1987 911 3.2 no start no spark

[porsche.racer]

Originally Posted by scarceller

One more test:

Unplug the DME harness from the DME and measure voltage at pin 4 on the DME harness. This pin MUST have 12vdc while cranking, does it go to 12vdc during cranking?

Pin 4 is tied directly to the ignition switch 'Start' it comes off the same line that feeds the starter.

If it does not then the fuel pump will never be energized.

Do this test and report result.

Pin 4 has 12v when cranking.

[porsche.racer]

Originally Posted by porsche.racer

Pin 4 has 12v when cranking.

Pin 4 has 12v

[ischmitz]

OK, as Sal says you have proper power to the DME and with the new sensors you need to move on and confirm if you have fuel and spark signals. At this point you will have to isolate whether you have a fuel delivery issue, a bad DME (no signals at all) or a bad DME (missing spark or fuel signal)

I guess you don't have the test light (yet). Without it do this:


(1) Smell the tail pipe and remember the smell. Then crank for 20 seconds and smell again. Do you note a distinct difference (smell of raw fuel)? If yes no need to do (3)

While you crank observe the tachometer needle and let us know if you see any movement. Report result.

(2) Unplug the main plug wire coming from the ignition coil at the distributor cap and hold its metal end about 10mm away from a metal surface at the engine while an assist cranks. This will tell you if you see a spark. Darkness helps. If this is positive move to step (3)

(3) Use starter fluid, break cleaner or similar and spray 10 seconds into the intake air filter, then try if it starts. If yes, you have an issue with fuel delivery (e.g. pressure regulator, filter, pump, etc).


Report back.
Ingo

[porsche.racer]

Ok guys, this is where it is.

I have no spark from the coil. I have 12v power with ignition on. This is a new tci coil Bosch. As I have the headers of I have fuel pressure and fuel coming out of the exhaust valves.

I tested 'flicker' while cranking on the coil with a multi meter and saw a 1,2,3 change in volts.

[ischmitz]

Sounds like you have fuel.

Measure resistance across the terminals of the coil - should be around 1 Ohm or similar

Then measure resistance from one terminal to the center HV contact of the coil - should be around 1K Ohm.

Report back

[porsche.racer]

Last question for the night.
When I test the coil when ignition is on I get power at both 15 and 1. 12v when I test to a separate ground on each. Is one not a ground??

[rick-l]

One side of the coil is at 12 volts and the DME completes the circuit to ground when it wants to charge the coil.


EDIT: Both sides of the coil will read 12 volts if the DME is not providing a ground.

[ischmitz]

One terminal get grounded for a very short duration (milliseconds) by the DME to charge the coil. And then the GND gets removed to release the spark. So when you measure the voltage with a DVM both terminals will show +12V.

You need an oscilloscope to show this effect.

Have you measured resistance from one terminal to the HV center contact?

[scarceller]

With key in 'RUN' the coil get's 12v at the '+' terminal but the '-' terminal will also read 12v because it's not grounded yet by the DME and is sort of left floating open. Since the coil is nothing more than a coil of wire with no current running through it it will read 12v at the '-' as well. It's normal to read 12v at both '+' and '-' at the same time with key in 'RUN' engine not running.

So what next? here goes:
1. connect a spark plug directly to the coil wire, remove the coil wire from the center distributor post and connect the plug directly to the end of that wire once removed from the distributor. Then ground the plug to a good ground point and crank engine, do you have spark?
2. if no spark, take notice if the tachometer needle is bouncing slightly while cranking. This isn't 100% accurate method but if you see it bounce then the DME is at least seeing the ref and speed sensor pulses.
3. you really now need the LED test light to place across the coil '-' and '+' and see if it flashes while cranking.
4. this one is controversial and some will say don't do it but if done very quickly you should not harm the coil. Disconnect the '-' terminal from the coil but leave the '+' connected. Turn key to 'RUN' and you will have 12vdc on the '+'. Then connect a spark plug directly to the coil (by-passing the distributor) and ground the plug. Now take a test wire and connect it to the '-' side of the coil, then very quickly touch a good ground with the other end of the wire, this grounds the '-' on the coil and the coil instantly charges. The moment the ground is removed the plug will fire. But when I say VERY quickly touch the ground with the wire I mean quickly, like poke the ground point. This coil has a charge time of about 2.2ms and if you exceed that you will super heat the coil.

[mysocal911]

Originally Posted by scarceller

With key in 'RUN' the coil get's 12v at the '+' terminal but the '-' terminal will also read 12v because it's not grounded yet by the DME and is sort of left floating open. Since the coil is nothing more than a coil of wire with no current running through it it will read 12v at the '-' as well. It's normal to read 12v at both '+' and '-' at the same time with key in 'RUN' engine not running.

