Another 'Intermittent Start' Problem

[Outback Porsche]

Hi All,

I'm a newbie to Porsche tinkering (and virgin post).

My problem is my 83 944 N/A will intermittently start. Once started it runs perfectly. Problem began at the beginning of the hot Ozzy summer (Oct07). After searching through the forum, I've run though the following (not in this order, I'm just typing as it comes into my head);

replaced the DME relay, checked the speed and timing ref signals with an o'scope (both within spec), checked the coil resistance. Jumped the fuel pump - it runs fine. Found that when the car doesn't start, that there is no spark (also no tacho bounce), cleaned up all the bad solder joints in the DME. Bench checked the condition of the Darlington transistor (in the DME) that drives the coil (it's O.K. - again, the car runs fine when it does start).

Recently found that I can make the car start by disconnecting the speed sensor (in the engine bay) and reconnecting it. Replaced the three pin connector on the engine wiring harness for both the speed and ref sensors (both sensors are 6 months old). Checked the continuity of the engine wiring harness back to the DME - it's O.K. Suspect a ground problem. Have cleaned the grounds down by the speed & ref sensors on the engine. Problem still exists, but still suspect a ground problem - Slowly going through and cleaning them all.

Any one else have seen this before, and have any other suggestions?

[Luis de Prat]

Originally Posted by Outback Porsche

Recently found that I can make the car start by disconnecting the speed sensor (in the engine bay) and reconnecting it.

The problem may be the sensor itself. Specifically the magnet that screws into the clutch bellhousing. Try replacing the whole sensor. Expensive, but will probably fix the problem after everything you've tried.

[Razorback1980]

Welcome to the forum....glad you are here.

For someone just starting to tinker, you've checked a lot of the right stuff.

Usually when the tach doesn't jump, it's the sensor. I'm wondering if the gap is barely in spec, but I really think the connector is the problem. Tighten the friction to make sure you have a solid connection there. Clean the connectors and coat them with dielectric grease. Did you check them with the oscilloscope when it was running or dead? Could still be the DME though.

[Outback Porsche]

'For someone just starting to tinker, you've checked a lot of the right stuff.'

Thanks to this forum and places like clarks garage, Bills 944 World, etc.


One thing I neglected to mention is that I swapped the speed and reference sensors - the problem didn't go away. It's unlikely that I've got two new bad sensors (not discounting anything though. After six months, I'm at my wits end).

Also, not totally discounting the DME. I did also neglect to mention that I tried starting the car with the DME open and hitting all the semiconductors with a blast of zero freeze. It didn't change anything either. Has anyone got a schematic for a ML1.1 DME, I could then properly bench check it.

I haven't yet verified the sensor gap - although the signal on the o'scope is strong at almost 3Vpp (running or dead there's no change) . Sort of indicates that it's O.K. I'm hopefully swapping out the oil separator this weekend (if the parts arrive). I'll verify the gap at the same time.

The last thing I did was to slightly bend the male pins 8 & 27 on the sensor cable, so that when they mated with the females, it would make a tighter connection. The car has been starting flawlessly for the past two days (I've had it working for two weeks before a failure - I'm not holding my breath yet). Time will tell.

I'd appreciate any more thought/ideas.

Cheers,
Jeff

[TibetanT]

G'day mate and welcome to the boards!

I believe your problem with starting had to do with the connectors for the engine harness, DME plug to the reference/speed sensors.
One of my restoration projects involved the same type of issues and it turned out to be the connections there.

I ordered some new connectors from Atlanta Auto, they do Porsche 944 parts really well.

Good luck with the car and happy driving.

[TibetanT]

Whoops! *********** is the proper name...you can Google them and see.

There just happens to be a Atlanta Auto, but that is NOT who I meant.
Sorry about that.

[Outback Porsche]

G'day all,

Thanks guys for the help so far.

Ed, I purchased my AMP connectors from EagleDay.com. Perhaps the connectors from *********** are a better quality? I'll see how long the current fix lasts - 3 days now and all is well (I shouldn't put the moz on it, should I).

Cheers,
Jeff

[Razorback1980]

If it quits, then you can always put an ohm meter on both sides of the connector to make sure you have a strong connection there. Sure sounds like that is the problem but it's so hard to tell.

Do you know the part number of your DME?

[Outback Porsche]

DME is;

Bosch 0 261 200 016
Porsche 944 618 111 02

[Razorback1980]

Thanks...let us know if it keeps failing..might have a spare DME to loan you but I'll have to check the numbers.

[TibetanT]

Yes! That is great.

AMP is ultimately the supplier of the best connectors in the electronics industry, IMHO, so you did good there.
I think/hope it solved the problem since three (days) out of three is very good statistically.

[Outback Porsche]

Crap! I couldn't get it to start this afternoon without doing the disconnect/reconnect thing.

On the brighter side, the parts arrived to swap out the Oil Separator - I know what I'll be doing in the morning......

I'll probably tinker with the sensor again and see what I can come up with next.

I'll let you all know how it goes.

Cheers,
Jeff

[sarbirus]

I have a similar problem with my 83 944... With mine I find that it will try to fire as the engine first turns over, but then will not even try to fire no matter how long you keep it cranking.. turn the ignition back to "off" and then try again and it does the same thing again.

A few times I got it started by litterally flicking it into the start position and then back to the ignition position quickly... often it will catch, fire and die, then eventually it slowly splutters into life and after a few seconds runs fine.

Got good compression, good leakdown, 2 new sensors, new ignition switch, plugs getting wet and it seems I am getting a spark only for the first split second of cranking the engine.

Have also tried a friends ECU, used a jumper leak to make sure the engine is earthed to chassis, changed the DME relay and tried all kinds of things.

Is this anything like your problem? anyone else seen anything like this??

[Razorback1980]

His is a bit different in that his tach doesn't bounce indicating some type problem with the sensor. Yours sounds more like a ground problem.

[UomoBello]

I have the same problem! except I just have to wiggle the sensor harness a bit and the spark comes back! It doesn't make any sense! I tightened the connector and put dielectric grease on it and it still does the same thing. It happens every couple of days. I feel your pain.

[Outback Porsche]

Sarbirus,

Your problem is a little different, as when my car stars it runs flawlessly.

To me it sounds like you have one of two possibilities;
1. No fuel pressure, or
2. A bad coil.

I gather you've also checked your rotor/distributor and plug leads? Plug leads should be no more than 10k ohms.

Jeff

[Outback Porsche]

UomoBello,

When I eventually find the problem, I'll let you (and everyone else) know.

Happy hunting.

Jeff

[mattdavis11]

The tach not bouncing can be a bad connection at the + battery terminal, and or up into the shielding. When you are jiggling the connectors, might you be placing you hand on the wiring loom?

I encased those two connectors in aquarium sealer.

Saribus, it sounds like your ignition switch is toast. Does the radio power up when the key is inserted? If it's an 83 and it doesn't there is something wrong with the switch. $12 plastic part.

[earlr85944]

id try ign sw on both cases for $12 worst that can happen is you have a spare....

[Outback Porsche]

Good suggestions. Looking into the harness.

I checked my ign sw. It's functioning perfectly. It's a good suggestion though.

Sarbirus, the ign sw provides 12V to the coil in both the start & on positions. It may be that you have 12V in the start position, but not in the on position.

Jeff