I have been researching quite a bit for my speedo recalibration and came across some information that piqued my curiousity.

The ABS system operates independently of the ECM/PCM in our cars. A guy on another board had a scanner on his car and was checking his speedo which was off due to a gear change, and when he switched to EBCM mode the EBCM computer was reading actual road speed. Which got me thinking.

How does the ABS sensor work??? Is it essentially the same thing as the VSS which is a magnetic pickup on a toothed ring?? If so, how does it send the signal to the EBCM??? Would it by chance be via pulses, and if so, would it happen to be the same 4000 pulses per mile??? If all this were to hold true, wouldn't we be able to tap the EBCM signal wire from one of the ABS sensors and take it directly to the speedo input wire for the speedo correction for a gear swap??

This is on a 93 before anyone suggests to have my computer reprogrammed.

Mods, if this isn't appropriate here, sorry about that.

 

I have a 95, but my speed is taken from the front drivers abs sensor. It was like that when I bought my car and I have not messed with it. If there is anything I can look at or find out about my car I will do what I can.

Richie
1994 Z28
1995 Formula

 

You're saying that your speedometer is tapped into the front right ABS Sensor???? That may be the information I was looking for in itself. If you could confirm that I'd be most appreciative.

 

Why are you doing this? The VSS isn't working for some reason?

I remember people doing this to prevent the limp home mode with high stalls on OBD1 apps and also people running A3 trannies so I looked it up in the archives. It appears that it works just fine for a VSS signal however the ABS system uses a 41 tooth reluctor wheel so you still have to adjust for the different frequency. Some people have also had to use an op-amp circuit to prevent an ABS INOP light.

 

I'm looking at the option of doing this because the piece of crap SLP Speedo Box I have won't work to save it's life. I keep getting a dead speedo at about 10-15 MPH with everything properly wired up and soldered. The only thing I haven't done is gone battery power and battery ground direct connections to get the cleanest power/ground signals I can. I'm just sick of screwing with it and if there's an easier fix by tapping the ABS system for an unaltered ground speed reading, then I'm willing to try it.

Any advice?

 

The speedometer is hooked into the right front ABS sensor, but I think the speedometer tables have been messed with to get it right. The reason my car is wired like this is because it has a TH-350 in it. I will look at the Tunercats file and the wires this weekend when I get home.

 

Don't bother. I can't do anything about that, as a 93 doesn't use the PCM and speedo together.

Does anyone know if the pulse signal from the ABS sensor is the same as the VSS???

 

Oh boy, it's been at least a couple of years since I've messed with those old fangled systems that weren't driven by a PCM

Have you tried a Dakota Digital calibration "box"?

If my information is correct the ABS system uses a 41 tooth reluctor wheel and of course the T56 uses a 40 tooth wheel. You need to do the math with your gears to find out where that puts you but you're still going to need to manipulate the signal to get an accurate speedo reading.

 

Have not tried the Dakota Digital box, but read a couple of horror stories about that one too. It's my next step if I can't get the SLP box going this week.

About the reluctor wheel, a 41 tooth compared to a 40 tooth should be fairly close, right??

 

Sorry if I'm butting in here, as I don't mean to hi-jack your thread. But I'm dealing with similar issues with my 95. I ditched the PCM and am still hoping to run the stock speedo with the T56 VSS. From what I've gathered, the VSS outputs a 6000 pulse/mile AC current, and then the PCM converts it to a 4000 pulse/mile DC current. Question is, how do I convert this signal without the PCM? Or am I way off with the VSS output?

Just hoping someone reading this can help. Please PM me so as not to hijack Fastbird's thread.

Thankyou.