Spark then no spark !!!

bobe · 08:57 AM

I switched the dme on my '84 (long story) the car fired right up ran for 20 minutes. I shut it off now won't start. Lots of spark when I turn it over I could feel it kick in after a couple of cranks then it stops for a second.
Voltage during cranking is at 10 VDC. When it tries to kick in voltage drops to 8VDC for a second stops cranking for a second then resumes cranking well at 10 VDC but no spark. Cycle repeats when I turn off the ignition and try again.

Does it sound like the starter ?? coil is new, dme relay is new, computer is known as good, tach jumps while cranking, (while spark is present) battery
is new. Excellent running car until this.

Thanks all.

944Spec_bound · 04-14-2008, 09:23 AM

Are you hearing the fuel pump kick on? Did you try pulling the relay and using direct jumpers to verify fuel pump operation?

I rigged up a long hot wire and worked backwards from the fuel pump toward the battery. Found I was losing voltage between the DME and the aux fuse panel, turned out to be the connector at the fuse panel.

there's a procedure on Clarks-garage.com

bobe · 04-14-2008, 11:41 AM

Yes it's all good.

bobe · 04-14-2008, 05:06 PM

but still not starting

Brando · Brando's Garage

Is the air flow sensor plugged in?

Also, is the DME from a same-year car?

Do you have an 83 model year or an 84? AFAIK there was even a small difference in the DMEs in that changeover year.

Also, you say the car cranks fine, runs for a short interval then dies out. It may be that you have an intermittent short or break in the speed or reference sensor wires. Double check your connections and connectors. Now is the time to check this since you say the tach bounces when it does have spark, but doesn't when it has no spark. Also, the sensors might be failing out.

http://www.clarks-garage.com/shop-manual/ign-02.htm

944Spec_bound · 10:59 PM

Quote:

Originally Posted by bobe

Yes it's all good.

I guess I should have read the title of your post. spark issue.

So, your losing spark while it's cranking or as soon as it tries to fire up?
Have you tried jump starting with another car?
I haven't done this on a 944, but the old school way I'd eliminate some things is pull the positive wire from the coil and run a hot wire straight to the coil from the battery, if it starts there's a few things it could be but you at least know it's electrical.
If you try this, be sure it's in Neutral though, it'll start in gear this way.

Hopefully some of the resident 944 gurus will chime in if I've just given you advise that will fry something!!

bobe · 04-15-2008, 02:04 AM

Thanks, The best way to describe what is happening is after two cranks
I could feel the the car trying to fire , then it seems as if the starter can't
continue cranking . Voltage on my gauge drops to below 8 volts for a couple
of seconds then the engine turns over and cranks well at a draw of 10 volts
but now with no spark. Turn the key off and retry, and it will do this evey time.

AKCJ · 04-15-2008, 10:16 AM

So the starter starts to turn it over then slows down or stops.

Seems like either the starter isn't getting enough juice or it's just bad/old.

Could be bad battery or bad battery cables/connections. You said it's a new battery but the voltage could be low due to a voltage leak or from just trying to start the car. You may be able to check the voltage drop from the battery post to the starter post. Also check the ground. Or change battery cables if they're really old. At least clean all the connections.

You can also try a cheapo amp draw meter on the batt cable while trying to start. A larger than normal amp draw would indicate a bad starter (or drag in the motor). Sorry, I don't know what normal would be in this case. Maybe pull the starter and have it tested?

After it gets cranking but then you have no spark - isn't there a threshold rpm before . . . now I can't remember. The DME doesn't turn on the spark or fuel pump until a certain rpm is reached? Sorry, I haven't had this problem yet so haven't researched it much.

bobe · 04-15-2008, 07:44 PM

I found an old receipt from PO. Mechanic stated starter weak. Guess I'll go
in that direction. Although this all started when I pressure washed the engine bay CAREFULLY last month.

944Spec_bound · 04-15-2008, 10:55 PM

Quote:

Originally Posted by bobe

I found an old receipt from PO. Mechanic stated starter weak. Guess I'll go
in that direction. Although this all started when I pressure washed the engine bay CAREFULLY last month.

I always try to avoid throwing parts at a problem until I find the bad one, but if you have a record of a weak starter, I'd eliminate it from the equation.

I'm curious to see if this fixes the problem, because I've heard people say their starter's drawing too much voltage and keeping the car from starting. But I can't remember that ever being the actual fix action, it always turned out to be something else after they replaced the starter.
I've never had one of mine fail like that. Mine usually blow the Bendix and won't stay engaged or they just freeze up.

but again, I'm not a mechanic, I've just been around alot of crappy cars.
(maybe I should add that discalimer to my signature)

bobe · 04-16-2008, 10:36 AM

STARTED !!! runs fine last thing now it needs to cool off for about 30 minutes to an hour then fires right up. I'm suspecting the senors.

Thanks everyone.

bobe · 04-17-2008, 09:16 AM

Jiggle sensor connectors fires up each time. Time to invest into a couple of sensors I guess .

Thanks for all the advise ....it's a great forum.

PXJ800 · 12:52 PM

Quote:

Originally Posted by bobe

STARTED !!! runs fine last thing now it needs to cool off for about 30 minutes to an hour then fires right up. I'm suspecting the senors.

Thanks everyone.

I've also been having some on-and-off startup problems with my '84 944... I'm getting a strong crank & spark (converted to MSD and am easily getting 12V and a strong starter crank, it's just not turning over). I'm replacing the DME relay this weekend and running through the sensors/wiring but if that doesn't solve my problem, I'll be checking the residual fuel pressure. The following article doesn't exactly apply to me but rings a bell. The trouble is that my startup problem is intermittent so it's a little harder to exactly identify the problem.

Sometimes I have no trouble with hot starts but they are more prone to need a few cranks, or possibly a 15-minute wait before finally giving me an easy startup, than cold starts.

Anyway thought it might help:

http://en.allexperts.com/q/Porsche-Repair-829/Porsche-944-Starting-Problems.htm