Van

Have you tried another coil? And/or done a continuity test for the coil wires (green gets +12 and black comes from DME). Another thing I thought, if you're getting a signal at the coil, but not at the plugs, maybe your rotor is out of phase... possibly the bolt that holds the rotor mount onto the cam "nut" is missing? Or maybe that "nut" isn't in the key-way?

PerryB

Van, thanks for responding.

I have not tried another coil. The current coil has the correct primary and secondary resistances. I did pull off the distributor cap and confirmed that the rotor is oriented correctly and is solidly affixed to the camshaft.

Also, in one of my previous post, the coil did fire in response to me rapidly energizing and de-energizing the DME. I believe that the coil works correctly. What seems to be happening is that the DME doesn't alternate grounding then un-grounding the coil which would cause it to fire.

The coil does get 12V from the ignition switch. The number 1 wire to the DME has continuity between the coil and the DME. The DME just doesn't fire the coil, hence my sending the DME off for repair.

Perry

Alex Sol

i had this no start problem and clearly had no spark. tried different coils,

here's something strange. had brand new wire set and the coil to distributor had no continuity. i replaced with an older spare set that had continuity.... still no spark

replaced the speed and reference sensors. was impossible to remove without removing the entire bracket (2 x 6mm hex bolts and hard to reach) - could it be your speed sensor(s) are new but faulty or you were sent the wrong ones.... i bought the bmw ones that are a little cheaper than the porsche sensors

gapped them according to clark's garage method with the washer glued onto the end of the sensor and set the bracket height.

prior to the no start, i had no tach bounce

as soon as i replaced the sensors, i had tach bounce and it finally started right away.

Alex Sol

oh i also pulled out the silver coil in favor of a black bosch... both had the resistance according to spec...

specsalot

Perry,

Try unplugging all your injectors and see if you can then get spark out of the coil. Perhaps you're got an elelctrical problem with an injector impacting electronics function? Just grasping at straws here. Something about your description of the behavior of your system doesn't set right. Sounds like a wiring issue. It is more common for the interface (wires) to fail which then trigger component failures.

PerryB

Specs, ALex Sol,
Thanks for your replies. I was out of town for a bit, then busy with work.

I had mechanic come over today to go over my car. He said that it seemed that the computer was at fault. I'm pretty sure that after a completely rebuilt computer and a double check by Specialized ECU on their unit that that is not the problem. He agreed, but could offer no suggestions as to what else it might be.

The computer grounds the coil but does not break the ground that will cause the coil to fire. Currently (no pun intended) the injectors are firing--something that did not happen when I originally had the problem. Why? Could the latest computer check have fixed the lack of injection but not the spark?

Specs, I really do not want to pull off the injector connectors. They look original and fragile.

It appears to me that either the DME is not working, or something is preventing it from alternating the coil ground. Normally, this would be a defective reference or speen sensor, but that's not the case here. I thought that there might be a short to ground in the wiring between the DME and the KLR that was grounding the ignition signal after it left the DME, or after it left the KLR. Nope.

Again, I will greatly appreciate any help.

PerryB

I've noticed that the oil level warning light is on. The oil level is correct. There's not some ignition interupting function that happens when the oil level is too low, is there?

Porschini

I'm just a lousy Italian car guy but I've been following this thread in utter amazement. I don't recall ever seeing the use of such a thorough and methodical diagnostic process that didn't result in running car (or at least the definitive diagnosis of why the car wasn't running). I gather the owner lives in a remote part of the Rockies; even so, I can't imagine there isn't another 944T owner (relatively) near him who would be willing to help out. Perhaps a PM to him, find out where he lives and maybe recommend a good mechanic in that area. Someone has to know of someone that can help this poor guy out.

Kevin Baker

I'm going to go on a very basic theme. Ice read as much of the technical testing part as I could without coffee this morning.

First wow! I think I would have given up by now.
Second I'm not trying to be the a-hole that my wife says I so naturally am.

Do exactly this. Unplug the speed sensor connector, leaving the speed sensor where it is. Unplug the reference sensor connector, leaving the reference sensor where it is. Now plug the "speed" sensor connector into the "reference" sensor and then plug the "reference" sensor connector into the "speed" sensor. Now see if you get tachometer movement.

When I did my engine rebuild, the one thing I did not label was the speed and refrence sensor connections. First crank got no tach, switched them. 1800 miles later off to do my first post rebuild oil change.

PerryB

Mr. Baker, Porschini,

Thanks for your replies.

I have swapped out the speed and reference sensor connections. No difference.

I have repeatedly checked the speed and reference sensors for correct output. I did these tests at the DME connection. With every test they have worked correctly.

Van

I know you have the main ground strap connected (because it cranks over), could the DME ground strap be disconnected or something?

Even though you're getting a signal on the oscilloscope, is it worth butting in a different set of sensors?

I once had a running issue under full boost. Both the AFM and TPS measured good on the bench with a multimeter, but changing both solved my problem. Somehow, one or both must have been out of tolerance enough that they were giving the computer conflicting information about how much air was going in, causing the computer to go into overboost protection.

Kevin Baker

Best of luck!! You are way deeper in it than I'm capable of. I am interested to see what the final diagnosis ends up being

william_b_noble

here are a couple of additional tests

1. disconnect the black wire from the coil and measure resistance - it should be open circuit or nearly so with power off - if it shows a low resistance, that is the problem, just need to find out what is causing it. The output to the coil low side is, if I remember right, an NPN transistor, it could be shorted.

2. with black wire still disconnected, put a pull up resistor to the +12 on the black wire and put your scope on it - see if you see any kind of pulses as you crank - if you don't, the DME isn't commanding the coil to spark. Then look at the base of the drive transistor and see if you see pulses there, if you do, it's a bad transistor, if not, keep looking

I've attached the schematic for the turbo DME for your reference.

PerryB

Van, Noble,

Thanks for your input.

I have double checked the DME ground at the top rear of the engine block--and in the process broke a spigot on the cycling valve, so I need a new one of those if anyone has one for sale--and it is tight and clean.

Mr. Noble, I'm not sure if I follow you in your instructions. Do you want me to pull off the black wire on coil terminal 15, this goes to the ignition switch + side, and test for resistance to ground?

I did this a while ago and I got 73.8 ohms, which suprised me, I thought that since this is the power into the coil it would not have any connection to ground. I started to pull fuses and relays to see what would happen. I pulled out relay G2 (ignition relay) and the resistance jumps to 2,136 ohms. I concluded that the power from the ignition switch also supplied some other component that connected to ground that put out the resistance. I'm not sure how to interpret this result.

Your second suggestion is above my understanding of electronics. If you could explain it more fully I will perform this test.

Lecrok

https://learn.sparkfun.com/tutorials/pull-up-resistors

This may help.

Nice tip, Noble.