No spark, No tach bounce, 2 new sensors

[Outback Porsche]

Originally Posted by Outback Porsche

If you want to check your coil, could could try this;

This is the disclaimer part. There is some risk here of being electrocuted by the back EMF (Electro Magnetic Force) of the coil. Care need to be taken (But it does make a funny story to tell your mates if you do electrocute yourself and any spectators will get a good laugh). Insulated pliers will help.

Rotate the engine so that you have electrical continuity through the rotor to a spark plug and remove the spark plug. Disconnect the wire on the coil from the DME (the green one). Whilst grounding the spark plug to the engine, tap the DME terminal on the coil with a flying lead that you have connected to ground. You should see a strong spark on the plug.

Cheers,

Forgot to mention, turn the ignition on.

[Big40ozs]

I updated the first post with my latest diagnosis.

[hpservertech]

It would be better if you just added another entry to where your at. That way we can follow the history.

How do you know the DME is good? Who rebuilt it? What was bad with it?

My issue was a bad dme. Replaced it with a known good one and I was good to go, then I sold the car.

[Outback Porsche]

Originally Posted by Big40ozs

Used volt meter and found green coil signal wire has ONLY 0.5 volts. (This is supposed to be near 12v right?)

Follow this up. This sound like the key to unlocking your mystery.

When then engine is not cranking, this should be 12V. When it is cranking the DME should pulse this to ground. If this wire is permanently at 0.5V, it's a problem.

The Coil could be bad, or
look for a short to ground on this wire (Remove the DME so it doesn't get spiked while fault finding).

Cheers

[Slam]

Ignition switch? Cheap and cheerful.

[Outback Porsche]

Originally Posted by Slam

Ignition switch? Cheap and cheerful.

Not a bad idea if it hasn't been changed recently. They are cheap and they fail often. A lot of current passes through this switch when you start up. I recommend changing this ever couple of years. Probably a good thing schedule with the belts.

But I don't believe this is your problem. The 12V to the coil passes through this switch, not the coil signal. You have the 12V at your coil, so it's probably OK. Change it out if you want (it won't hurt), but hunt for the short on the green wire.

Cheers,
Jeff

[mikepellegrini]

Diagnosing no spark conditions are tedious and maddening. I feel your pain. You just gotta be methodical and stick with it and eventually you'll figure it out.

You replaced both sensors - did you move the sensor bracket at all? Is there a chance they're gapped wrong? That'd do it.

[Big40ozs]

Originally Posted by hpservertech

It would be better if you just added another entry to where your at. That way we can follow the history.

How do you know the DME is good? Who rebuilt it? What was bad with it?

My issue was a bad dme. Replaced it with a known good one and I was good to go, then I sold the car.

I'll add my updates to new entries, thx for the tip.
I took the DME to a company to have it checked.
They replaced the A/D converter, which processes the Ref & Speed signals.

Originally Posted by Outback Porsche

Follow this up. This sound like the key to unlocking your mystery.

When then engine is not cranking, this should be 12V. When it is cranking the DME should pulse this to ground. If this wire is permanently at 0.5V, it's a problem.

The Coil could be bad, or
look for a short to ground on this wire (Remove the DME so it doesn't get spiked while fault finding).

Cheers

Just to clarify, I disconnected the green wire and checked it with a voltmeter to ground with someone cranking the car. It was showing 0.5 volts.
If you chk the coil, both terminals will show 12V due to continuity between the internal coils during cranking.

This signal wire is part of the problem, may point to a faulty DME. Or a short.

Next I will check the signal using an oscilloscope, just to see if it is flat, or if I really have some type of pulsed signal.

Also contacted the company that rebuilt my DME, told me to chk for injector pulse.
I will chk this to confirm if that circuit of the DME is working, but I am concerned more with Spark right now.

[Outback Porsche]

Originally Posted by Big40ozs

Just to clarify, I disconnected the green wire and checked it with a voltmeter to ground with someone cranking the car. It was showing 0.5 volts.
If you chk the coil, both terminals will show 12V due to continuity between the internal coils during cranking.

This signal wire is part of the problem, may point to a faulty DME. Or a short.

Next I will check the signal using an oscilloscope, just to see if it is flat, or if I really have some type of pulsed signal.

Sorry, I misunderstood that you had 0.5V with the coil in circuit.

Removing the green wire and checking for a pulse will not give you a conclusive result. The Darlington driver (transistor) in the DME, that outputs the pulse signal requires the 12V from the coil to be in circuit for it to work.

Good idea to get an oscilloscope to look for the signal with the green wire connected to the coil. A quick and nasty way to see if you may have a signal is to check it with a DVM across the coil. The pulses will be too quick for the meter to measure, but the meter should average the measurement or flicker slightly lower than 12V. If you can easily get an oscilloscope then it is the better option.

You can use a Noid light to check the injector pulses, but if you don't have any tach bounce then you won't have any injector pulses either.

You may still have issues with your speed sensor. while you have the o-scope, check the speed signal at the DME plug.

Cheers

[Big40ozs]

The car now runs great.

Summary of what I did recently AFTER confirming the ignition switch works, the DME/engine wiring harness is ok, and the coil works.

