IAC question
Thread Starter
Instructor
Joined: Jan 2013
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From: Friendly WV
IAC question
My 90 L98 runs great but has a miss, stumble, I don't know what, at idle and just off idle. I'm not having any luck running it down. I thought it might be a bad IAC so I read the resistance on the coils in the IAC with my VOM. Both sides are made and read the same resistance. Now here's my question, even though both sides are made and read the same, could it still be bad?
Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
Code check said it had low voltage to the TPS and something else. I wrote it down but I'd have to find my notes to see what it was. Anyway, I checked both and they both had 5 VDC. I honestly haven't found anything specific. I found a vac leak and thought I had it but not. I've bought new intake gaskets as it seems worse when everything is warmed up. I thought it might be a vac leak around part of the intake that opens up as it warms up. I haven't put it in the car yet though.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Check the voltage on all three terminals of the TPS.
Ground your meter to a bare metal part of the engine. Ignition on, engine NOT running (unless you like having moving parts near where you're trying to work). The top terminal should measure 5 volts (reference voltage from ECM). The middle terminal should measure about 0.5 volts (the FSM calls for 0.54 volts, but we're just trying to find out if there's a fault here). The bottom wire should be 0 volts (ground).
If the middle and bottom wires are 5 volts then the ground is missing somehow.
Ground your meter to a bare metal part of the engine. Ignition on, engine NOT running (unless you like having moving parts near where you're trying to work). The top terminal should measure 5 volts (reference voltage from ECM). The middle terminal should measure about 0.5 volts (the FSM calls for 0.54 volts, but we're just trying to find out if there's a fault here). The bottom wire should be 0 volts (ground).
If the middle and bottom wires are 5 volts then the ground is missing somehow.
Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
Check the voltage on all three terminals of the TPS.
Ground your meter to a bare metal part of the engine. Ignition on, engine NOT running (unless you like having moving parts near where you're trying to work). The top terminal should measure 5 volts (reference voltage from ECM). The middle terminal should measure about 0.5 volts (the FSM calls for 0.54 volts, but we're just trying to find out if there's a fault here). The bottom wire should be 0 volts (ground).
If the middle and bottom wires are 5 volts then the ground is missing somehow.
Ground your meter to a bare metal part of the engine. Ignition on, engine NOT running (unless you like having moving parts near where you're trying to work). The top terminal should measure 5 volts (reference voltage from ECM). The middle terminal should measure about 0.5 volts (the FSM calls for 0.54 volts, but we're just trying to find out if there's a fault here). The bottom wire should be 0 volts (ground).
If the middle and bottom wires are 5 volts then the ground is missing somehow.
Team Owner
Joined: Jul 2004
Posts: 28,472
Likes: 3,290
From: Hartford WI
3 wires. 1 is the 5V reference, the other is the ground and the 3rd is the output. I suppose if you hook it all up, you can tap into the line and get a voltage coming out of the TPS. I prefer to use a scanner so I can see what the ECM sees but tapping the wire to the ECM works too.
Pro
Joined: Feb 2010
Posts: 677
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From: Metairie LA
Back probe the TPS sensor while it is connected and key on (engine not running). Middle terminal (positive) and ground (negative) should read 0.54 VDC. Operate the throttle by hand slowly - it should smoothly raise the voltage to 4.5 to 5.0 VDC at full throttle.
You stated that you had a code for low voltage TPS. A bad TPS will cause all kinds of problems.
You stated that you had a code for low voltage TPS. A bad TPS will cause all kinds of problems.
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Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
I hate to just change parts and see what happens. I pulled the EGR tonight and it was filthy. I cleaned it well and made sure the diaphragm was working. I put it back in, no change. I had high hopes for that. I'll not give up. It's getting personal now.
Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
Back probe the TPS sensor while it is connected and key on (engine not running). Middle terminal (positive) and ground (negative) should read 0.54 VDC. Operate the throttle by hand slowly - it should smoothly raise the voltage to 4.5 to 5.0 VDC at full throttle.
You stated that you had a code for low voltage TPS. A bad TPS will cause all kinds of problems.
You stated that you had a code for low voltage TPS. A bad TPS will cause all kinds of problems.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The classic way to back probe a connector is with a straightened paper clip. I prefer a hat pin or needle because they have pointed tips. You need to slide the needle into the connector from the wire side. You need to work it around a bit (it tends to get hung up on things inside the connector shell) to get it to go in far enough to make a connection with the terminal in the connector shell. Then you can measure things while the connector is plugged in.
Or get one of these guys (this is what I use):
Or get one of these guys (this is what I use):
Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
The classic way to back probe a connector is with a straightened paper clip. I prefer a hat pin or needle because they have pointed tips. You need to slide the needle into the connector from the wire side. You need to work it around a bit (it tends to get hung up on things inside the connector shell) to get it to go in far enough to make a connection with the terminal in the connector shell. Then you can measure things while the connector is plugged in.
Or get one of these guys (this is what I use):
Or get one of these guys (this is what I use):
Team Owner
Joined: Jul 2004
Posts: 28,472
Likes: 3,290
From: Hartford WI
Thread Starter
Instructor
Joined: Jan 2013
Posts: 150
Likes: 6
From: Friendly WV
Team Owner
Joined: Jul 2004
Posts: 28,472
Likes: 3,290
From: Hartford WI