Wits end, 944 NA no spark

[Sahara]

This is a very nice 2 owner 1988 944, never had electrical gremlins or incompetent mechanics. Last year it had an occasional starting problem but after cleaning and applying Gibbs oil ( a non acidic penetrating oil) to the speed and reference sensor plugs the problem went away.

Drove 4 blocks to a store, when I came out it would not start. Cranks fine. No tach bounce. Found some help to push it home and started troubleshooting. The results are:

• Car ran perfectly, then refused to start
•
• Checked engine continuity to ground - Good
• Checked battery voltage - Good, 13.25 V
• Cleaned all battery terminals
• Checked coil by resistance - Good
• Checked coil for primary voltage - Good, 12V with ignition on
• Checked coil and spare coil with the test from Clarks, put a spark plug at the end of the coil wire, removed coil negative wire, tapped a wire between the coil negative and battery negative - Spark AT BATTERY, not at coil, both coils
• Checked, cleaned Cap and Rotor - Good
• Checked, cleaned plug and coil wire connectors - Good
• Checked DME Relay - good
• Checked resistance of Speed and Reference sensors at plugs- Good
• Checked the above at DME plug - Good, same readings
• Checked continuity between sensor plugs and the DME connector - All Good
• Checked continuity between DME connector 1 and coil neg - Good
• Checked for voltage at DME connector pins 18 and 35 - Good, 12V
• Checked all engine compartment electrical plugs - all Good,clean, shiny
• Checked Alarm plug - already jumpered out
•
• Connected timing light to coil wire:
Cranks, no spark, no tach

• Put 3 legged jumper to bypass DME relay, left it in from this point , unplugging it between tests

• Replaced Speed and Reference sensors
Cranks, no spark, no tach

• Replaced coil with a known, checked good spare
Cranks, no spark, no tach

• Replaced DME with an allegedly good one from an 1987
Cranks, no spark, no tach


What am I missing? Or have I been sold a defective DME? What else could be the cause of no spark?

[nynor]

damn. that is a hard one. i was going to guess DME, but you tried that already. i am going to go with a bad connection at speed/reference senders. just a stab in the dark. i saw that you messed with them, but i think there is something hinky there.

[exitwound]

You mentioned you checked the resistance at the speed & reference sensors. How did you test this? Male ends or female ends?

[Sahara]

To answer your question on the sensors I checked resistance BOTH at the male end of the lead from each sensor AND at the corresponding pins at the DME plug.

The fact that I get the same readings at both locations tell me that the wiring is good and the sensors are in spec, at least as far as resistance.

To be absolutely sure I also checked continuity from the female side of the sensors to the corresponding pins at the DME connector.

[Razorback1980]

Here are a couple things that come to mind for me. No tach bounce and I lean toward speed and ref sensors.

The HV wire going from the coil to the distributor could be bad...pull the number one spark plug wire and put it in the coil and put a spark plug in the other end and lay it on the manifold and see if you have spark.

Ignition switch also will sometimes have 12V to the coil in the on position but not in the start. I would either put a test light on the coil wire and see if it goes out during cranking or you can jumper a wire from the battery to the coil and check for spark.

The reference sensor should show a 2 volt pulse when cranking, the speed sensor should show a constant 2 to 2.5 volts when cranking.

The only other thought I had was checking the set screw in the flywheel to make sure it's still there. It's rare they fall out but have heard of some that broke or fell out. I pull the pan underneath to see and turn the flywheel around. I can't remember how many there were in a 1988.

Of course you can test the coil by putting the number one plug wire in the coil and the spark plug in the other end and grounding it and then jump the coil to the battery and ground the #1 pin I think it is and see if you get spark. Coils are rarely the problem though. I really think it's related to the speed and ref sensors or DME. George from 944 Ecology I think it is can test the DME if we don't find anything else. I don't know of a DME test myself but some others here might.

