EJZero1

86 951
M-Tune

Was running great until sudden onset of intermittent no-start, progressing to no start at all.

I have no spark coming from the coil, but the coil is good (Swapped coils from another 944.) and the DME relay is good as well.

Based upon tach behavior while cranking, I replaced the SR sensors, but still have no spark. My car doesn't have the metal bracket holding the sensor to harness connectors I've seen in other cars. Furthermore, these connections were both wrapped in electrical tape. When I unwrapped the tape, the connectors on the sensors crumbled in my hands. So much so that I had to pick pieces out of the harness connectors.

Given the condition of the plugs on the sensors, and my persisting no-start situation, my next thought is to replace the sensor harness as suggested by the numerous other no-spark threads I've read here.

Anyone have any other suggestions on troubleshooting?

Tiger03447

If the pins in the connector were shot, which it sounds as they were, you can purchase new pins, cut back the wire and recrimp the new pins on the wires. If the plastic harness connector that contains the two pins, is shot those can be had also. Or, you can replace the whole mess with a new one. (mess). Up to you..I think there are some guys who live in the Denver area who can help out. An electronics store might be able to come up with the pins...sometimes, less is more..jus sayin..

EJZero1

One of the boots (don't remember which right now) on the harness connectors was brittle and cracked. Pretty good section of exposed wire in there, which adds credence to my harness suspicions. I'll have a look around next time I have time to do so, and probably do some continuity check from those connectors to the DME plug.

drive135mph

Hey I'm fighting kinda the same issue. before you get after the harness it might be worth connecting it up and checking cranking voltage at the DME


Also I discovered that quite a few VW and Audi's share that flat three connectors for a number of uses. and are readily available on ebay or in your local salvage yard. As a little bonus they have a wire retaining clip that you push down that is far easier to disconnect than the factory 944 retainer. I did this repair on my last 944 with good crimped connections and glue filled shrink wrap. It lasted 80k when i sold it running just fine

happy hunting

EJZero1

Just a photo update...

You'll notice the difference between the TDC sensor connection which would at least appear normal/healthy. The speed sensor connection is a different story. Notice the rubber boot is torn, with a nice bit of exposed wire. Thinking I might have found the smoking gun, I broke out my multimeter and started checking continuity. While the juncture between the female plug and the wires behind it seems "flimsy", all three pins and wires indicate good continuity. Hit the key a couple more times thinking maybe the "wiggle the connectors" fix might work, but no dice. While I was checking things, I verified I didn't cross the wires during sensor replacement.

I guess my next step is to check continuity from the connections to the DME.

What do we think of the suspect plug?

thomasmryan

its hot back there so I would do both.

the harness ends are twenty bucks.

PaulD_944S2

First test should always be continuity to sensors from the DME.
This confirms the wiring and connectors.

Then do the AC voltage test.
This confirms the coil integrity, gap spacing, and signal to the DME.

If you don't have intermittent tach bounce, then power to the DME, internal DME fault, or if Turbo, KLR issue. Alarm module can cause fail to start also, but bypass jumpering is different for 89 and up vehicles, if you elect to try that. With an 86, just follow Clark's' Garage for details.

Even if the DME relay is new, always test using the jumpers; that way you absolutely know that the circuits are energized.

EJZero1

Finally had some time to do some troubleshooting.

Ran the resistance/voltage diagnostics per the video posted above.

RPM sensor pins 8/27.
Resistance should be Between 600-1600 ohms.
Mine read 874.

Voltage while cranking should read more than 1 volt
Mine read 2.

TDC sensor pins 25/26.
Resistance should be between 600-1600 ohms.
Mine read 888.

Voltage while cranking should read less than 1 volt.
Mine read .2 volts.

As you can see, all of my readings fell well within the expected parameters.

Checked gap of sensors, just to be sure, and gaps spot on.

So now my question is, do these readings verify good continuity from sensors to the DME, or is that not a given? The condition of the connector at the harness end of the rear sensor still has me wary, but as you can see, it passed all of the tests. Next week, will try to swap out DME/KLR units with known functional ones.