So what next? here goes:
1. connect a spark plug directly to the coil wire, remove the coil wire from the center distributor post and connect the plug directly to the end of that wire once removed from the distributor. Then ground the plug to a good ground point and crank engine, do you have spark?
2. if no spark, take notice if the tachometer needle is bouncing slightly while cranking. This isn't 100% accurate method but if you see it bounce then the DME is at least seeing the ref and speed sensor pulses.

"I have no spark from the coil. I have 12v power with ignition on. This is a new tci coil Bosch. As I have the headers of I have fuel pressure and fuel coming out of the exhaust valves."

"I have no spark from the coil. I have 12v power with ignition on. This is a new tci coil Bosch. As I have the headers of I have fuel pressure and fuel coming out of the exhaust valves."
3. you really now need the LED test light to place across the coil '-' and '+' and see if it flashes while cranking.
4. this one is controversial and some will say don't do it but if done very quickly you should not harm the coil. Disconnect the '-' terminal from the coil but leave the '+' connected. Turn key to 'RUN' and you will have 12vdc on the '+'. Then connect a spark plug directly to the coil (by-passing the distributor) and ground the plug. Now take a test wire and connect it to the '-' side of the coil, then very quickly touch a good ground with the other end of the wire, this grounds the '-' on the coil and the coil instantly charges. The moment the ground is removed the plug will fire. But when I say VERY quickly touch the ground with the wire I mean quickly, like poke the ground point. This coil has a charge time of about 2.2ms and if you exceed that you will super heat the coil.

The simplest test is to just leave everything connected as normal and use a test light,
one side grounded, to check both pins of the coil while the engine is cranking.
One of the coil pins will cause the test light to flash if the DME ECM is pulsing the coil.
If the coil is being pulsed, but there's no spark the coil is bad. If the coil is not being
pulsed then the problem is related to the DME ECM, given that the fuel injectors
are proving fuel.

"I have no spark from the coil. I have 12v power with ignition on. This is a new tci coil Bosch. As I have the headers of I have fuel pressure and fuel coming out of the exhaust valves."

If the DME ECM is the problem, the most likely repair is discussed here;
911 Carrera 1987 Engine Cut Out Issue

[porsche.racer]

Update:

Still no start. I have completed every test except for the LED test light on the coil (I don't have one, will get one).

However, I did notice that my afm (maf) bracket is broken and the maf has slight physical damage due to bouncing and hitting the air intake manifold. I don't thin it's serious damage, but if it is damaged could that cause no spark no start?

[scarceller]

The AFM will not cause a no spark issue.

These DMEs are known to fail in the coil circuit because of craked solder joints. If you have another 84-89 911 test car you could take your DME and put it in the test car. If the test car does not start you have bad DME.

But NEVER put a good DME in a questionable car, don't take the DME from the good car and put it in your car. Not a good idea.

[scarceller]

Originally Posted by mysocal911

The simplest test is to just leave everything connected as normal and use a test light,
one side grounded, to check both pins of the coil while the engine is cranking.
One of the coil pins will cause the test light to flash if the DME ECM is pulsing the coil.

Don't mean no disrespect but putting one end of the test light on ground may not work for finding coil pulses. The best is to put the '+' of the test ligt on '+' of the coil then the test light '-' on coil '-' this works well with LED test light. Reason is that when a coil first start to charge it has 12vdc across it as it charges the vdc starts to reduce to 0vdc once fully charged. So when the DME commands the coil to start charging it grounds the coils '-' side and the coil has 12vdc across it and voltage starts to decrease from 12 to 0 as it charges. After 2.2ms the coil is fully charged and at 0vdc.

If however you put the test light '-' on ground and then test each post at the coil you'll find both posts at 12vdc. Try it, reason for this is that while the coil is not commanded to charge the '-' side of the coil is left floating so both sides will read 12vdc. This test approach may work but here you'll have the test light lit and then go slightly dimmer as the coil charges. Basically you'll be looking for the inverse in the light, waiting for it to go out or dim and this is harder to see than waiting for the light to light.

You can try both approaches but I preffer simply measuring the voltage across the coil it's easier to see a test light come on then go out.

[ischmitz]

I yet am waiting for the simple resistance measurement of your coil.

- between both terminals
- between one terminal and the center HV contact.

This shouldn't take more than 5 minutes and will tell if your coil is broken.

Ingo

[ischmitz]

Originally Posted by scarceller

The AFM will not cause a no spark issue.

