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924S Won't Start - Help

My brother and I have been trying to figure out what's wrong with his 1987 924s for about a month now. The car starts intermittantly, usually when it's nice out. When there's humidity (or rain) though, chances are it won't go. Sometimes it'll start, then die a few seconds later. Usually it will either start or not.

The issue seems to be with delivery of spark, fuel seems fine.

Here's what's been done so far:

Switched DME Relay - no resulits
Switched flywheel and reference sensors - no results
Switched coils - no results (there is power to the coil, no grounding signal though)
Checked plugs/wires, new cap & rotor
Checked for dirty/loose grounds
Removed computer & checked contacts & any signs of moisture

Here's what we're still thinking:
Distributor (how do you check this?)
Ignition switch

Any other suggestions? We're running out of ideas
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Old 06-14-2004, 07:50 AM
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Ignition on; 12 volts at the coil and at the injectors.

No voltage at the injectors bad DME relay.

Cranking; fuel pump should be running. The pump will stop just after you stop cranking. No fuel pump; check the fuse.

Check the speed and reference sensors. Both sensors should have about 2k ohms across their windings. Center pin to one of the side pins. The other side pin is a floating ground and should not have continuity with the other pins.

Check the sensor to flywheel clearance. 0.8 mm between the rear sensor (speed) and the flywheel starter ring teeth. About the thickness of a dime.

Reference sensor (front) is what triggers the coil through the DME. This could have a bad connector. See if the connector is loose. If it is use a pull tie to hold it in place for testing. Note that these are coaxial cables and extremely difficult to splice.

Air flow sensor. There is a wiper on a carbon coated ceramic plate that feeds back the position of the flap. This can be contaminated if the black plastic cover is loose. See www.frwilk.com.

There is no "distributor" just a rotor and a cap. The electronics (DME) does all of the work.

Alarm can cause problems too.
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Hugh - So Cal 83 944 Driver Person
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Old 06-14-2004, 09:44 AM
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I had an old MG that wouldnt start when it was humid/raining. It was because moisture was getting under the Distrib Cap.

I used to have to take a hair dryer and dry it out.

See if its wet in there.
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Old 06-14-2004, 09:47 AM
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Where the heck did I read about a special dist cap that has a hose connected to it for doing what your hair dryer did.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-14-2004, 10:00 AM
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??? Wish I had something like that 15 years ago. It was a real PITA!

Of course it had points though.
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Old 06-14-2004, 10:02 AM
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SoCal - I'll run through this check list.

Thanks much. I believe I've eliminated a bunch of these things like the cap. They were the first things to check... Haven't looked at the airflow sensor yet. The thing is that it just cuts out, like a sudden loss of spark, not a funky poor-running leading to dying which I would associate with bad airflow which probably leads to a bad mixture.

How about the computer brain and or Ingition switch?
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Old 06-14-2004, 10:40 AM
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Photo of DME

What do you think about the discoloration on the DME board? Could this be a sign of old condensation and water damage? Nothing looks too corroded, but a little off.

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Old 06-14-2004, 12:40 PM
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Ignition switch is definitely a possibility. These are currently failing in the early 944's.

Rout a test light to the power side of the coil and place it where you can see it.

Another quick test is to stick a spare spark plug in #1 plug wire and lay it on the intake. If this arcs while cranking this will tell you that there is power to the coil, the reference sensor is working, the DME is working, the wire from the coil to the dist cap is good, the rotor is turning (no broken timing belt or lost screw) and the wire from the dist to #1 is good.

As to the AFS; your reference to moisture is what makes me question this. But the connectors to the sensors are moisture sensitive too.
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Old 06-14-2004, 12:43 PM
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DME looks questionable. One of the "fixes" is to resolder all of these connections.
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-14-2004, 12:46 PM
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SoCal - how do you check that sensor to flywheel clearence? On mine, when the bracket broke off, the rear sensor was the one that wore on the ring gear, but the front one didnt. After checking the resistance, the front one showed about 1Kohm and the rear 3.5Mohm - would this cause it to not fire the coil, but still turn the fuel pump on?
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Old 06-14-2004, 01:53 PM
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I'll try some more of these tests and see if I can swap the DME computer with another known good version.