Any other ideas in the meantime?

Here's a quick vid of the tach bounce I'm seeing.

mel_t_vin

EJ, that is some crazy tach bounce. Have never seen so great a needle fluctuation on my car...or any car, for that matter. Have you been able to locate a known working DME/KLR local to you area?

My suspicion...issue lies with the faulty connector on the harness. Have you separated both sensor pigtails from their connectors to see what the internal male/female pins look like?

EJZero1

Good morning, Allen, and thanks for chiming in.

Aside from the ugly look of the one connector, the connections look fine. The sensors are brand new, and the plug/pins at the harness end look fine. There is certainly the option of replacing the suspect connector, but would like to troubleshoot with good DME/KLR first before cutting into my harness.

No luck with sourcing good units around my area just yet.

PaulD_944S2

Referring to your post # 8, your tests confirm the sensors and wiring to the DME is good.

Three other things to check:

1. Make sure that the Speed & Reference sensors are not reversed at the engine; i.e. they are plugged into the correct sockets that go back to the DME. Unfortunately, Porsche did not make the plug/socket different between the two and it is easy to get them reversed. You can unplug one and confirm which circuit is open at the DME. You may want to consult the shop manual.

2. Test the continuity of the ignition coil wire (green) from the #1 pin of the DME (not the relay) to the coil (refer to shop manual to confirm my memory). I don't know if your '86 has the connector that is in front of the fuse/relay panel on the engine side of the firewall, if you do, open it up and check the pins. It is not sealed and sometimes the pins can get corroded.

3. When you tested for spark was it the coil lead or a plug wire? Confirm the timing belt is good and the rotor in the distributor is turning.

EJZero1

Thanks for the inputs. I really appreciate any and all direction. I want to drive my 951 again!

In order of your suggestions:
1. Pretty sure the sensors are not reversed. I marked the harness prior to removing the old ones. Additionally, the voltage at the pins associated with each sensor in accordance with the video matched the prescribed parameters. I'm not above swapping plugs just to make absolutely positive, though. Is there any risk to cranking with the sensors reversed?

2. Will give this a shot today. Does this test dictate the key needs to be in any position? On? Cranking?

3. Checked for spark at the coil to distributor wire using an in-line tester. Verified timing was intact by checking both TDC marks at flywheel and cam gear.

Thanks again for the inputs. Please keep them coming!

PaulD_944S2

1. Pretty sure the sensors are not reversed. I marked the harness prior to removing the old ones. Additionally, the voltage at the pins associated with each sensor in accordance with the video matched the prescribed parameters. I'm not above swapping plugs just to make absolutely positive, though. Is there any risk to cranking with the sensors reversed?

A) It would not hurt to confirm. Would hate to see you not able to drive your turbo because of this! No risk to sensors or computer while cranking if plugs reversed.

2. Will give this a shot today. Does this test dictate the key needs to be in any position? On? Cranking?

A) Ignition off, just measure for continuity from the DME computer to the coil. The connector on the firewall is in between. I see you have M-Tune, not sure if M-Tune coil is pin #1 (original DME is #1) so find M-Tune pin that is for the coil.

3. Checked for spark at the coil to distributor wire using an in-line tester. Verified timing was intact by checking both TDC marks at flywheel and cam gear.

A) OK cam timing, timing belt, and rotor operation confirmed with no spark at coil or plugs, correct.

EJZero1

I believe we may have a winner.

In going through some trouble shooting with Paul, I swapped sensor connectors. No start, as expected. Swapped the connectors back, and hit the key just for the heck of it....and it started.

Course of action as soon as I get time will be to repair/replace both connectors at the harness ends.

Genuine replacement or "suitable replacement"?

PaulD_944S2

Awesome!

If cost difference is not a lot, go genuine. But I would wait a while to see if there are any other problems before attacking. Don't want to make anything worse. While the shield is showing, the wiring has integrity hence no rush to replace.

You may want to tyrap or secure the wiring and connectors to eliminate any possible problems from vibration or flexing.