Porsche 924s 1987 no injector pulse

[alexjames]

Hi I own a Porsche 924s 1987 that's automatic. I have done quite a bit of work on it replaced all the sensors on the engine block including speed and reference sensors and gapped the spacing between them and the fly wheel. I have sent the DME in and got it refurbished. I also replaced the engine and the exterior wiring harness that goes to the lights and signal lights etc. However my car cranks and will turn over if I spray starting fluid into the car but will not continue to keep running. I checked injectors with noid light and I'm getting no signal. I have 12V on both sides of injectors which is normal because the DME grounds one side to complete the circuit. The speed and reference sensors are good. Any help would be appreciated. Thanks Alex

[DannoXYZ]

Disconnect all injectors and test for voltage on injector connectors again.

[alexjames]

Originally Posted by DannoXYZ

Disconnect all injectors and test for voltage on injector connectors again.

I disconnected all the connectors and get 1.35v on one side of the connector for all four injectors. More so now that I crank the vehicle I get no tach bounce I’ve changed nothing since yesterday. And there are new speed and reference sensors in?!?!?

[944 Ecology]

There are two possibilities:

1) The speed and the reference sensors are swapped. Not an uncommon error.

2) The distance from the end of the speed sensor to the starter ring gear is not 0.8mm (.032 inch). Regap the sensors (I use a piece of cardboard that is .032, so that I do not have to disturb the measurement, once made.)

[Jfrahm]

I did not see you say you'd replaced the DME relay (a common failure point) or at least tried the DME relay jumper wire:
DME Relay Information and Testing

-Joel.

[alexjames]

Originally Posted by 944 Ecology

There are two possibilities:

1) The speed and the reference sensors are swapped. Not an uncommon error.

2) The distance from the end of the speed sensor to the starter ring gear is not 0.8mm (.032 inch). Regap the sensors (I use a piece of cardboard that is .032, so that I do not have to disturb the measurement, once made.)

I tried switching the sensors still nothing.
I hooked both sensors to a oscilloscope and got a wave for both but the reference sensor wave was not as high or frequent as the speed sensor wave. Both were above 2.5V which should be passing right???
Is there a way to check if my DME is sending a signal to the injectors? Is having 1.35V at one end of the injector harness normal or does it need to be 12V?
Thank you for all the help!!

[944 Ecology]

Originally Posted by alexjames

I tried switching the sensors still nothing.
I hooked both sensors to a oscilloscope and got a wave for both but the reference sensor wave was not as high or frequent as the speed sensor wave. Both were above 2.5V which should be passing right???
Is there a way to check if my DME is sending a signal to the injectors? Is having 1.35V at one end of the injector harness normal or does it need to be 12V?
Thank you for all the help!!

Compare the waveforms to that in the Factory Workshop Manual, and measure the waveform at the DME, not at the sensor. The wiring harnesses (after 30 years) have not held up well.

The quick way to check for injector pulse is to buy a noid light (from NAPA or such) and plug it into the harness.

That said, if you're not getting a tach bounce, chances are that it is something on the input side of the DME.

[alexjames]

Originally Posted by 944 Ecology

Compare the waveforms to that in the Factory Workshop Manual, and measure the waveform at the DME, not at the sensor. The wiring harnesses (after 30 years) have not held up well.

The quick way to check for injector pulse is to buy a noid light (from NAPA or such) and plug it into the harness.

That said, if you're not getting a tach bounce, chances are that it is something on the input side of the DME.

I bought a noid light and got no flash on all four injectors. I sent my DME to specialized ECU repair and they sent it back rebuilt. I will test the wave output through the harness tomorrow. But the wave output through the sensor looks good. When the key is off the injectors have 12V on both sides going to the positive. When I turn the key to the on position the voltage drops to 10.8V on both sides to the positive. Could this drop be the battery not being able to hold a load? Or could it be a ground somewhere in the harness itself. Where would I be able to find the factory waveforms is there a website that has them?
Thanks for everyone’s help

[alexjames]

Originally Posted by Jfrahm

I did not see you say you'd replaced the DME relay (a common failure point) or at least tried the DME relay jumper wire:
DME Relay Information and Testing

-Joel.

And yes I replaced the relay and have good foul pressure in my lines.
Thanks for all the help and advice Alex

[TibetanT]

Good troubleshooting so far!

However, this is one of three (3) things:

1) Battery is not good; should be about 12.3Volts or higher for the fuel injectors to fire.
To test this: turn on the front lights in a garage or against a wall so you can see how bright they are. Next, try and start the car. Do the lights go dim?

If the lights go dim, your battery is dying. Get a new battery.

2) OR, the ECU/DME brain is not the correct one for your car...or, worst the refurbishment done to it was not successful.

Is there someone near you that has the same type of car? Same year, same make and model? If "Yes" then, take your ECU/DME brain out of your car and install it in the good running car. If the good running car does not start, then the ECU/DME brain is bad.

3) Frame grounds are corroded. Use a good flashlight and check two (2) places; first, right near the reference/speed sensors on the block. Make sure these are clean.
Next check the frame grounds under the dash, and also everywhere within the engine compartment.

Hope this helps you.
Good luck

[alexjames]

Originally Posted by TibetanT

Good troubleshooting so far!

