No fire - where to start ?

[Tori]

Car has sat for a few months,... Went to start it today and am getting no spark.

I've had a few random issues in the past where it would stall on me mid drive, sometimes restarting right away, sometimes i plug and unplug the DME, or swap it. Seemed to be when on a rough road or after traveling on one.

Luckily now it's currently broken while in the garage so maybe i can finally find this intermittent problem.

So far i've swapped 3 different DME relay's, and swapped the computer chip. Wiggled all connectors and inspected those of the DME relay and Computer.

Where should i go next ?

UPDATE: CAR HAS BEEN FIXED WITH A REPAIRED DME

[mikesarge]

Just the basics- fuel and spark.

Turn the key- do you hear a fuel pump? When you crank, is there spark? Just go on down the line

[Dave Colangelo]

Could be the crank sensor. You can check the internal resistance on the sensor with a multi-meter (1K Ohms maybe(?) dont quote me on that reading).

What do you mean swapped the "computer chip", the fuel map/tune chip?

I had a 924S which had the same Bosch Motronic ignition system (same ECU basically) that had the transistor on the low side of the coil go and lead to a no spark situation. you wont see this as a burnt component or anything it was just a dead transistor. Electrical components do have a cycle limit and don't last forever. You will need an oscilloscope to diagnose correctly but if memory serves you should always have +12V on the (+) side of the coil and the pulse signal will be from the (-) side allowing it to ground basically. A scope should allow you to see the square wave when cranking to see if the ECU is pulsing the coil correctly. I had the ECU rebuilt by an outfit in Florida at the time (not sure if they are still around). If you have a buddy with a similar car, swap ECU's and test.

Im older and wiser now and could likely desolder the part and match it with some equivalent from digikey but thats not for everyone. The ECU is fairly easy to open up and the boards are 80s style through hole so easy to repair if need be.

Make sure all your grounding points are corrosion free and providing good continuity. Can be a common electrical issue on these cars that causes intermittent problems, loss nuts on the screws can also cause shaky connections.

Regards
Dave

[mysocal911]

Originally Posted by Dave Colangelo

Could be the crank sensor. You can check the internal resistance on the sensor with a multi-meter (1K Ohms maybe(?) dont quote me on that reading).

What do you mean swapped the "computer chip", the fuel map/tune chip?

I had a 924S which had the same Bosch Motronic ignition system (same ECU basically) that had the transistor on the low side of the coil go and lead to a no spark situation. you wont see this as a burnt component or anything it was just a dead transistor. Electrical components do have a cycle limit and don't last forever. You will need an oscilloscope to diagnose correctly but if memory serves you should always have +12V on the (+) side of the coil and the pulse signal will be from the (-) side allowing it to ground basically. A scope should allow you to see the square wave when cranking to see if the ECU is pulsing the coil correctly. I had the ECU rebuilt by an outfit in Florida at the time (not sure if they are still around). If you have a buddy with a similar car, swap ECU's and test.

Im older and wiser now and could likely desolder the part and match it with some equivalent from digikey but thats not for everyone. The ECU is fairly easy to open up and the boards are 80s style through hole so easy to repair if need be.

Make sure all your grounding points are corrosion free and providing good continuity. Can be a common electrical issue on these cars that causes intermittent problems, loss nuts on the screws can also cause shaky connections.

Regards
Dave

The coil driver transistor rarely fails! Most likely it's a circuit board intermittency.
Check with this SoCal company; https://www.systemsc.com/products.htm

[PeteKz]

How do you know its loss of spark? How did you test for it?

When dealing with no-fire problems, I usually first test by spraying starting fluid in the intake. If it fires, then the engine is missing fuel. If it doesn't fire then you are most likely missing spark (it's also possible that the engine is flooded). It's a cheap and fast test. Try it.

[Tori]

Originally Posted by mikesarge

Just the basics- fuel and spark.

Turn the key- do you hear a fuel pump? When you crank, is there spark? Just go on down the line

I said I don’t have spark. ???

