mannys6speed

I bought my 94 z28, and it was parked for about a year. The speedometer would only go up to 15 mph. So I bought a new Speed Sensor, and now it marks like only at half of the real speed. I dont know how to fix this???

Injuneer

If I understand this correctly, with the old speed sensor, no matter how fast you went, the speedo would not go over 15MPH. With the new speed sensor, it will vary at about 1/2 actual speed - i.e. indicates 20MPH at a true 40MPH, 40MPH at a true 80MPH, etc.

Does the odometer record the correct miles driven?

mannys6speed

Yes thats correct, and if i drive two miles, it says that i drove 1 on the tripometer, and on the mileage on the car. I think that the only reason this happened was because the car was parked for about a year or two. It supposably says that it has 85,000 when i bought it.

jmd

Hmm. There was an 11 tooth reluctor whereas 94 should have 17 tooth.

If the PCM has a tune for different gear ratio that could do it. But half is odd in either case.

Can you swap pcm's with another car?

mannys6speed

I can't switch pcm's , i would have to buy another one, Do you think I might need to get another speedometer??

jmd

Put a scantool on it to see how the pcm speed compares to the speedo. If it's not different, the speedo is fine.

Pop the vss out of the tailhousing and count teeth of the reluctor.

Maybe the 17 in the trans. is correct but the pcm has an auto tune for 40t. That's where trying to borrow a local 6 speed owner's pcm for a quick drive would pinpoint the tune as offf without paying for a tune.

mannys6speed

I took it about a week ago to get a new waterpump and opti and the mechanic scanned my car and he said that i have a bad speed sensor, but it's new.

jmd

Yep. Clearly stated in post 1.

Injuneer

Get an OBD-I scanner on it, and see what the PCM thinks the vehicle speed is, as suggested by jmd. Starting in 94 the VSS feeds the pulses directly to the PCM. The PCM converts the pulses to the vehicle speed, based on the programmed pulses per revolution (a function of the # of reluctor gear teeth), tire diameter and rear axle ratio. Then is sends the signal to the speedo/odo.

If the PCM indicates the correct speed, it's a problem with the cluster. If the PCM indicates the same 1/2-speed as the speedo/odo, its a problem with the reluctor wheel or with the programming.

Clarify - did you buy the car and park it for a year, or did you buy a car that had been sitting for a year?

If you had a problem with the old speed sensor, and a code was set, did you "clear" the code after you installed the new speed sensor? If you don't clear the code, it will stay stored in the PCM until you drive something like 40 or 50 "drive cycles" and the problem has not repeated. That might explain why the mechanic found the VSS code. Clear it (pull "PCM BAT" fuse for 30 seconds) and then have it scanned.

mannys6speed

When I bought the car it was parked for over a year. I never cleared the code off.

Injuneer

From your screen name, can we assume it's a T56? Is it possible that someone installed a T56 from a 1993 Camaro or Firebird? That would explain the presence of an 11-tooth reluctor, rather than the expected 17-tooth.

If the PCM was tuned for an automatic, with the 40-tooth gear, you would have numerous codes for the missing automatic transmission electronics.

mannys6speed

I bought the car from the second owner, everything on the car was original.

jmd

Well, troubleshooting involves more than posting in a thread so let us know when you've done anyhing suggested.