If I'm getting power to the coil with the ignition on, and it cranks properly, would that eliminate the ignition switch?
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Old 06-14-2004, 02:00 PM
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Quote:
Originally posted by echrisconnor
I'll try some more of these tests and see if I can swap the DME computer with another known good version.

If I'm getting power to the coil with the ignition on, and it cranks properly, would that eliminate the ignition switch?
Mostly. See if there is power to the injectors. This will confirm that the first set of contacts in the DME relay are closing too.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-14-2004, 02:13 PM
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Shouldn't I see pulsing power at the injectors when cranking, not just with the ignition on? Since this isn't a continuous injection system.
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Old 06-14-2004, 02:30 PM
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Quote:
Originally posted by Texas944
SoCal - how do you check that sensor to flywheel clearence? On mine, when the bracket broke off, the rear sensor was the one that wore on the ring gear, but the front one didnt. After checking the resistance, the front one showed about 1Kohm and the rear 3.5Mohm - would this cause it to not fire the coil, but still turn the fuel pump on?

The sensors are "good" from one to just over 2 k ohms. The voltage of the signal that is generated from the starter teeth and the pin passing the sensors is the key.

The sensors are set up first using the rear sensor (speed) to fly wheel starter ring teeth with the bracket adjustment.

To set up the front sensor (reference) you have to take it out, put some putty on the end, put it back in and turn the engine over by hand. Remove the sensor and use a tooth pick to check the thickness of the remaining putty. You then adjust the pin (set screw) to the proper height. Some have made tools to fit into the sensor hole and measure the height of the pin.

So you could have a dead on clearance for the rear sensor but be totally off on the front sensor. Fortunately setting up the rear sensor also sets up the front in most cases. If you get an aluminum flywheel be sure to check this.

The way the fuel pump works as I have come to understand it is the speed sensor (rear) signal keeps the fuel pump running once the engine is running. Before that the DME "primes" the fuel pump as the starter solenoid signal wire is split off to the DME -- pin #4. Wilks has a good dissertation on this www.frwilk.com.
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-14-2004, 02:31 PM
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Could be a serious vacuum leak. I was baffled recently after changing belts - would run for a couple seconds then quickly die. Turned out to be the vacuum connect on the bottom of the rubber intake bellows between the throttle body and AFM had popped out of place.
Did you mention the condition prior to this incident, ie, what maintenance was done, etc.
Good luck
Old 06-14-2004, 04:55 PM
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OK, more answers to tests.


**No arc on plug 1 when cranking**

There is 12v to the coil and 12 v to the injectors with ign on.

All sensor connections look good and have been recently adjusted and tested.

Vacuum leaks don't seem to be apparent. Also, when the car runs, it runs beautifully -- nice, smooth and powerful.

Now checking on whether the fuel pump is running while cranking.

Thanks to all, I appreciate all the great help.

Chris
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Old 06-15-2004, 09:06 AM
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Let's back up to no spark. Could be as simple as the wire from the coil to the dist. Could be that the screw on the rotor came loose and fell out.

You can move the plug wire from the dist cap to the coil and see if the coil is firing.

You also need to ohm out the sensors to see if they are still viable. Then you need to check the clearance on them.

0.8 mm on the rear sensor to the flywheel starter teeth. About the thickness of a dime.

If any of the connections for the sensors are cracked then it's likely that you have an intermintent connection. You can tie the wires up in different directions to see if this cures the problem.
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-15-2004, 09:15 AM
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OK, a little more progress.

Swapped coil to dist wires and tried to spark directly from the coil. No results.

Sensors are brand new, changed by the shop which was working on this problem. The old sensors were about a year old, and seemed fine, but have been replaced anyway.

Gap shouldn't have changed in the progress. But we're checking.

Connectors have been played with and don't seem to make a difference.

HELP! DME box?
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Old 06-17-2004, 08:46 AM
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If you have 12 volts at the coil and at the injectors and no spark then it looks like it is pointing toward the DME.

BUT! I would first see if the sensors are generating the correct pulses for the DME. Have to be 2 volts or more.

Not all DME's are the same. Make sure it has the same part number.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Never break more than you fix!
Old 06-17-2004, 09:04 AM
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How does one test the pulse from the sensors? We've checked resistance across the sensor, but not output.

So, you're starting to think it's the DME box?
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Old 06-17-2004, 10:23 AM
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