MAF Troubleshooting - bad ecm?

[Bronson with Bronco]

I swapped a 4.0 from a '92 Explorer into my '84 BII last winter. I'm (slowly) trying to diagnose some high idle issues I've had all along and am currently working with the MAF sensor to see if that might be the issue.

On the MAF plug there are four pins. As I understand it, it's set up like this:
Red - 12V for heat element
Black - ground for heat element
Blue - sensor signal
Tan - ground for sensor

Red and black wires have no issue. However, with key on engine off I'm getting around 6V to ground on the tan wire. I expected this to be 0V to ground. I ohmed out this wire from the MAF sensor to where it plugs into the ECM and got 0.8ohms. So I'm thinking that the wire is good but there is an issue inside the ECM. Anybody have experience with this that can work through this with me?

 

[franklin2]

Tan is not the ground for the sensor. It is actually the sensor signal. The blue is the reference voltage from the ECM to supply the sensor. The sensor varies the voltage on the tan wire back to the ECM. If you unplug the sensor, you will get a feedback voltage on the tan wire from the ECM. Think of the sensor actually lifting or pulling down this voltage from the ECM on the tan wire, that is how it works.

 

[Bronson with Bronco]

So the 6V I'm seeing on the tan wire with the sensor unplugged is normal?

 

[WutUp Clegg]

What happens when you unplug the sensor while the engine is running? If nothing happens, after a few seconds of unplugging it, then there is a good chance it's bad. If the idle changes, then the MAF is more likely working.

 

[SenorNoob]

At 6v I'd think you'd have a code set. IIRC the reference voltage is 5v and usually goes down from there.

Tolerance of multimeters is a thing though.

 

[Bronson with Bronco]

With the engine running, unplugging the sensor drops the idle and it dies after 10 seconds or so. So I guess I'll leave the MAF alone and continue on.

I tightened up a few things on the air box and it seems to have helped (also no gasket was present between the MAF and air tube headed to the intake). Could it also be the ECM is programmed for an auto trans (which the donor vehicle was) and I'm running the stock manual trans from the BII?

 

[SenorNoob]

How high is your idle? IIRC, manual 4.0 should idle at 750 and automatic is 850 when warm. Both 1100 cold.

 

[bobbywalter]

are you positive the throttle is fully returning?