Carrera AFM measurments.

[jpnovak]

I am helping to troubleshoot an 86 RoW Carrera that is not running properly.

The engine barely starts with the AFM plugged in. if you unplug the AFM the car starts immediately and then oscillates due to lack of reference voltage.

The AFM measures good on the bench. Here is my specific question.

I measure 4.5 +/- 0.1V on Pin3 of the AFM plug with key-on. This is the reference voltage. Pin 2 should be a voltage input to the DME. I measure almost 2V on pin 2 with key-on. This voltage changes with changing throttle position - presumably because the idle switch is clicking on/off.

I think there is an issue with the DME that is giving feed back to Pin2 on the AFM plug. Can anyone tell me what the spec is for Pin2? This should go to pin9 on the DME.

Will reset idle mixture and speed once AFM is working properly.

[DRACO A5OG]

It is possible the potentiometer arm may need to be re-tensioned by slightly bending the arm down on to the strip, while in there check the strip is not gouged. measure the full swipe to insure there is no dead spots. Do not over tension the arm, too much tension will wear the strip down.

[steely]

I could be wrong, but I think AFM pin 3, the ref, comes from 9 of the DME, and AFM pin 2, the wiper is output to DME 7 (not 9). One of us may have it swapped. I am looking at the AFM with pin 1 on the right. The AFM connector may be labeled/embossed with the actual DME destination pins if I recall.

My understanding is that the pin 2 is just input to a mux on the digital board with no feedback.
In the AFM, pin 2 is the wiper and will sweep the track Jim mentioned from just above 0v to Vref (4.5) (the spec is a range) at a fully open barn door (96 degrees).

+1 on the (careful) wiper repositioning for nice results.
hth

found a pic CREDIT FR Wilk (arg - which I can't upload in EDIT mode)

[steely]

[jpnovak]

thanks guys. I realize that the AFM probably needs to have its wiper arm repositioned. That is part off the process here. The current AFM does not have any dead spots when measuring the output from the AFM.

The problem is the plug(wire) side. I measure a voltage on Pin2 on the plug, not plugged into the AFM. Since this is an input the voltage should come from the effective voltage divider inside the AFM. Therefore there should be no voltage on Pin2 of the plug.

We have tested several AFMs on the car. I suspect the motronic ECU but do not have one to swap as a test.

I took apart the ECU and did not see any broken solder connections or anything out of the ordinary for a PCB. The mica(?) insulating paper looks like it has seen better days though.

[r_towle]

Have you tested the O2 sensor if it has one?
If you have a bung for a sensor, you may want to get one installed and take some reading from the sensor while the car is warm and idling....that may show you more (mixture) results that could point out the issue.

Lastly....clean every connection...they can make troubleshooting a mess as the age (ECU connection) and get dirty.

YMMV

Rich

[Tippy]

I am selling both of my DME's today.

I would have totally let you borrow one of them to try.

[jpnovak]

There is no O2 sensor. There was a bung on the cat (added during federalization?) but it is welded shut.

I might have to weld in a new bung and plug in the LM1 to see what ails it.

Tippy, You have a PM. Thank you for the gracious offer.

Jamie

[Lorenfb]

"but I think AFM pin 3, the ref, comes from 9 of the DME, and AFM pin 2, the wiper is output to DME 7 (not 9)."

That is correct. Pin 9 of the DME ECM provides the 5 volt reference which must be
maintained as the flapper moves in the AFM. Pin 7 of the ECM should vary from
about 1 volt (idle) to 4.5 volts at the max of the flapper.

[jpnovak]

Loren, Thank you for responding.

Yes, I had pin7 and pin9 reversed. I was going from memory without the wiring diagram in front of me.

I understand that Pin 7 should have an input voltage out of the AFM from 1V to 4.5V. Basically, this is the output of the effective voltage divider that is the barn door and carbon resistor of the AFM arm.

Can you tell me what the measurements at DME Pin7 are when the AFM is NOT connected? DME Pin 9 is the reference voltage that measures 4.5 V. I am concerned that there is an internal short that is applying voltage to DME Pin7 when there should not be any voltage (it is an input).

Thanks.

[Lorenfb]

"Can you tell me what the measurements at DME Pin7 are when the AFM is NOT connected?"

It's zero volts. Seriously doubt a short unless high voltage from a a plug wire jumped into
the AFM wires.

[Lorenfb]

"Can you tell me what the measurements at DME Pin7 are when the AFM is NOT connected?"

It's zero volts. Seriously doubt a short unless high voltage from a a plug wire jumped into
the AFM wires.

[jpnovak]

Loren, Thank you for confirming that it should be 0V at pin 2 of the AFM plug (DME Pin7).

This is exactly why I suspect an issue with the ECU. I measure nominal 2V at Pin2 of the AFM plug (DME Pin7) with key-on. Something is feeding voltage into Pin7 and it should not be. Time to look more closely at the ECU.

Do you have a circuit diagram for the ECU? This will help me isolate specific segments of the ECU. I would suspect that the voltage applied to Pin7 (AFM output) should go straight to the main chip on one of its I/O pins. Does anyone have a pinout for this chip?

Thanks in advance. getting closer to finding the issue.

[Lorenfb]

"Do you have a circuit diagram for the ECU? This will help me isolate specific segments of the ECU. I would suspect that the voltage applied to Pin7 (AFM output) should go straight to the main chip on one of its I/O pins. Does anyone have a pinout for this chip?"

It's been posted many times on Pelican. Do a search.

[rick-l]

It goes into pin 26 of this part (.1uF to ground). 10 n Amps leakage but I would be hard pressed to say what this input should look like if connected to a high impedance i.e your meter.

http://www.national.com/ds/AD/ADC0808.pdf

[jpnovak]

Ok, the input goes through an ADC.

I have ways of testing the signal. Thanks Rick!

I think its time to put the ECU on the bench, power it up and start feeding it some signals to trace.

btw, the link you posted does not go through but I was easily able to find a datasheet for the National ADC0808.

[rick-l]

see if this works
DME - Schematic Download

[jpnovak]

last night I opened the ECU and completely removed the boards. The bottom of the board had some questionable corrosion. This might be the source of the unwanted voltage feedback. Its all cleaned up now.

Will try to test soon and report back.

If it still has issues then I will source an ECU to swap and/or look for a bad multiplexing chip. I also notice a limiting resistor and capacitor to ground on the input side. Will trace and check these.

[rick-l]

That resistor and capacitor is a low pass filter.

For a car that is running but just running poorly there would seem to be a lot more to check before suspecting the reliable ECU. Why do you think the mass airflow sensor is bad?

[Tippy]

One of my DME's had a bunch of corrosion from the flux. I was told it would not be a problem and cleaned it all off with alcohol and cotton swabs.

It didn't fix my issue so it most have not been a big deal after all like I was told.