[Tori]

Originally Posted by PeteKz

How do you know its loss of spark? How did you test for it?

When dealing with no-fire problems, I usually first test by spraying starting fluid in the intake. If it fires, then the engine is missing fuel. If it doesn't fire then you are most likely missing spark (it's also possible that the engine is flooded). It's a cheap and fast test. Try it.

I have an in-line spark tester. Goes in-line of a plug wire.

[Tori]

Originally Posted by Dave Colangelo

Could be the crank sensor. You can check the internal resistance on the sensor with a multi-meter (1K Ohms maybe(?) dont quote me on that reading).

What do you mean swapped the "computer chip", the fuel map/tune chip?

I had a 924S which had the same Bosch Motronic ignition system (same ECU basically) that had the transistor on the low side of the coil go and lead to a no spark situation. you wont see this as a burnt component or anything it was just a dead transistor. Electrical components do have a cycle limit and don't last forever. You will need an oscilloscope to diagnose correctly but if memory serves you should always have +12V on the (+) side of the coil and the pulse signal will be from the (-) side allowing it to ground basically. A scope should allow you to see the square wave when cranking to see if the ECU is pulsing the coil correctly. I had the ECU rebuilt by an outfit in Florida at the time (not sure if they are still around). If you have a buddy with a similar car, swap ECU's and test.

Im older and wiser now and could likely desolder the part and match it with some equivalent from digikey but thats not for everyone. The ECU is fairly easy to open up and the boards are 80s style through hole so easy to repair if need be.

Make sure all your grounding points are corrosion free and providing good continuity. Can be a common electrical issue on these cars that causes intermittent problems, loss nuts on the screws can also cause shaky connections.

Regards
Dave

Yes, the chip in the ECU. The one everyone swaps for a Steve Wong Chip.
Already checked the main ground- body to trans.

[wazzz]

To make it clear, the DME is actually the ECU. When you say you swapped the DME, I guess you mean the DME relay. If you have no spark, you don't need to try and swap the DME relay, since it has no relation with the ignition.
You need to check the basics. First make sure the ECU is powered on when the ignition is switched on. You can check this by listening in the engine compartment. You should hear the idle control valve (ICV) humming. If not that means the ECU has no power or is completely dead.

Next, if the ECU is indeed alive, you need to check if you have fuel.
If you have no spark, maybe you have no fuel either. In this case it would point to a failing sensor (maybe). The ECU won't allow any spark or fuel if it doesn't get proper signals from both flywheel sensors at startup. No spark no fuel is typical of a failing sensor or a failure of the IC inside the ECU that is used to format these sensor signals for the processor.

[ant7]

Originally Posted by wazzz

To make it clear, the DME is actually the ECU. When you say you swapped the DME, I guess you mean the DME relay. If you have no spark, you don't need to try and swap the DME relay, since it has no relation with the ignition.
You need to check the basics. First make sure the ECU is powered on when the ignition is switched on. You can check this by listening in the engine compartment. You should hear the idle control valve (ICV) humming. If not that means the ECU has no power or is completely dead.

Next, if the ECU is indeed alive, you need to check if you have fuel.
If you have no spark, maybe you have no fuel either. In this case it would point to a failing sensor (maybe). The ECU won't allow any spark or fuel if it doesn't get proper signals from both flywheel sensors at startup. No spark no fuel is typical of a failing sensor or a failure of the IC inside the ECU that is used to format these sensor signals for the processor.

[mysocal911]

Check here for troubleshooting guidance;

http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

It's that simple!

[Tori]

Originally Posted by wazzz

To make it clear, the DME is actually the ECU. When you say you swapped the DME, I guess you mean the DME relay. If you have no spark, you don't need to try and swap the DME relay, since it has no relation with the ignition.
You need to check the basics. First make sure the ECU is powered on when the ignition is switched on. You can check this by listening in the engine compartment. You should hear the idle control valve (ICV) humming. If not that means the ECU has no power or is completely dead.

