After messing with a surging idle on my 91 740 for a week or two I finally found the throttle body (TB) switch was not seeing the closed position. Previously I had removed and cleaned the TB and checked that the switch was clicking when the TB was opening and it was. But when I reattached the short linkage arm it was a tad short and must have held the switch open and the result was a surging idle. So happy story I fixed it. However in checking different things I removed and tested the idle air control (IAC) valve. I put 12 volts to it and it popped open and took the 12 volts off and it closed. Repeated several times and it worked great. So my question is how does the IAC valve achieve different idle settings if it has an open and closed position only? While I had the TB switch in the not closing mode the IAC valve held different idle RPMs at different times. I could unplug the IAC valve electrical connector and the engine would idle right down. So how's it work?

Hi Kaz, you need a 'scope to see the pulses to the IAV,your LEDtest is showing exactly what it should.The valve is fed pulses with varying on/off ratio to maintain 950 rpm idle. Is nothing in these Volvo electrics easy?
I find that if I boot the car the idle rises,put this down to oil from the pcv contaminating the valve.Someday I'll get one from the scrappy and take it apart to see if oil can get into the coil space.Let you know!

Hi Kaz, you need a 'scope to see the pulses to the IAV,your LEDtest is showing exactly what it should.The valve is fed pulses with varying on/off ratio to maintain 950 rpm idle. Is nothing in these Volvo electrics easy?
I find that if I boot the car the idle rises,put this down to oil from the pcv contaminating the valve.Someday I'll get one from the scrappy and take it apart to see if oil can get into the coil space.Let you know!

I have the same model 91, with rex/regina. I am undergoing
an investigation of how it works also (unstable idle in my case).
I did the same test, putting 12 volts on the valve popped
full open. The power coming out of the connector however
varies which varies how much it is open (12 volts would
natural extend it all the way). I put a power supply
on it and watched the inside plunger move in and out (lower
than 12 volts), pretty interesting actualy. At the same
time, a responsive valve from closed to open doesnt necessarily
mean it moderates well, though a good cleaning i find helps
that out a lot.

I am glad you got your idle better. In my case, I wired an
LED to the valve connections to see how it changed in normal
driving conditions. I have noticed the LED pulsating rapidly
(not normal) and concluded that my ECU is sending bad/non stable
voltages to the valve.
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-J-