1990 Volvo 740 non turbo:
Very hard starting. Idle jumps hi 2000rmp down to 500rpm then stalls.
Disconnected TPS plug and stabilizes at 1000rpm and is drive-able
with sluggish response and poor shifting.
Testing TPS I found that the two outer pins both have + 5Volts
and the center pin is ground.
Both of the outer pins come from the ECM. disconnected Plug at ECM
there is no continuity between the two outer pins.
Is this an ECM failure or is something else causing this.
Thanks for your help.

 

No

Yes, Don't think I ever had to replace an ECM on those cars - although techs/people will often think/diagnose "it's the computer" when it's something simpler than that.

A sticking idle motor/iac/air valve/whatever your name for it is - will create idle problems. The PCV system pushes oil fumes past the throttle plate - and the throttle plate gets a black build up on the edges reducing air flow at idle - that the ancient idle motor tries to fix. Have you cleaned your throttle housing and reset the idle switch as a first step?

 

Thanks for responding.
I should have included that the throttle body and idle air control have both been cleaned. I am under the impression that the throttle position sensor has a variable output to the ECM. And that one of the three wires is 5volts and the other changes from .5 to 4.5volts depending on the amount of opening of the throttle body.
My question is if i have 5 volts on both of these wires how van the ECM know how much fuel to send to the injectors.
thanks again

 

I believe the ECM uses the engine temp, air temp, RPM, and air mass meter to control the injectors. Modified by the O2 sensor after a minute or so. Throttle position - not so much, might give an extra squirt like an accelerator pump with a carburetor.

Unless it's a Regina car - there's no air mass meter, but there is a manifold pressure sensor - and those cars always ran like crap, with no power compared to the Bosch cars.

Do you have any codes? https://www.volvoclub.org.uk/faq/EngineOBDCodes.html

And ok the throttle is clean - did you set a base idle after that? (engine stays running with the air going through the throttle plate when warmed up with the hose to the idle motor clamped/blocked off - I realize the those hoses are fragile now, but they were not when I used to do that)

The ecm will shut off the injectors if the idle speed is too high if it thinks it's supposed to be idling (a valid signal from the throttle switch telling the engine to idle is present) So 2000 rpms at idle, ecm shuts off injectors, 500 rpm injectors come back on, then repeat.

 

Set throttle sensor to switch on opening of plate.
will try to block off idle air hose
Thanks again