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Is my ISC motor shot? ('90 4.5L)

1001 Views 1 Reply 2 Participants Last post by  thu
I performed the ISC max extension test according to the manual: Ignition ON, enter E.5.3) I'm supposed to get 1.15-1.20 volts at the TPS. I probed each of the three pins of the TPS and got 5.07v from the gray wire (pilot voltage which is correct), 0.12v from the blue wire, and 0.09v from the black wire. Nowhere near 1.15v
Problem with car is a high off-throttle idle "sail"; however it will idle down around 5mph. I am not getting that 70mpg benchmark when coasting, so I know the off throttle idle is too damn high. I am getting no codes, except E52. I have cleaned the throttle bore and performed idle relearn several times. Idle does drop at about 5mph, after standing on the brakes!
Any thoughts on the low voltage? ISC shot? Thanks in advance!! Matt
1 - 1 of 2 Posts
Exact same thing happened to me. I ran the PCM switch test and found it to be the TPS. Simple diagnostic to do and so was the replacement. Pay particular attention to test E.7.2, 'Throttle Switch'. Hope this helps.

There are also other sections in the FSM regarding the TPS, see if you can dig for them in the FSM.
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