 'scope (and win free drinks from bets). The Tech-II lies. Thanks, Vetronix. I'm taking a serious look at PC-Based systems with the ability to simultaneously use a PC-based dual trace 'scope (like the PicoScope) to verify PCM data BEFORE it hits the A/D or D/A I/O converters and signal conditioning. I'm thinking that in about ten years, they'll ALL be that way, and this clunky hardware used now will be nice door stops or paper weights. Some of them are already that way, and are FULLY J2534/CAN compliant, plus backward compatible, and less $$ than a handheld.

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