Hi Bricksters,

I was driving my 84 240 yesterday when it lost power, it would not rev past 2500rpm. It didn’t die when I pulled off the roadway - it just wouldn’t rev. The only thing I could think off was the AMM went bad. I removed the connector and the car ran better in “limp home mode” so I made it back to the house.

Using my Bentley manual, the AMM checks out except for the
“Test idle mixture potentiometer” part. Ohm-wise it reads OK
but, the adjusting screw does not change the resistance. The manual says replace, is that it?

Also, I want to check the wiring harness to the ECU. What terminals on the ECU read the AMM? The manual is fuzzy on this.

And….Who has the best rebuilds and price on L.H. 2 AMM’s?

P.S. The hot air intake from the engine was disabled long ago so I am not worried about frying the new one.


Thanks,

Randy Anderson

Sometimes -- not always, of course -- when the fuel pressure regulator fails, the car will run somewhat better in "limp home" mode.

The "limp home" mode switches the fi ecu to a preprogrammed fuel delivery mode that becomes very lean as you apply additional throttle. The system runs OK at idle and low speed, but has no way of knowing how much fuel is required for acceleration (it's not measuring anything), so delivers no extra fuel. Thus, the engine leans out.

A common failure mode for the fuel pressure regulator is a pinhole in the diaphragm, allowing raw gas to be sucked into the intake manifold through the vac sensing hose. This will send LOTS of extra gas into the engine.

If you use "limp home" in this situation, you have two compensating errors -- extra fuel from a bad regulator and an otherwise lean condition from an unplugged AMM. This can create moderately better running at moderate highway speeds.

And it can cause you to mis-diagnose the AMM.

Check the small regulator vacuum hose for ANY signs of raw gas or gas vapors.

Check in the 7 - 9 FAQ and there is a place in Gardnerville, NV that is mentioned and I have used. Do have a spare here I believe as kids had 2 -4 of the 83 -84's that fried AMM's until we sold them.

dick