928 won't start, no spark

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928 won't start, no spark

Postby thofmann on Sun Sep 27, 2015 6:52 pm

Hopefully the experts on the forum can point me in the right direction, since I am out of ideas. Please read the entire post before suggesting to check fuses or connectors. I'm an electrical engineer with access to fairly advanced test equipment.
When trying to start my 928 (early 1986, 5-speed) there is no spark. All the inputs (temp sensor, TPS, supply voltage) to the EZF ignition unit as measured on the 25-pin connector check out ok. The crank sensor signal looks exactly as in the factory service manual, both in shape and absolute voltage. Yet, there is no output on pin 16 to the LH-jetronic control module or on pins 1 and 13 to the ignition trigger unit.
I have an arbitrary waveform generator (AWG) that can be used to simulate the crank sensor signal (1 big spike followed by 98 smaller ones). With the generator I could bench test the EZF unit and it worked right away. HOWEVER, it would not work below 300rpm. It is possible to jump-start the unit with some tricks to show that it is capable of running at speeds as low as 50rpm, but it wouldn't work under conditions similar to an engine start.
Over time I acquired 3 EZF units and they all behave in exactly the same way: works great at >300rpm, doesn't work below.
With a fully charged battery the starter gets the engine up to 190rpm. I checked the starter brushes and cleaned the solenoid magnetic switch contacts, but it didn't help. The battery is a bit old, but 190rpm feels more than fast enough for a normal engine to start.
As a last test I put everything back together, pulled the fuel pump fuse and fed the AWG output into the crank sensor connector. With the ignition switch set to run and the AWG set to >300rpm, an inductive pick-up timing light showed that the ignition was working. The tachometer even showed the correct speed (when setting it to 2000rpm). Alas, no spark at less than 300rpm.
At this point I see only two possibilities: all three EZF units are broken in the same way to cause this weird behavior, but the chances for that seem astronomical. Or, Porsche programmed the EZF not to start below 300rpm for reasons beyond my imagination. However, I would assume that many 928s don't have a starter strong enough to reach 300rpm and this should therefore be a known problem. Yet, I couldn't find anything on the internet about this quirk.

Any help is greatly appreciated,
Thomas
1975 911S coupe, hibernating
1986 944 turbo, back from the dead
1986 928 5-spd, temporarily out of service
thofmann
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Re: 928 won't start, no spark

Postby Greg Phillips on Mon Sep 28, 2015 5:10 pm

Greg Phillips
SDR Past-President @ 2014 Instructor of the Year
1982 911SC coupe & 1986 911 Carrera Cabriolet
1986 928S & 1989 928S4
1992 968 M030 1985 944Spec & 2005 Boxster (Pat's car)
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Re: 928 won't start, no spark

Postby thofmann on Tue Sep 29, 2015 9:07 am

Hi Greg,

Thank you for taking the time to look up the links.
Unfortunately, no smoking gun yet.

A quick internet search showed that a cranking speed of 150-200rpm would be normal and that 300rpm would be hard to achieve.
I forgot to mention that the car ran fine for many months and towards the end required a fully charged battery to start.
Looking back, I don't think the engine ever cranked faster than it does now. Therefore, it appears that the EZF didn't have
the 300rpm limit in the past.
Maybe it is not that unlikely that all my 3 units (1 original, 1 found in the junkyard, 1 bought off ebay) have the same fault. After all,
in the LH control units, for example, the fuel pump enable circuit tends to break a lot.

Thomas
1975 911S coupe, hibernating
1986 944 turbo, back from the dead
1986 928 5-spd, temporarily out of service
thofmann
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Posts: 11
Joined: Tue Aug 09, 2005 12:21 pm


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