Hi All,

I have been working on a 91 Z28 I picked up for my stepson who is turning 16 in February. The fans were completely hacked up and not working at all, I cut out all the relays they put in and patched up the wiring and replaced the factory relays and the fans are now working as designed by the factory.

Now I am working on the code 21 I keep getting. I know 91's are speed density and that the TPS is non-adjustable. That is what is driving me nuts. I replaced the TPS and the condition is the same. Also when I tried to back probe the TPS as described in the articles I have searched I get approx 3.5 volts with the key in the on position. I removed the TPS and measured it without it being connected so that the arm would be in it's natural position and the result was the same. Also when I move the arm it goes up to approx 4.0 or so and then start going back down. It never gets to 5 before it starts going down again. It appears to operate in a linear fashion but reaches a point and then starts going in the opposite direction from a voltage stand point.

The engine is not original, it is a 350 out of a vette that was built by Arizona Speed and Marine. So I have no specs on the cam etc, doesn't sound to nasty though. So tuning/PCM could be in play here as well. I have not yet removed the PCM to check the part # or the prom info.

I have read that if the base voltage is above .9 to 1 volt at startup that it could be causing the problem. I have no idea how to test this or how to reduce the voltage if that is the case.

I wonder if this could be a ground issue?

Anyway if anyone has an insight we would greatly appreciate it.

Eric

 

try another sensor maybe and mines adjustable on a 91z tpi but yes it should be 5v in the c pin i think

 

With the ignition turned 'on', the voltmeter reading should be 0.55 volts, with an allowance for 0.075 volts either way. If your getting close to a 3.5 volt reading, one of the voltmeter probes is probably connected to the "C" terminal, which is a 5 volt reference wire (I would also look into that 3.5 reading, sounds like a weak reference wire connection). You stated that it read in a linear fashion, and this is definitely good. A bad TPS will obviously spike. Double check your connections again, then set your TPS to 0.55, respectively. If the code remains, look into that weak 5 volt reference reading....

 

Thanks to everyone for the feedback. I probably won't get a chance to spend much more time on this until next weekend but I will look into voltage and weak signal.

Eric