Rochester Quadrajet TPS problem
Thread Starter
5th Gear
Joined: Oct 2019
Posts: 5
Likes: 0
Rochester Quadrajet TPS problem
Hello:
This is a bit embarrassing as I don’t own a Corvette, but I’ve been scouring the internet for a solution to this Quadrajet prob. Please excuse my intrusion.
The rig I’m working on is an 89 Chevy Caprice with the 305 olds engine. 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 for reading this
This is a bit embarrassing as I don’t own a Corvette, but I’ve been scouring the internet for a solution to this Quadrajet prob. Please excuse my intrusion.
The rig I’m working on is an 89 Chevy Caprice with the 305 olds engine. 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 for reading this
Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
Welcome...at least it's a Chevy! I used to have a TBI L-G4 305 engine in my '83 Firebird. 82- up Camaro has the same engine as well. CA Corvettes of 1981 had 305 L-G4 (makes a nice Bar-bet).
I think the TPS must be tuned to 5.25 volts. It's just a potentiometer.
Buccaneer can help more.
Unkahal
I think the TPS must be tuned to 5.25 volts. It's just a potentiometer.
Buccaneer can help more.
Unkahal
Racer
Joined: Feb 2002
Posts: 378
Likes: 19
It has been a while since I changed a TPS on a feedback Q-Jet, but as recalled, the varying voltage pin is suppose to read in millivolts and it seems correct that when disconnected, it should be 0 vdc.
At this point the common trouble shoot step is to check the wiring. If it was my problem, I would try an locate another computer to try out to see if you're still seeing 3.39 at the center pin, followed by ohming out the wiring from the computer to TPS connection. Would be helpful to have a wiring schematic to double check whether there was a wire color change at the computer.
At this point the common trouble shoot step is to check the wiring. If it was my problem, I would try an locate another computer to try out to see if you're still seeing 3.39 at the center pin, followed by ohming out the wiring from the computer to TPS connection. Would be helpful to have a wiring schematic to double check whether there was a wire color change at the computer.
Thread Starter
5th Gear
Joined: Oct 2019
Posts: 5
Likes: 0
Thanx!
I'm tracking down a wiring diagram, but this wiring has been fiddled with some so it may be tricky. I'm not much familiar with the "computer" system on this car. Would that be the ICM? the Ignition Control Module inside the distributor cap? I replaced it and there was no difference in the TPS readings, still 3.49.
I will figure out the wiring and test it, but it seems odd that if there were a "voltage leak" that it would come in at 3.49 volts. But I dunno, all new to me.
Thanx Much!!
I will figure out the wiring and test it, but it seems odd that if there were a "voltage leak" that it would come in at 3.49 volts. But I dunno, all new to me.
Thanx Much!!
Melting Slicks
Joined: Feb 2017
Posts: 3,232
Likes: 1,232
From: PHX AZ
Jebby...that's a wicked COP-OUT. 5 times the work and expense, just to find a bad wire????
Q-jet and manifold=15mpg
TBI EFI=26mpg
Lets help him go-forward, not BACKWARD.
He's got a wiring issue, and lack of a $57 GM service manual.
Unkahal
Q-jet and manifold=15mpg
TBI EFI=26mpg
Lets help him go-forward, not BACKWARD.
He's got a wiring issue, and lack of a $57 GM service manual.
Unkahal
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Thread Starter
5th Gear
Joined: Oct 2019
Posts: 5
Likes: 0
Jebby.. i wouldn't mind doin that, more in my league, but the state of Nevada would frown on it too.. tryin to get by smog stuff.. and Yes, may have to spring for the Shop manual.. cant find a decent wiring diagram. haynes one sux.
Thread Starter
5th Gear
Joined: Oct 2019
Posts: 5
Likes: 0
Thanx Guys!!
been very helpful! I tried several other forums on chevys, hot rods, caprice.. an your the only ones have a clue on this (which I dont). I'm learnin much about this motor and carb.. other places like to bash the Q-jet and the 305.
Jus had back surgery so it will be a bit, but I'm continuing research and formulating a plan (that could be a mistake : )
I'm an ol road dog musician and I really dig this big wagon an want to get it up to road-worthy snuf. Planin on haulin a small trailer.. got under 90k miles on it an aside from sittin to long has been takin care-of.. It gets a lot of reaction. A corvette would be nice tho..
Jus had back surgery so it will be a bit, but I'm continuing research and formulating a plan (that could be a mistake : )
I'm an ol road dog musician and I really dig this big wagon an want to get it up to road-worthy snuf. Planin on haulin a small trailer.. got under 90k miles on it an aside from sittin to long has been takin care-of.. It gets a lot of reaction. A corvette would be nice tho..