scarceller

Both sensors trigger from the flywheel not the cam, no sensors on the cam for 84-89 cars.

The ref sensor triggers one time per crank rotation and is used to determine a reference point for firing the ignition. It triggers at about 60degrees BTDC and uses this point as a starting point to calculate coil fire events. The coil fires exactly 3 times per crank rev.

Speed sensor counts flywheel teeth and fires 129 times per crank rotation. This sensor basically tells DME Engine RPM and is also used to calculate coil fire events at a rate of 3 coil fires per rev.

scarceller

The exact spec from the Factory Books is 960ohms +/- 96ohms

Book reads like this:
1-2 = 960ohm +/- 96 ohms
1-3 = > 100,000 ohms
2-3 = > 100,000 ohms

If you are in the 1,000 ohm range your good.

Lorenfb

"I'd go through the Bentley trouble-shoot sequence."

- Ed -

The best advice given in the whole thread (and even that posted by a
former Porsche independent shop owner), i.e. the 'shotgun approach is
a waste of time and money.

Bottom line: At this point (three pages of posts), maybe it's time to take
the car to a qualified Porsche shop and NOT one who swaps parts and
charges for them like many Porsche mechanics who have retired did
because they lacked key technical abilities.

"It triggers at about 60degrees BTDC and uses this point as a starting point to calculate coil fire events."

Actually, it is not required for running the engine once the engine starts,
and as a result it can be used for diagnostic troubleshooting.

scarceller

Back on post 6 he already found the most likely problem a bad Speed Sensor. He measured the resistance as open circuit so we know that sensor is bad, it's not a guess! At a minimum he needs to replace the failed sensor.

Lorenfb

"With 12vdc on the '+ terminal you can now place a test wire onto the coil's '-' terminal and take the other end of the wire and hold it to a good ground, count 2 seconds and remove it from ground. The spark plug will fire as soon as you remove it from ground. Do NOT apply ground to the coil for more than 2 seconds as you'll super heat it."

Not a good idea, as one could damage the coil, i.e. the 911 3.2 coil has about .7 ohms resistance,
check here under 'Typical Ignition Coil Values' - Bosch TCI;
http://www.systemsc.com/diagnostic.htm

The above test results in over 100 watts of power being dissipated by the coil which could affect
its long term reliability.

scarceller

I agree that placing 12vdc across the coil COULD damage it if you allow it to really heat soak. In my prior post I listed checking for coil pulses from the DME first and use the 12v coil test if you find that coil pulses are verified.

If you have DME coil pulses then the problem most likely is the coil and this 12v test is a quick way to verify what is suspected to be a bad coil.

You pointing this out now has me thinking that it would be best to use a inline resistor when doing this test. A 10 ohm wire wound resistor would limit coil current to just above 1 amp, much safer power/current levels.

The question was asked how can I verify a coil?
Loren, do you know of a better safer way to test a coil that can be done easily?

Lorenfb

"The question was asked how can I verify a coil?"

Replace it with a known good coil (for testing only), e.g. an old VW coil from
Pep Boys, Auto Zone, etc. - about $20 - $25.

scarceller

Given the price I say fair enough, try a known good coil.

While on this topic have you found any other coils that perform as well or better than the stock coil?

SiberianDVM

I replaced the new Brazilian coil with my old German coil early on in the process, so I'm pretty sure it's not the coil.

I appreciate all the help, and the new sensors will be here Friday. So we should know by Saturday afternoon.

scarceller

My bet is once you replace that speed sensor she'll start right up and you'll be happy again!

scarceller

Loren, you are correct that the engine will run fine once started even if you unplug the ref sensor. But the speed sensor can't miss counting any teeth, if it does the timing starts to drift. In the code the ref sensor is looked at and acknowledged every time it triggers via a processor interrupt. The interrupt re-syncs the timing even if it's not needed.

SiberianDVM

Got home from work Friday evening at 1945. By 2030 I had both new sensors installed, despite the hordes of Asian Tiger mosquitoes sucking me dry. It's the moment of truth: Bwunhilde lives!!!

Thanks for all the help, everyone. I'll spend the morning putting all the little bits back into place, then taker her for a test drive.

theiceman

outstanding ...

SiberianDVM

She's running like a champ!

Amber Gramps

Awesomeness. Preemptive replacement of sensors needs to be high on everyone's list of things to do.