Sometimes you can tap on the top of the DME and get them to work momentarily. There are some solder joints that come loose occasionally.

[ballysdad]

My vote is defective dme.. Ian from 944online will check it for you too. The only other test you can perform is to put an oscilliascope (sp?) on the reference sensors to see output.

[944 Ecology]

Originally Posted by ballysdad

My vote is defective dme.. Ian from 944online will check it for you too. The only other test you can perform is to put an oscilliascope (sp?) on the reference sensors to see output.

+1 While the resistance test on the flywheel sensors can tell you if a sensor has a broken wire, the only test that really tests the sensors is the oscilloscope test referenced in the factory service manual.

[John_AZ]

Another vote for the bad DME. The PET lists different numbers for the DME for specific years and models.



George may disagree and has more knowledge about interchanging DMEs.

You could try the "DME Resolder" technique:

http://forums.rennlist.com/rennforums/924-931-944-951-968-forum/289396-dme-repaired-w-pics.html?referrerid=204
OR
http://forums.rennlist.com/rennforums/924-931-944-951-968-forum/310806-early-dme-repair-write-up-with-pics.html?referrerid=20449

GL
John_AZ
1988 924S + 1987 924S

[944 Ecology]

No, I don't disagree with the chart. It is probably pretty good for checking to see if the car will start.

I still think it is equally likely that it is one of the sensors as much as it might be the DME.

I also test them for my customers, no charge except return shipping. (I keep several test cars around just for that purpose.)

[jmd_forest]

Stripped a few teeth on the timing belt maybe???

jmd_forest

[Sahara]

Thanks to all for the suggestions. To clarify my initial troubleshooting:

When I said I replaced the speed and reference sensors I replaced them with new spares, which also tested good on resistance.

When I tested the spare coil I used its coil wire. The spare has the right resistance specs but is from another breed of car.

What I have done to follow up on your suggestions:

Here are a couple things that come to mind for me. No tach bounce and I lean toward speed and ref sensors.

I initially replaced them with each other, then replaced them with new spares.

The HV wire going from the coil to the distributor could be bad...pull the number one spark plug wire and put it in the coil and put a spark plug in the other end and lay it on the manifold and see if you have spark.

I can not see how the plug wire would ever fit in this coil. My testing with a different known good coil and wire still produced no spark.

Ignition switch also will sometimes have 12V to the coil in the on position but not in the start. I would either put a test light on the coil wire and see if it goes out during cranking or you can jumper a wire from the battery to the coil and check for spark.

The coil had 12 V positive terminal to ground while cranking.

The reference sensor should show a 2 volt pulse when cranking, the speed sensor should show a constant 2 to 2.5 volts when cranking.

Being limited by 2 hands, no o scope and no alligator clips small enough I could not check it. But the chance of 4 sensors being bad?

The only other thought I had was checking the set screw in the flywheel to make sure it's still there. It's rare they fall out but have heard of some that broke or fell out. I pull the pan underneath to see and turn the flywheel around. I can't remember how many there were in a 1988.

Thanks so much for that idea ! I checked, the flywheel has one set screw. I think that is correct.

Of course you can test the coil by putting the number one plug wire in the coil and the spark plug in the other end and grounding it and then jump the coil to the battery and ground the #1 pin I think it is and see if you get spark. Coils are rarely the problem though. I really think it's related to the speed and ref sensors or DME.

The plug wire does not fit the coil. I tested two coils as per Clarks, both came out good on resistance. HOWEVER NEITHER of them, each with its own coil wire, with 12 V positive in, would spark a grounded plug. With the coil negative detached, grounded plug in coil wire, when I held a wire to the coil negative and tapped it to the battery negative THE SPARK WAS AT THE BATTERY. Is this Clarks procedure a valid test? The spare coil is running my other car so I asume it is fine.


The only other test you can perform is to put an oscilliascope (sp?) on the reference sensors to see output.

Don't have an O scope or the budget for adding one.

