Very rough idle/Accelerating after DME harness cut..

[Mony]

Just got the 1986 911 3.2 registered 3 weeks ago and been driving the car daily all this time. You see my legs are really tall, So I wanted to retract the seat all the way back more than the stock position to sit comfortably. So told my fabricator to remove the seat and find a way to position the seat all the way backwards.

He did that all well, though he drilled right through the DME wiring harness.. Well, I did not know that. I started the car and suddenly a very very rough idle. And a tach that is not working. I was confused, just one hour ago the was very smooth. I thought the car was low on fuel, I refueled it. Still very rough, the car would just rev and rev then the rpm would go down to almost stall. One minute and then the car would give up and stall.

I thought my Idle control valve was seized, removed it and cleaned it, checked the three pins if it was closing and opening all was working. Then I moved to the Idle micro switch, I can hear the button clicking once the throttle is not moving and at "idle."

Then I ruled out the idle because when I drive the car and row throw the gears, the car basically hesitates at all speeds. Every three seconds there is a violent jerk like someone just hit the brakes. Rpms gos up easily when I apply the throttle but the hiccup is basically too violent to be fuel related issue. I noticed the half eaten wiring harness and I fixed all the wires individually and was confident that was my problem. The Tachometer was fixed but still the same rough idle/engine.

After that I was really confused, I fixed my messed up wiring harness and changed my DME relay to a spare relay I had. The problem was not fixed.. What could happen if the wiring harness shorted? What fuses/sensors could be burned?

I basically checked all these sensors and still I could figure out my hesitating engine:
- Checked both speed/reference sensors both has about 1153 ohms and looking good.
- Disconnected oxygen sensor and reconnected no difference.
- Removed the idle control valve and cleaned it.
- Tested the micro switch didn't see any difference.
- Changed DME relay
- Fuse box looks okay all new fuses

Now I removing the Actual DME computer and I am trying to open it and see inspect visually to see if there is cracked joints or anything.

So what could be my problem here? was my car running just find before the fabricator drilled through the harness. Fixed my wiring and still the same. Is it possible that the same time that this happened other stuff broke in the process? The only two possible culprits would be the AFM sensor or a sudden vacuum leak. How do I check AFM sensor?

I am literary stuck here.. Next week there is a track day and I wasn't expecting this sudden fallout.

Thanks for the help in advance, heres a coupe of pictures.

[86 911 Targa]

Originally Posted by Mony

Just got the 1986 911 3.2 registered 3 weeks ago and been driving the car daily all this time. You see my legs are really tall, So I wanted to retract the seat all the way back more than the stock position to sit comfortably. So told my fabricator to remove the seat and find a way to position the seat all the way backwards.

He did that all well, though he drilled right through the DME wiring harness.. Well, I did not know that. I started the car and suddenly a very very rough idle. And a tach that is not working. I was confused, just one hour ago the was very smooth. I thought the car was low on fuel, I refueled it. Still very rough, the car would just rev and rev then the rpm would go down to almost stall. One minute and then the car would give up and stall.

I thought my Idle control valve was seized, removed it and cleaned it, checked the three pins if it was closing and opening all was working. Then I moved to the Idle micro switch, I can hear the button clicking once the throttle is not moving and at "idle."

Then I ruled out the idle because when I drive the car and row throw the gears, the car basically hesitates at all speeds. Every three seconds there is a violent jerk like someone just hit the brakes. Rpms gos up easily when I apply the throttle but the hiccup is basically too violent to be fuel related issue. I noticed the half eaten wiring harness and I fixed all the wires individually and was confident that was my problem. The Tachometer was fixed but still the same rough idle/engine.

After that I was really confused, I fixed my messed up wiring harness and changed my DME relay to a spare relay I had. The problem was not fixed.. What could happen if the wiring harness shorted? What fuses/sensors could be burned?

I basically checked all these sensors and still I could figure out my hesitating engine:
- Checked both speed/reference sensors both has about 1153 ohms and looking good.
- Disconnected oxygen sensor and reconnected no difference.
- Removed the idle control valve and cleaned it.
- Tested the micro switch didn't see any difference.
- Changed DME relay
- Fuse box looks okay all new fuses

Now I removing the Actual DME computer and I am trying to open it and see inspect visually to see if there is cracked joints or anything.

So what could be my problem here? was my car running just find before the fabricator drilled through the harness. Fixed my wiring and still the same. Is it possible that the same time that this happened other stuff broke in the process? The only two possible culprits would be the AFM sensor or a sudden vacuum leak. How do I check AFM sensor?

I am literary stuck here.. Next week there is a track day and I wasn't expecting this sudden fallout.

Thanks for the help in advance, heres a coupe of pictures.

Which wires were damaged?

I have the factory manual.

[janessa642]

Originally Posted by 86 911 Targa

Which wires were damaged?

I have the factory manual.

cant you see if you have factory manual???

[Mony]

Originally Posted by 86 911 Targa

Which wires were damaged?

I have the factory manual.

I do not remember which two wires exactly but I will check today.

You reckon maybe if it was a speed sensor wire, the sensor would give up and short?

[DanielDudley]

How exactly did you fix your wiring ? Is it possible that he drilled it twice ?

[ischmitz]

Is the ICV vibrating with the ignition ON?
What about the CHT, what happens when you unplug it during idle?
What happens when you unplug the AFM during idle (IAT and AFM signal)
Does the note change when the micro switch is closed or opened?
(and the list goes on).....

