I too had a problem similar to what you are describing. I really should have entered my solution into the brickboard after I did the repair last year, but you have now prompted me to get it done.

My car 1984 245DL B23F engine with LH2.0 M46 Transmission, 235,000 miles
ECU Bosch part 0 280 000 510

I only know it worked with my vehicle. I have no other Volvo's to experiment with.

Cause

In 1999 I replaced my engine wiring harness because of crumbling insulation. I was sloppy and swapped the IAC (Idle Air Control) valve with the TPS (Throttle Position Switch) for short time. That I believe was the cause of the problem.


False leads

I replaced the IAC motor both a junk yard and new unit but each time the engine would rev up to about 2500 RPM. The only way I could keep the car drivable was to stuff a rag in the inlet hose to the IAC. That reduced the amount of air bypassing the throttle. But it made cold starts quite troublesome.


The Real Problem

The problem was with the ECU (Engine Control Unit) in the circuit that controls the IAC. I had fried a power transistor when I hooked it up incorrectly.

The IAC has two winding; one to open the valve and another to close it. Each of the two windings are connected as the collector resistors in two common emitter amplifiers. The NPN power transistors that drive the IAC are located in the ECU. When I swapped the connectors for the IAC and TPS, I shorted collector resistor on the "close" transistor. This transistor fried allowing the ECU to open the IAC, but not close it.

When I opened up the ECU I immediately found a transistor that was so burnt it crumbled into pieces. Fortunately the "open" transistor was available. It is listed as "PH ON-895." I tried a few weeks of web surfing, a visit to the library and a few electronic stores but I could not find anyone who could get me 1) an exact replacement, 2) a data sheet, or 3) a equivalent transistor. I was able to find a couple of companies that would sell me a replacement transistor, but the catch was they had at best a $100 minimum order. My only other option was to have the ECU repaired for a about $300. And all they would need to do was replace a $1-2 part.

This is one of the problems with Automotive Electronics. The Auto companies, Volvo included, buy so many parts that they can demand their own custom labels on generic components. This transistor is just one example. The other gripe I have with Automotive Electronics is that they are poorly documented for the end user.


Finding A Replacement Transistor

Since I could not get any help with the transistor specifications, I decided to measure them myself. The device was verified as an NPN bipolar transistor
on a curve tracer (a rare and fancy oscilloscope that is used to test transistors) The specs I measured and inferred are as follows:

Hfe (or Beta) 85 (at 800 mAmps)
Case TO-220
Ic (max) < 1 Amp (12V/20 Ohm IAC resistance)
Vceo (max) 15 Volts (deduced from max alt volts)
Power (max) 2 Watts (TO-220 case w/o heatsink)

I purchased from Digikey (www.digikey.com) a transistor
that had better specs:

Manufacturer Panasonic
Part Number 2SD1474
Hfe (or Beta) 300-2000 (at 1 Amp)
Ic (max) 6 Amps
Vceo (max) 60 V
Power (max) 2 Watts w/o heatsink
Vce(sat) 0.5 V

I considered the last spec, the collector-emitter voltage at saturation rather critical when picking a replacement I wanted this value as low as possible since it is related to the total power dissipated by the transistor, and I
did not wish to burn up the new transistor.

Power = Vce(sat) * Current through IAC
0.5 W = 0.5 V * 1 Amp

The Solution was to just replace the burnt out transistor with a new one. Just as a precaution, I purchased two transistors and replaced both the opening and closing circuits. I wanted to make sure the electronics were balanced.

Total cost

Parts 2 transistors at $1.29/each $2.58
Labor 30 minutes to extract ECU, resolder, reinstall


Hopefully my experience can help, Check your ECU, open it up and look for burnt traces, or transistors.