Next, if the ECU is indeed alive, you need to check if you have fuel.
If you have no spark, maybe you have no fuel either. In this case it would point to a failing sensor (maybe). The ECU won't allow any spark or fuel if it doesn't get proper signals from both flywheel sensors at startup. No spark no fuel is typical of a failing sensor or a failure of the IC inside the ECU that is used to format these sensor signals for the processor.

Thank you for the correction of DME v. ECU. Yes, i did mean DME relay.
I'll confirm my ICV is humming, and continue. I know i have fuel as i could smell it.

While researching where and how to get to the crank sensor, i found a real good thread that talks about a common failure point inside the DMV - power resistors that either fail or their solder joints fail. I'll get a look at those this coming weekend when i can get the car down from the lift again.
Cheers for now....

[Tori]

Originally Posted by mysocal911

Check here for troubleshooting guidance;

http://forums.pelicanparts.com/porsche-911-technical-forum/621262-911-3-2-no-start-troubleshooting.html

It's that simple!

oooooh, that's perfect. Thank you.

[mysocal911]

Originally Posted by Tori

Thank you for the correction of DME v. ECU. Yes, i did mean DME relay.
I'll confirm my ICV is humming, and continue. I know i have fuel as i could smell it.

While researching where and how to get to the crank sensor, i found a real good thread that talks about a common failure point inside the DMV - power resistors that either fail or their solder joints fail. I'll get a look at those this coming weekend when i can get the car down from the lift again.
Cheers for now....

Who said that? That's NOT the case for the 911 3.2 DME ECM ('84-'89). Probably YouTube DIY "experts".

[Tori]

Originally Posted by mysocal911

Who said that? That's NOT the case for the 911 3.2 DME ECM ('84-'89). Probably YouTube DIY "experts".

This is what i was reading and referring to

https://forums.pelicanparts.com/porsche-911-technical-forum/667659-replacing-3-2-reference-senders-engine-car.html

[mysocal911]

Originally Posted by Tori

This is what i was reading and referring to

https://forums.pelicanparts.com/porsche-911-technical-forum/667659-replacing-3-2-reference-senders-engine-car.html

Yes, that's a very rare failure, as explained in post #28.

[ant7]

This might be a daft question but; have you checked the rotor arm in the distributor ?
Reason being, if it has the inbuilt resistor type, it may have become faulty,[ I had one go down in my 3.2] just a thought, and an easy one to try, just swap it out for a new one.
Ant.

[Tori]

I have not,... didn't know the rotor was made that way.... Thanks for the tip

[ischmitz]

If it was me I'd get an LED test light, a NOID light, or better yet a low-cost oscilloscope for under 30$.

It will tell you if you get spark and fuel pulses from the DME (ECU) and if not can help diagnose bad flywheel sensors when both are absent. If only one or the other is missing just work down the path where you verify the DME signal is present, then check the components downstream:

no fuel: is there fuel pressure, is the injector stuck
no spark: is the coil good, is the the distributor working

As others said the 3.2 Motronic is relatively simple to troubleshoot once you understand the relation of components and there are many helpful articles.

Curious what you find

[Schulisco]

If it's neither the ECU/DME or one of both flywheel sensors take the ignition coil also into concern. They die slowly due to heat and / or vibrations, mostly on CIS cars with the CDI as the coil is mounted on top of the cooling fan case. But I also have few credible reports from dying ignition coils on 3.2 Carreras too!
The behaviour of the car with some stalling issues may fit pretty well in this manner.

But also broken soldering points on the ECU board after decades of working can cause exactly the same behaviour.

In case of the coil the test is pretty easy: Get a replacement from any car with a single ignition coil with transistor igntition for testing purposesand see if the stalling engine persists...

Last but not least: Use the three wire bridge to rule out the DME relay as this may also fail causing a bad start and/or stalling issues:
https://forums.pelicanparts.com/porsche-911-technical-forum/1031064-84-carrera-non-starting.html#post10482778
https://dorkiphus.net/porsche/showthread.php?t=16859

Thomas