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Rochester Quadrajet TPS problem

Hello:
Newbie here hoping I might get help, this Throttle Position Sensor prob has me stumped!
The rig I’m working on is an 89 Chevy Caprice with the 305 olds engine. The code reader is reading 21, TPS voltage to high and 45, rich exhaust (make sense)

I have been consistently reading about 3.39 volts (on the middle connection that should have the voltage variation) no matter what position the throttle is in. I replaced the TPS and also the ICM (as it is called on the 305 olds engine) and the reading is the same, with a very small variation when the throttle is operated.

I then unplugged from the TPS and checked the voltage coming from the plug, the 5 volts is coming to the top wire, and BEHOLD the middle wire (that should have the varying voltage) is reading 3.39 volts when disconnected. Shouldn’t this read 0 or close when the TPS is disconnected? I suspect this is over-riding anything the TPS is trying to send.

When I first checked this is when I replaced the ICM. Still the same. I don’t have any visible shorts and suspect some type of Voltage Leak which is why I replaced the ICM, AC Delco parts. Any ideas?

Thanx much..

Quote:

Originally Posted by nickmostly (Post 436089)

Hello:
Newbie here hoping I might get help, this Throttle Position Sensor prob has me stumped!
The rig I’m working on is an 89 Chevy Caprice with the 305 olds engine. The code reader is reading 21, TPS voltage to high and 45, rich exhaust (make sense)

I have been consistently reading about 3.39 volts (on the middle connection that should have the voltage variation) no matter what position the throttle is in. I replaced the TPS and also the ICM (as it is called on the 305 olds engine) and the reading is the same, with a very small variation when the throttle is operated.

I then unplugged from the TPS and checked the voltage coming from the plug, the 5 volts is coming to the top wire, and BEHOLD the middle wire (that should have the varying voltage) is reading 3.39 volts when disconnected. Shouldn’t this read 0 or close when the TPS is disconnected? I suspect this is over-riding anything the TPS is trying to send.
what rpm should my car idle at? Access Autoguysland site for more info if you concern.
When I first checked this is when I replaced the ICM. Still the same. I don’t have any visible shorts and suspect some type of Voltage Leak which is why I replaced the ICM, AC Delco parts. Any ideas?

Thanx much..

I see you post in another forum. hence, have you got the solution for this issue yet? You also refer this link for more info if it helps or not.
https://www.thirdgen.org/forums/carb...-problems.html

With an ohmeter, unplug the connector, read the 5 volt reference side to the return side (3.39 volts you are reading), move the throttle off idle stop, the resistance isn't changing is my guess, that means the wiper on the resistor is bad, causing it to only read 3.39 volts.

Thanks Kevin:
I'm W/O a clue in this system, I have already performed the test you suggest, and it still reads 3.9. Is the resistor you speak of a red square one in the distributor? I saw this when I replaced the ICM, attached to the ICM. It looked as if it might have been replaced before, but I hadn't any idea what it was.
Thanx again.

It is attached to the throttle arm, and moves a wiper across a fixed resistor. That changes the reference voltage depending on where the throttle is, i.e. half way, 2.5 volts etc. If it reads 3.39 regardless of the actual throttle position it needs replaced.

https://cimg7.ibsrv.net/gimg/chevrol...dd75d66925.png
already replaced this.. bench tested it and it works fine..

the 3.9 is coming from the center wire that plugs into this
overiding the signal the tps sending. I'm trying to figure out where the 3.9 is coming from. seems it should be reading 0. Thanx

Is it reading high unplugged? Could be a crossed wire or connection from the reference to feed back, not impossible, but not really likely

yeah.. not likely at all.. but there it is.. I'm tryin to find a decent wiring diagram and do sum circuit testing.. may spring for the shop manual, hate to tear into the harneses.. I was hoping sumone out here would have the magic bullet fix : ( thanx much!