Good day,

1990 16v 740 GLE, with all new parts, but poor wiring engine harness I suspect...:

upon warming up the car, the idle is way high like 1500 to 2000 constant. When I unplug the AMM the car returns to normal idle (I know bad practice not good!). The parts in question have been replaced: AMM, IAC, ECU, ECT, TPS. still condition is present where when the car is up and has been running 4 minutes or so, the idle never settles to where it should. I wondered if it was indeed the wiring in the harness is damaged somewhere in the cable wrap, and what the symptoms might be if there wasn't the right voltage being supplied at the right times to the IAC. Any advice on this?

 

Voltage to the iac is not constant, it varies as temperatures change. I no longer have access to what that voltage range should be, but surely it is in the Volvo green books. Also, if someone here has a functioning version of VADIS or DICE, perhaps they could research what the voltage ranges should be. Have you used a dvm to see if there is any voltage to the unit? I had a similar issue on an 84 turbo car and deove around for a couple of months with hose clamp pliers, adjusting as needed for the ambient temps. No solution, but got rid of the high rev's I couldn't stand...

 

BTW, in my case the solution was a failed circuit withing the ecu. Everything else within the ecu worked flawlessly.

 

warm up the engine, switch off all accessories (lights, a/c, etc) and unplug the IAC and the engine should idle about 600 rpm in park/neutral. any faster, then someone has jiggered your throttle in a mistaken attempt at fixing a bad idle, or the throttle is sticking open, perhaps due to too much residue buildup.

the IAC is controlled by pulse width modulation, not a variable voltage. more/wider pulses == more air, fewer/narrower pulses == less air. when the ECU sees the throttle position switch is at 'idle', then it uses the IAC to regulate the engine RPM via these pulses.

thats a long list of parts you replaced... the replacements were all new correct part numbers Volvo OEM for a B234F ? or was this a mix of used stuff and who knows what? What testing was done to verify the replaced parts were defective and that the new parts rectified the problem?

 

BTW to the op, have you tried pulling obd codes? Occasionally they actually point in the correct direction.