legacycentral bbs

Background: 1994 Subaru Legacy SS, now with an NA engine, and used as a rally car, fully caged and has been raced for years.

Now that is out of the way, here is my question. Is the speedometer needed for a VSS signal? And is the VSS signal important to the ECU functions?

Reason I ask is that ever since I have gotten the car the speedometer doesn't work because the little transfer piece that attaches into the transmission was broken. I bought a new one, but never put it on because of how much of a PITA it is. After my first rally, I kind of liked not having a speedometer, kept me focused just on the road and the revs, and really had no concept of how quickly I was actually going.

So when I pulled everything out of the car for an NA engine swap, I also pulled out the transmission and could have put the transfer/adapter thingy back on, but decided not to. Now I have noticed that the car has a ton of afterfire (which I kinda sorta love) but the gas mileage has also taken a big hit. I pulled the running engine out of a 1991 Legacy L that I drove down from Chicago and got 28mpg on the way back. Same ECU as it was in the parts car, same wiring, same intake, etc. etc. Right now the only thing that I can think of that would be causing the MPG to be incredibly poor and the massive amounts of afterfire is either a vacuum leak, or the fact that I don't have the VSS signal going to the ECU. But thought I would check in with you guys.

http://www.youtube.com/watch?v=U5wkOIILBIE

http://www.youtube.com/watch?v=ngejQ43OF_8&

The speedometer signal regulates a lot of important ECU functions. I wouldn't be surprised if your issues were caused at least in part by the speedo signal being missing. It's a big enough deal that when I was having some problems with a previous Sport Sedan with a messed up speedo adapter that was the suggestion to fix as a troubleshooting step before anything else.

Yes, the ECU becomes unhappy without the VSS.

To add, the ECU will run the coolant fans all the time, and it does some things with timing/fuel mixture.

If you don't want the speedometer in the dash you could run a newer style speed sensor that goes directly into the transmission and then wire the output directly to the ECU. It may not be calibrated exactly the same, but it doesn't really matter as long as the ECU has an input signal.

Added the speedometer cable back in for now. Car definitely runs a lot better now without all the excessive backfire. Haven't driven it enough to notice if MPG is back where it needs to be. Might just cover it up with duct tape while I am on stage to keep me from glancing down at it.

Later on when I have the transmission out again and the redoing all the chassis wiring I will probably add in my VSS from my old WRX transmission and switch to a tach only dash, doing away with the cluster completely. Unless you guys know if there are other things the ECU is referencing from the cluster?

MConte05 wrote: Unless you guys know if there are other things the ECU is referencing from the cluster?

Not to my knowledge. The VSS sensor sits behind the speedometer portion (Ive got a thread somewhere with detailed pics).... in case you wanted to relocate it somewhere with the swap.

Yeah, the only reason the ECU needs the cluster is for the VSS. It does send the tach signal to the tachometer, but that's only an output.