+1 While the resistance test on the flywheel sensors can tell you if a sensor has a broken wire, the only test that really tests the sensors is the oscilloscope test referenced in the factory service manual.

See above. Also I replaced both sensors with each other and then with new spares to the same result.Then for good measure I put the old ones in again.

You could try the "DME Resolder" technique:

Thanks for that great link.I opened up my DME, all the joints looked good but I resoldered the ones for the transformers, the ribbon cable, the motronic connector and any others that seemed not quite right. However it still does not spark.

I did not follow up on the idea that the belt may have jumped, it seemed too unlikely to have happened after a 4 block drive around the corner at 25 mph.

So I still have crank, no spark, no tach. NOW WHAT?

[slow944s]

Do you hear the fuel pump running when you try and start the car? I was driving my 87 to work on Friday and it quit also. Hopefully mine is a bad fuel pump relay. Do you think you might have fried the ECU? Just trying to give some new ideas as to your failure. Good Luck.

[Razorback1980]

I think my next step would be to fedex to DME to one of the two places mentioned to see if it is good. It sure looks like you have checked everything else I can think of. I'll have to check to see if the flywheel is suppose to have one setscrew or two.

[John_AZ]

Originally Posted by Razorback1980

I'll have to check to see if the flywheel is suppose to have one setscrew or two.

I thought I saw 3 on mine. 2 for reference and 1 for the TDC sensor. No matter, I do not think the problem would be a set screw came loose.

John_AZ

[ballysdad]

Originally Posted by Razorback1980

I think my next step would be to fedex to DME to one of the two places mentioned to see if it is good. It sure looks like you have checked everything else I can think of. I'll have to check to see if the flywheel is suppose to have one setscrew or two.

This wouldn't be the first DME that was sold defective. I would stop and send it out sounds like you checked everything needed to start car and get spark to the coil. The only thing missing sounds like the signal from the DME.

[John_AZ]

If you prefer to go the "Rebuilt DME" path...
http://www.systemsc.com/index.html

Many choices

John_AZ

[Sahara]

Since I did not have the budget to tow it to him I called up Autolab ( Mahwah, NJ) to beg for suggestions.

After reviewing what I had done so far he suggested that, lacking an O scope or helper, I use a noid light to check each injector for pulse. No pulse, no DME I/O from sensors. If no pulse, before going through the gyrations of turn the engine a few degrees,slide under and check flywheel for a dowel pin count and do it again and again, check the sensor clearance.

What !! I asked. It died after driving around the corner. How could the clearance change?

So I made up a remote starter switch as per FR Wilk - HOWEVER on this 1988 the connection for the starter side of the harness is a female bullet connector, not a spade connector. I used a 3/4 hp washing machine lid switch in line so I could have very momentary contact.

No injector pulse.

So I pulled the sensors and started to loosen the bracket. Oh, something is wrong. No M6 (10 mm head ) hex head and allen head bolts. Two M8 ( 13 mm head ) hex bolts with 7 washers each on them. The bracket is also ground down to fit the bolts. Damm PO.

The bolts loosened and the bracket pivoted up. 3/4 of the bracket. One end was attached to the pivot bolt, another end was attached to the hold down bolt and the middle section was missing. Apparently if was fastened under stress and finally sheared.

So much for logical troubleshooting - a self readjusting clearance bracket.

ANYONE ON THE NY METRO AREA HAVE A SPARE BRACKET ?? A holiday weekend without mail service and I am grounded by lack of a bracket.

Thanks to all who have helped with suggestions on this nightmare.

[kurtdaniel]

Originally Posted by ballysdad

My vote is defective dme.. Ian from 944online will check it for you too. The only other test you can perform is to put an oscilliascope (sp?) on the reference sensors to see output.

i guess so too...

[Arizona_928]

so was it the bracket that held the refference and speed sensor?

[callmethewander]

yes, that's what he said.