Let's play - diagnose the Cavalier for Todd

[gunslinger]

Most likely not the switch. You've probably got an open in the wire going from the switch to the ECM.

 

[tjthorson]

There's a TSB for this:

Number: 84-273-6E

Section: 6E

Date: Nov., 1988
Subject: SLIGHTLY HIGHER IDLE SPEED

Model and Year: 1983-84 CAVALIER WITH 2.0L TBI (VIN CODE P) ENGINE AND POWER STEERING
TO: ALL CHEVROLET DEALERS


Condition:

Idle speed in the range of 1200 to 1625 RPM, in closed loop, in park or neutral, with the steering wheel not fully locked either right or left, may be experienced. This slightly raised idle speed has no adverse affect on driveability or vehicle performance, but may be objectionable from a noise standpoint.
Cause:

An open circuit in the signal to the Electronic Control Module (ECM) from the power steering pressure switch. Unlike the 1985 and later models which use a normally OPEN switch, the 1984 and earlier J-cars with the 2.0L engine use a normally CLOSED power steering pressure switch. If the ECM sees an open at any point in this circuit, it will interpret the open as a power steering load, and will raise the idle speed.
Correction:

Disconnect the power steering switch. If there is no change in idle speed, this condition is the cause of slightly higher idle and the switch should be replaced. If the idle speed changes, the cause of the higher idle speed should be investigated through normal diagnostic methods.
Refer to the 1984 Service Manual, pages 6E2-76 and 77, which is the Idle Air Control (IAC) Diagnostic Chart. The wiring diagram will lead to possible areas which may be open in the circuit, such as an open power steering pressure switch, an open A/C compressor clutch coil, an open A/C high pressure switch, an unplugged connector, loose ground connections, or a cut or pinched wire. Repair as necessary.



Straight from alldata

Need a little more help. The "manual set" idle is driving me crazy. It idles around 500rpm when cold - then once it warms up, in park it idles at 1000. if I try to bring the warm idle down to 800 or so (for 650 in drive) - then it wont idle cold. I think I really need to get the ECM back in the loop.

Does anyone have the pinout diagram for the IAC so I can at least close the PS request circuit and maybe get the IAC counts back to normal so I can let the ECM control idle again?

 

[TurboJim]

Like I said before, you dont have to replace anything yet. Get out your scanner, look at the PS pressure pid. Turn the wheel and look for a change of state. If there is none, unplug it it, if still no change, short the plug, if still no change, then replacing the switch wont do squat. You'll need to find the cause of the non change of state. Broken wire, bad connector, bad ECM whatever. I wouldnt waste any more time taking parts off the other car, or going to NAPA until you've diagnosed the problem.

You know that the ECM wants counts that high. Now you need to think of what would cause the ECM to raise the idle. PS request, AC request, Park/Neutral input, possibly cooling fan (but I doubt it) and see which one is causing the idle to be raised.

Replacing parts is an expensive way to fix it, especially when in the case of that TSB posted above, a part is not the cause of the problem!

In the immortal words of Jim T ....oh wait, thats me, Test, dont guess. Shouldnt take more than 1hr to figure out the deal.

 

[tjthorson]

Like I said before, you dont have to replace anything yet. Get out your scanner, look at the PS pressure pid. Turn the wheel and look for a change of state. If there is none, unplug it it, if still no change, short the plug, if still no change, then replacing the switch wont do squat. You'll need to find the cause of the non change of state. Broken wire, bad connector, bad ECM whatever. I wouldnt waste any more time taking parts off the other car, or going to NAPA until you've diagnosed the problem.

You know that the ECM wants counts that high. Now you need to think of what would cause the ECM to raise the idle. PS request, AC request, Park/Neutral input, possibly cooling fan (but I doubt it) and see which one is causing the idle to be raised.

Replacing parts is an expensive way to fix it, especially when in the case of that TSB posted above, a part is not the cause of the problem!

In the immortal words of Jim T ....oh wait, thats me, Test, dont guess. Shouldnt take more than 1hr to figure out the deal.

Jim, I think you may have been replying to an earlier posting? I clearly have a break somewhere in the wiring causing a PS request. My cartridge I am using on my OTC doesnt show the PS request as one of the parameters - so I cant tell if the ECM is showing a PS request. That was why I asked for the ECM pinout, so I can find the PS switch wire at the ECM, close the circuit (simulating a non PS request) and look to see if the IAC counts drop. If it does, then I can drop the min idle screw back down and plug in the IAC solenoid and get the ECM controlling idle again. Then - I will just run a new wire from the ECM to the PS switch if I cant find the break outside the loom somewhere.

I did try to short the switch - it had no effect on IAC counts.

Again - I might have figured this out earlier - except for some reason - the OTC4000E cartridge I am using doesnt show the PS request.....

A/C request has been "no" all along....