I checked the output for my new ref and speed sensors at the harness using an oscilloscope.
The pictures are linked below for everyone's reference, the output was strong.
I checked for coil pulse and had none.
I checked for injector pulse and had none.
I checked for tach bounce and had none.
My DME harness was getting 12 volts, and had good grounds.

So basically my DME inputs were great, and I had no outputs at all.
Contacted the ECU specialist and had them check the DME again, and it was fried. They gave me a replacement, and the car started.

So overall I have replaced the original ref and speed sensor.
I also replaced the DME relay.
Had the DME rebuilt. (Company was great to work with, took the unit back and gave me another working one. They have a 5 yr warranty on their work, it came in handy.)

[Big40ozs]

Thanks to everyone that posted and to the forum for all your help.
I am very happy to have the car running, now I can enjoy it a bit before I start work on my steering issue, and sloppy shifter.

[earlr85944]

Congrats on getting it going!
Get a good used DME and use it for troubleshootingt in case you have a no start, saves alot of headaches.

[Roger 911]

Resurrecting an old thread here. We had an intermittent stall/no start issue with our '86 951 that we traced to the speed ref sensor, right near the plug. The wires were cracked and frayed. If you wiggled it, you could get it to start. If it was running and you wiggled it, you could get it to stall. So, I bought the reference sensor replacement harness (for both sensors). The sensors had already been replaced. I installed this per the instructions and now, no start. No tach bounce either. I went back and verified all the connections. Everything is correct. I verified continuity from the sensor plugs to the DME pins. All good.

The car ran great before the harness install. tI started it a couple hours ago to move it to a different staall in the garage. Started, ran fine, then I shut it off by wiggling the speed sensor wire. This would seem to confirm for me that all other sytems should be OK. I have to believe that it is related to the wiring of the two reference sensors, but I can't imagine what it is. And essentially everything electrical that could be replaced on this car, has been replaced or rebuilt over the last couple of years (DME, all sensors, AFM, relays, wires, coil, fuel pump, etc., etc,). Baffling.

Roger

[mhariush]

Check and clean grounds. The battery ground on bell house and the sensor ground on engine crank case. Those made my S not give a spark.

Does your car have factory alarm? If so, try to bypass it if it doesn't have the 928 part number. See Clark's Garage. This made my S not give spark as well.

Good luck.

Note: I see that you did get it running, after I posted. Still stand by my two check points.

[Bradical]

When you say "verified connections" you mean that you verified that the sensors were connected to the correct plugs, right? I know you did, but for the conversations sake or anyone reading who doesnt know, the symptoms shown here can be caused by the sensors being swapped- the sensors are indentical in form but not function.

[Roger 911]

Originally Posted by Bradical

When you say "verified connections" you mean that you verified that the sensors were connected to the correct plugs, right? I know you did, but for the conversations sake or anyone reading who doesnt know, the symptoms shown here can be caused by the sensors being swapped- the sensors are indentical in form but not function.

Yes. I verified not only that the sensors were plugged in to the right plug, but verified that each wire was in it's correct place from the plug down to the DME. I did this to make sure that the new wiring harness wasn't mislabeled.

Roger

[Roger 911]

Originally Posted by mhariush

Check and clean grounds. The battery ground on bell house and the sensor ground on engine crank case. Those made my S not give a spark.

Does your car have factory alarm? If so, try to bypass it if it doesn't have the 928 part number. See Clark's Garage. This made my S not give spark as well.

Good luck.

Note: I see that you did get it running, after I posted. Still stand by my two check points.

I know grounds are a common problem, but it was running a couple hours earlier, with only the aforementioned frayed reference sensor wire as an issue. It seems odd that replacing the sensor wires would trigger a ground issue.

No alarm.

Roger

[fiily]

Originally Posted by Bradical

When you say "verified connections" you mean that you verified that the sensors were connected to the correct plugs, right? I know you did, but for the conversations sake or anyone reading who doesnt know, the symptoms shown here can be caused by the sensors being swapped- the sensors are indentical in form but not function.

Sorry about the Thread-jacking


So correct me if I am wrong in my thinking. I can use 2 identical reference or speed sensors and install them(1 as a reference/1 as a speed sensor in their respective location in the reference/speed sensor bracket), and the DME would use these identical reference or speed sensors accordingly to their respective plugged in connection to the DME harness?

Or like you mentioned, the form might be the same, but they are totally different functioning products.

[Roger 911]

Originally Posted by fiily

Sorry about the Thread-jacking


So correct me if I am wrong in my thinking. I can use 2 identical reference or speed sensors and install them(1 as a reference/1 as a speed sensor in their respective location in the reference/speed sensor bracket), and the DME would use these identical reference or speed sensors accordingly to their respective plugged in connection to the DME harness?

Or like you mentioned, the form might be the same, but they are totally different functioning products.

Yes, the sensors are the same part #. They perform different functions. Obviously, if the wrong plug is attached to a sensor (speed to position, or position to speed), it won't work.

Roger

[earlr85944]

I didn't read the whole thread but double check the sensor gap, I had an intermittent no start when it was off a hair