Nothing in there should be making any noise, the ECU is purely electronic. its a circuit board, with an Intel 8039(*) 8 bit microcontroller chip, with various input/output circuits for the sensors and actuators (power transistors for the injectors and various relays the ECU controls, a/d converters for the ECT and AMM.




(*) the LH2.2 ECU has a 8039 processor, which is an 8048 with no ROM and more RAM. the LH2.4 ECU is a faster 8051 family CPU, and a very similar board.


typical LH2.x boards (these are from a porsche 928 but are nearly the same as the Volvos)

 

You are, of course, right, Pierce. Thanks. I finally found the source of the noise by using a stethoscope. It's coming from a black, cylindrical object forward/above the ECU. The cylinder is about 3" in diameter and ~ 2.5" high. Could this be a climate control fan? I'll try to post a photo.

 

740 ventilation, with the glovebox removed. not mine, I cribbed this photo from somewhere.



the drum on the far right is the fan cage, the fan motor is installed from below. the thing to the left is the AC evaporator, then the heater core is in the center console, with all the air ducting coming off it.

if you turn your AC off and switch your fan to 0, it /should/ be off. theres' a lot of vacuum controls of all the air flaps, you could be hearing a vacuum leak, I suppose. The flap for recirculating vs fresh air is right above the fan.

 

Thanks, Pierce. I don't see my little, black cylinder in your photo. It should be forward/above the ECU and to the right of the big gray drum. Any idea of what my little black cylinder is?

 

without seeing it, I'm having trouble guessing.

 

It was the blower motor/fan. The small black cylinder was the actual motor assembly sticking down. Thanks.