87' 944 NA No Power to Coil

[Icey1174]

I've had a no start issue for a while. Basically just parked it in my shop for the past few years because I did not have time to work on it. My shop is dry, clean, and climate controlled.

All new properly gaped plugs, new wires, new cap, new rotor, and new coil (all from three years ago).

My ground was bad to the coil so I ran a new, clean ground to the body and have continuity there now.

With the key in the ignition and turned to the on position, I am getting no power at all to the coil at the 12V wire nor the ground.

Next step is to trace the 12V to find and correct the problem. This does not look like an easy task.

I did not check the coil. I did not have a multi meter with me, but the coil is brand new.

Is it possible to just rewire? If so, where do I rewire to?

Any pointers or help would be greatly appreciated!

[944 Ecology]

The 12 volts comes from the ignition switch into the fuse block at pin g23 and out of the fuse block at pin c32, a black wire. The ground comes from the DME/ECU pin number 1, through pin 7 on the 14 pin connector, then directly to the coil, a green wire.

Hope this helps...

[Icey1174]

Does the ground have to come the DME or can the coil be grounded to the body? Could this be a DME issue as both the ground wire and hot wire are bad?

Thank you for your help!

[944 Ecology]

The ground has to come from the DME.

I found the following explanation on another forum:

In the 944 (and nearly any electronically-controlled ignition), the coil is supplied with a constant +12 volts whenever the ignition is turned on. The DME controls the "ground-path", by a large transistor (the silver colored can-style transistor in the DME). To operate the ignition coil, it first must be charged, which the DME does by turning on the transistor, providing a ground for the coil - completing the circuit. Then, once the coil is charged, the DME turns-off the transistor, which grounds the circuit causing the coil to fire.

[Icey1174]

Thank you! That is a very complete explanation that makes a lot of sense! So I could have a bad wire or a bad DME? I'll be working on it in the next few days.

[walfreyydo]

Get a multimeter on the DME ground signal (at the DME plug) and see if its grounding. If not, bad DME, if so, bad wiring. I would guess Pin 1 is the one that sends the ground signal to the coil.

More info here: https://www.clarks-garage.com/shop-manual/ign-04.htm

Pin output (I think this is for an 8V NA?):

[Icey1174]

Thank you!

[Icey1174]

Is this the same for a NA 944?

[walfreyydo]

Actually I think this is for a turbo, but if you google Porsche 944 DME diagram you can dig deeper - KLR signal is the giveaway.

I think PIN 1 would still be the one you want to check, turbo or NA.

[Monkey Wrench]

I think that since the ground is being pulsated at a high frequency you'll need a scope to see it properly. some of the fancier Digital multimeters may have added functions but the common simple ones cant really track a fast pulsation they just become confused. it might show as an AC component or similar.

try just taking a little bulb like you'd use for a dashboard lamp or similar. hook it across the coil wires , hop in and turn the key you should see it flashing at cranking speed. when it's running this will be a pretty high frequency so your lamp then might look partially lit.

the primary coil needs to be interrupted, each time power is interrupted a pulse for spark in the high tension coil is generated. it's frequency relates to RPM speed. I thik it will be 4 times the RPM.

the little lamp is connected in parallel with the low tension coil. being connected in parallel means it is sharing the load and the resistance of that particular circuit will be decreased while the little test lamp is in use because you have added an alternate current path.

If you were to use a big bulb, it will cause more power draw as it will add a lower resistance, so I'd keep that bulb to a low watt one so you aren't stressing the circuit.

its just a very quick and simple way to verify the coil has a pulse. If you can ascertain whether or not you have this pulse it then tells you what to troubleshoot next.

if it is absent the car wont start. if it is present the car will most likely be getting htat pulse to the plugs.

I think this will work on most cars, just try sometime and then you will know what to expect if it fails to start. if you find your coil pulse is missing when cranking , then you can go back into the troubleshooting stuff mentioned previously.. If you do have pulse then this ignition pulse being missing may not be the cause of trouble. I just think it saves time to verify that. No point going into checking pinouts and stuff or replacing the DME or the ampligfier thing between the DME and the coil, if the pulse is present.

you should find thing mounted to the body , around the fender, it amplifies the signal fromt he DME to the coil, I believe this is on the ground side. I think you should see hot at one of the coil terminals with key on.

if one terminal is hot so is the other because they are connected by way through the primary winding of the coil. I think then since the digital meter is sensing the voltage and not the current, you will measure voltage on either connector. you can unplug the coil wires, and measure the resistance through the primary coil.

I think it happens often that people replace coils, thinking, hoping it will help, and then it ends up not being a problem with the coil. The only one Ive ever seen go bad was one that actually had rusty terminals and just needed cleaning up. in all my life Ive never had a coil go bad on one of my own cars.