However, this is one of three (3) things:

1) Battery is not good; should be about 12.3Volts or higher for the fuel injectors to fire.
To test this: turn on the front lights in a garage or against a wall so you can see how bright they are. Next, try and start the car. Do the lights go dim?

If the lights go dim, your battery is dying. Get a new battery.

2) OR, the ECU/DME brain is not the correct one for your car...or, worst the refurbishment done to it was not successful.

Is there someone near you that has the same type of car? Same year, same make and model? If "Yes" then, take your ECU/DME brain out of your car and install it in the good running car. If the good running car does not start, then the ECU/DME brain is bad.

3) Frame grounds are corroded. Use a good flashlight and check two (2) places; first, right near the reference/speed sensors on the block. Make sure these are clean.
Next check the frame grounds under the dash, and also everywhere within the engine compartment.

Hope this helps you.
Good luck

Thank you for all the help I went ahead and tested all the grounds on the engine wiring harness with a multi meter and they all tested good. I inspected and cleaned both grounds under dash and engine block. I will pick up a new battery in the morning. Is there a way to test the DME computer itself without sending it out or should I put a new one in?

After cranking the car and testing the wiring my tach is no longer bouncing even though this morning it was bouncing do I have a bad connection or could a fuse have blown? Is there some type of relay that goes between the sensors and tach along the way that may have gone bad?

Both reference and speed sensors are brand new and still give me a wave output. Is there any other sensor responsible for the tach bouncing

Thanks for all the help Alex

[alexjames]

After spending many hours tracing wires and checking my speed and reference sensors I am left with this.
The sensor output I got through the harness was a little weaker than testing them at the thermal but they both still passed with over 2.5V. However the reference sensors frequency was far less than the speed sensors frequency but both were above 2.5V of alternating current. Then I checked my wring harness voltage.
Every time I check the voltage on it day by day it changes some days I would have five volts others I would have ten others I would have 12 to start and than it would dip down to 1.5 volts. I initially thought this was the case because I was using starter packs to give the car power but after putting in the a battery it continued to fluctuate.
So I then tired testing the resistance from either side of the connector to ground with the key out but the battery connected. I got 5 Ohms and 2 Ohms on the sides of the connector. This tells me that my injector harness is bad going from my DME relay to my injectors right? Because one side of the harness is always connected to the positive of the battery and the other never directly connects to ground until it passes through the DME.
And if so is there a place where I can buy just the injector section of the harness?
Thanks for everyones insight Alex

[alexjames]

After many more hours of climbing and tracing wires I wanted to let everyone know how I solved my problem. I first cut the red wire on the nine pin connector that goes from the foul pump relay and connects to the engine wiring harness. I believe it was the second pin on the nine pin connector. IMPORTANT NOT TO CUT THE BLACK WIRE THAT ALSO ATTACHES TO THE SECOND PIN ONLY CUT THE RED WIRE. Then I ran a wire from there to the positive side of all four injectors. After that I cut the two wires running from the DME (Pin 14,15) and ran one to the first two injectors and the other wire to the other two injectors. Tried to start and it fired right up.
For anyone experiencing no stat and the speed/reference sensors check out the computers good and all hope seems lost. Tap either side of the injector connector with a multimeter set to the Ohms setting. Then have the other cable tucked to ground both sides of the injector should have infinite resistance. If you get anything but infinite resting regardless which side of the injector you use you have bad wiring. Ive heard good things about Lindsay racing harnesses or if your cheap and impatient like me buy 16 gage wire and do it you self.

[kdjones2000]

Great troubleshooting.

Here is what I found in my injector harness when I was building my race car. The main wire going to all 4 injectors had corroded at the crimp/weld:

[Johnny O]

I own an 88 924S automatic and did a lot of upgrades myself with the sensors, fuses, plug wires, I also bought 2 new dme's from F9, and I put the original in the glove box plus the spare one, just to be safe. I experienced a lot of what you are describing and for me, as it turned out I replaced the positive battery cable because of green death in the cable, but it was my negative battery cable that was causing the intermittent starting issue. So with both battery cables replaced the car starts no issues, even my gauges are working properly. I can say if you replace the negative battery, if yours is like mine attached to the back of the engine/flywheel housing. Stuff a rag or something to cover the open slot to the flywheel. Taking the nut off and cleaning the ground was easy, putting it back on was a problem (hence) Dropped nut forum on here. It's easy to drop the nut, but it's hard as hell to get it out, I ended up buying a borescope, but it still took a lot of time to get it out. Good luck mate...

[DannoXYZ]

Originally Posted by alexjames

For anyone experiencing no start and the speed/reference sensors check out the computers good and all hope seems lost. Tap either side of the injector connector with a multimeter set to the Ohms setting. Then have the other cable tucked to ground both sides of the injector should have infinite resistance. If you get anything but infinite resting regardless which side of the injector you use you have bad wiring. Ive heard good things about Lindsay racing harnesses or if your cheap and impatient like me buy 16 gage wire and do it you self.

Yes, crispy old cracked wiring is common issue with these cars. You can easily replace segments or entire harness yourself with new wires and connectors. I put together list of connectors & part# here: DIY Harness Connectors

Please report any changes or errors. thx