These DMEs are known to fail in the coil circuit because of craked solder joints. If you have another 84-89 911 test car you could take your DME and put it in the test car. If the test car does not start you have bad DME.

But NEVER put a good DME in a questionable car, don't take the DME from the good car and put it in your car. Not a good idea.

Sal, I have seen shorted fuel drivers that dump raw fuel into the engine once the key is turned on. And shorted ignition drivers that drive coils to solid GND. Maybe a good idea to test for that before putting a questionable DME into a good car.

I think as a rule of thumb either swap can cause harm if you keep trying for a long time. I usually advice to do the test real quick. If it works - great. If not stop and turn ignition off so you don't loose more parts in the process.

Ingo

[scarceller]

Ingo,

Good points, either of these shorted condition could cause harm, especially a boat load of fuel into the intake, would not take much to hydro lock our small cylinders.

BTW - I have been repairing more and more broken solder joints on the Coil transistor these days.

You can also bench test the coil with a 12v supply (spare car battery), a 10ohm wire wound ballast resistor, short plug wire and a spark plug. It's a simple bench test.

Originally Posted by ischmitz

Sal, I have seen shorted fuel drivers that dump raw fuel into the engine once the key is turned on. And shorted ignition drivers that drive coils to solid GND. Maybe a good idea to test for that before putting a questionable DME into a good car.

I think as a rule of thumb either swap can cause harm if you keep trying for a long time. I usually advice to do the test real quick. If it works - great. If not stop and turn ignition off so you don't loose more parts in the process.

Ingo

[mysocal911]

Originally Posted by scarceller

Don't mean no disrespect but putting one end of the test light on ground may not work for finding coil pulses. The best is to put the '+' of the test ligt on '+' of the coil then the test light '-' on coil '-' this works well with LED test light. Reason is that when a coil first start to charge it has 12vdc across it as it charges the vdc starts to reduce to 0vdc once fully charged. So when the DME commands the coil to start charging it grounds the coils '-' side and the coil has 12vdc across it and voltage starts to decrease from 12 to 0 as it charges. After 2.2ms the coil is fully charged and at 0vdc.

If however you put the test light '-' on ground and then test each post at the coil you'll find both posts at 12vdc. Try it, reason for this is that while the coil is not commanded to charge the '-' side of the coil is left floating so both sides will read 12vdc. This test approach may work but here you'll have the test light lit and then go slightly dimmer as the coil charges. Basically you'll be looking for the inverse in the light, waiting for it to go out or dim and this is harder to see than waiting for the light to light.

You can try both approaches but I preffer simply measuring the voltage across the coil it's easier to see a test light come on then go out.

1. Who said to use a LED test light?
2. When using an incandescent test light that test works perfectly.
3. What do mean try it? I've done that for years, i.e. grounding the test light. You need to try it!

"Reason is that when a coil first start to charge it has 12vdc across it as it charges the vdc starts to reduce to 0vdc once fully charged.
So when the DME commands the coil to start charging it grounds the coils '-' side and the coil has 12vdc across it and voltage starts to
decrease from 12 to 0 as it charges."

That statement is not correct. The coil does not saturate (zero volts across it), as the coil driver is switched off before saturation.
Besides once the coil driver turns off the coil voltage rises to about 350 volts on the primary which is more than enough voltage
to flash an incandescent test light.

A coil does not function like a resistor, although it does have a very low resistance, i.e. the voltage does not vary as the current
changes, only when it reaches saturation. Although it's series resistance does have a small effect on the voltage of the coil,
the rate of change of the coil current determines the coil voltage. Time to put an o-scope on the coil signal. The DME ECM
calculates the charge time (dwell time) of the coil driver.

Voltage (coil) = L (inductance) X d I (change of current) / d T (change of time)

[rick-l]

I am staying out of the light debate because so much depends on the actual device you select. One relies on the 50 volts across the coil for a millisecond during the burn time and the other relies on the light going out during the 3 millisecond coil charge time.

The DME doesn't control dwell time to charge the coil it just provides enough time for the differential pair amplifier that drives the big darlington transistor to limit the current to 8 amps. Loren told me it didn't work this way. He must not have understood the circuit.

[mysocal911]

"The DME doesn't control dwell time to charge the coil it just provides enough time for the differential pair amplifier that drives the big darlington transistor to limit the current to 8 amps."

The statement is contradictory!

[rick-l]

Dwell angle in a points car set the charge in the coil and varied with rpm.

The DME sets the current to 8 amps for every spark so dwell time has nothing to do with the energy in the spark.