Vehicle Speed sensor question

[Survivor87]

My '87 GN turned on the SES light the other day when the wife was driving it. It had put up code 24 (VSS), and then when I drove it I noticed the speedo wasn't working. Determined it was a broken cable. I have a couple of questions, though. This is a two-piece cable with a reducer just in front of the firewall. I was told this was part of an overlay package from an unknown vendor (145 MPH speedo face). My Buick parts guy ordered me a cable, saying he thought it was a two-piece unit. It is P/N 88959481. Of course it came in as a single piece and he says it shows to be correct for the car. Was there a cable set-up from some other application that is supposed to work with this overlay, or do I have the wrong part from GM? The short cable from the firewall to the dash is the one that is broken.
The second question is: What does the ECM do with the data from the VSS? The reason I ask is because I have observed the vehicle speed reading on a scanmaster to be off (slow) as much as 7-10 MPH versus actual speed. I'm sure this is because of the reducer slowing down the cable so the overlay will read right. Wondering if this could have an adverse affect on performance or driveability. Appreciate any advice on either of these questions.

Steve Allen

[kenmosher]

As far as the second part of your question ... the reason why you are seeing a slower speed on the ScanMaster is the reducer on the speedo for the overlay (as you postulated). In the stock chip, the biggest factors will be lockup and cruise strategies.

With some of the aftermarket chips, they use Vehicle speed to scale boost in different ranges, for lean cruise, and for fancy fueling/timing strategies based on speed/load.

[Survivor87]

Thanks, Ken. I'm reading in between the lines that it won't cause problems on a mildly modified car? It has a factory chip in it now, but I will soon be going to a Translator Plus and extender chip. Maybe the guys at fullthrottlespeed will want to know about that when I order. Still hoping to find an answer on my cable situation. Will check back later.

[edv2]

I used to have an 83 T-type w/ cruise control, that had a short cable and a long cable that went into the cruise unit. I did away with this by getting a cable from salvage yard that was 2 piece from a 84-87 cutlass w/ a 307 engine. Hope this helps!

ED V. :)

[Harry Mash]

My 87 has the two piece cable that connects together in frount of the drivers side firewall. In 87 the cable no longer inter connected with the cruise control.Years ago I bought the 145MPH. speedometer face and cable speed reducer from Poston. When I found what it did to the cruise and converter lockup I Traded it back to them for some other parts.

[Survivor87]

Hey thanks for the input, Ed and Harry.I will take a look at my set-up again and decide whether to keep the overlay or not. Personally I would just as soon have the stock GN display, or maybe a GNX if I could find one. I was concerned that I might have problems getting that overlay off without damaging anything.
Steve