I would get a pin-out of the 35-pin connector and then trace and measure from each pin of the connector to the respective aggregate connector in the engine bay. Check signals to the ICV, CHT, AFM (including IAT), speed sensor, reference sensor, and idle switch. If one of these wires is compromised you will have poor running.

Then if all that checks out it's time to get the DME checked out. Depending on what was shorted in the harness you could have damaged one of the input or output channels in the process.

Ingo

[Mony]

So I took you advice and I gathered a lot of info from removing/plugin connectors but not sure how to decipher it.

Originally Posted by ischmitz

Is the ICV vibrating with the ignition ON?

Yes when ignition on it vibrates softly.
----------



So I am basing these tests in these three connectors:
1- Cylinder head temp
2- Speed sensor
3- Reference sensor
Am I right?

Originally Posted by ischmitz

What about the CHT, what happens when you unplug it during idle?

As soon as I unplug it, the car runs like crazy and just revs really high.

Originally Posted by ischmitz

What happens when you unplug the AFM during idle (IAT and AFM signal)

The car becomes more sloppy and runs rougher, escalating between 1500 rpms and 500 rps at idle. When I plug it back in, its a bit smoother bit still rough and sloppy.

So when I unplug it car runs sloppier.

Originally Posted by ischmitz

Does the note change when the micro switch is closed or opened?

Yes when I apply little bit of throttle and switch opens the cars notes and and revs a tad bit higher. when switch is pressed it becomes calmer.


Two more things:
1- When I unplug the speed sensor the car dies instantly, is that normal?
2- When I unplug the reference sensor there is no difference at all. The car idle and roughness does not change at all.
Do I have a bad reference sensor? The reference sensor is used for starting the car. But is it used when the car is running? I had a GMC that had a bad crank sensor that had similar issues to the Porsche. Did I identify a broken sensor?

Originally Posted by ischmitz

(and the list goes on).....

Any more tests you can tell me I can do and gather more info?

[Mony]

Originally Posted by DanielDudley

How exactly did you fix your wiring ? Is it possible that he drilled it twice ?

Just checked, he drilled once only..

[mysocal911]

Originally Posted by Mony

2- When I unplug the reference sensor there is no difference at all. The car idle and roughness does not change at all.
Do I have a bad reference sensor? The reference sensor is used for starting the car. But is it used when the car is running?

Yes, once the engine is running, the reference sensor is no longer needed to develop the timing.
What you just did is a good test for the speed sensor, i.e. if revving the engine without the reference
sensor results in normal running and doesn't cause the engine to die, the speed sensor is most likely OK.

[pmax]

Originally Posted by Mony

I noticed the half eaten wiring harness and I fixed all the wires individually and was confident that was my problem.

I would go back to this .... check which wires were damaged and post pics of how you fixed them.

[Mony]

Originally Posted by mysocal911

What you just did is a good test for the speed sensor, i.e. if revving the engine without the reference sensor results in normal running and doesn't cause the engine to die, the speed sensor is most likely OK.

Can you please elaborate? If the engine revs and doesn't die that mean that the speed sensor is ok?

[Mony]

Originally Posted by pmax

I would go back to this .... check which wires were damaged and post pics of how you fixed them.

Yeah gonna do that now and double check my work.

[mysocal911]

Originally Posted by Mony

Can you please elaborate? If the engine revs and doesn't die that mean that the speed sensor is ok?

That indicates that the speed sensor is providing a good signal, not intermittent, and the
DME ECM is not losing the timing count, i.e. having to use the reference sensor to determine
TDC. If the speed sensor is flaky, revving the engine without the reference sensor connected
will cause the engine to die.

[86 911 Targa]

Originally Posted by janessa642

cant you see if you have factory manual???

No-Not on the pictures.

And, please, no snide remarks.

[Mony]

Originally Posted by mysocal911

That indicates that the speed sensor is providing a good signal, not intermittent, and the
DME ECM is not losing the timing count, i.e. having to use the reference sensor to determine
TDC. If the speed sensor is flaky, revving the engine without the reference sensor connected
will cause the engine to die.

Thanks for clarifying that. Just tested it and it seems like my speed sensor is ok. Disconnected my reference sensor and the engine didn't die and kept idling.

One more thing I noticed If I may ask you guys, when the car is on the Idle valve controller is vibrating. not ignition, but when the car is running the and at idle, the idle valve controller is vibrating softly. is that normal?

[Mony]

So this is my wiring. It seems to be coming from the front (Fuse box?) to the dme relay. not the harness. I was wrong on that.

-(Top) 2 wires where cut completely: 2 black wires, one with a violet stripe and one fully black. ( Don't mind the red wires, the wires were cut and I couldn't join them without adding some extra wire from outside the car, so the top two are BLACK and not red).

- (Bottom) 3 wires where wounded: One black, one red with a yellow/green stripe, one fully red.

So in total of 5 wires were damaged. Could someone help identify these 5 wires?
All fuses in the fuse box are ok.







Seem that couple of red wires join into one here? Factory or someone else played with the wires right before the dme relay before me?

[Mony]

Thanks for the help in advance guys..

[Jeff Burger]

did you check to make sure none of the pins in the motronic control unit and connector did not get damaged by the same tech?

[ischmitz]

If the black 0.75mm wire was shorted to one of the red wires for a prolonged time you have likely damaged your DME. Specifically the fuel injector driver. As a result your mixture is off. The black wire is the shift light output and extremely sensitive to feeding in an external signal.

I recommend getting your DME checked out or doing a swap test.

[86 911 Targa]

^^^^^
Hope this helps.

Best,

Gerry