Crank Sensors - Gradual Decline?
 

Hello:

I have removed the aftermarket alarm from my 1984 Targa. I have also performed the requisite tests on my factory alarm.

Still no start.

I think I have it narrowed down to either the ECU or the reference sensors. Since my starting issues were intermittent prior to the final shutdown, I am at a loss. I tested my sensors per the Bentley manual and they were within spec. But Gerry(fellow Pelican) and my tech are both telling me that the Multi-Meter test is not conclusive.

So my question is - Will a crank sensor cut out intermittently prior to complete failure?

Thanks for reading.

Measuring ohms is not conclusive. You need to use an oscilloscope to see its waveform. Change it, it is relatively cheap.

My Tech, Gerry from this board, and now you, are all pointing to the sensors, rather than the ECU. Maybe I should just change them, as they are now over thirty years old.

Worked for me.

Did your's just all of a sudden cut out? Or did you have a few times where you had a hard time starting your car?

I've found several bad ignition coils recently on carreras. Everything checks out and there's some orange spark but no start. New coil, blue spark, starts right up. I keep them in stock now just like DME relays.

What do the coils for a 964 run?

Have you tried putting your ECU in someone else's car? It's a pretty easy check compared to replacing the reference senders (especially with the engine in)

Wholesale from my supplier about $65.

Actually, my tech offered me a spare ECU. He said the chances of my car damaging his unit are very slim. It does look like a PITA to change out the sensors with the engine in-situ.

Note well that the current Bosch coils for the 964 (silver in color) are made in Brazil and have a very spotty record. Many 964 owners are changing to the 993 coil pack. Seek here and the other forum for good details; the largest gotcha is you need new coil-to-distributor wires as the length (and I think the connectors?) are different.

John, I had bought a generic 12V coil from our local Napster. It did not work. Do I need a special coil for my car?

Not so hard to change sensors. Also better to swap in a known good ECU into your car than a possible bad one into a running car.

To answer the OP: yes, entirely possible. Sensors can become gradually worse over time. Heat and vibration kill them. Ohm them out and for the speed sensor measure the AC voltage ~2.5V it generates while cranking. For the reference sensor measure the pulse with an LED test light or an o-scope.

And further, a bad ignition coil is possible but an entirely different failure mechanism. If the speed sensor or the reference sensor are not working there will be ZERO spark signals an d ZERO fuel signals. The DME will not go into RUN mode.

A bad coil, missing coil or intermittent coil will prevent proper spark but not impact fuel signals. So the diagnostics should first confirm whether the DME recognizes flywheel motion (NOID light on fuel injector plug). Then check if you get a proper spark. Following a lot of these posts here is a rough breakdown as I see it.

- no fuel, no spark: 45% it's one or both of the sensors, 45% no power (alarm, DME relay), 10% it's the DME itself
- no fuel, spark: likely the DME, can also be shorted or stuck injectors
- no spark, fuel: usually the DME but I can see a bad coil or ignition components downstream

Ingo

For the cost of one cheap BMW substitute flywheel sensor you can find a used oscilloscope on Craigslist.

- Replace, replace, replace = BAD
- Test, test, test = GOOD

Yes, the sensors can fail intermittently before flat out quitting.

We had an 85 with the original sensors. I would be zipping along and the engine would just shut off (tach dropping down to nothing) and then catch and keep running. A couple of "slow" starts occurred intermittently then one day BAM. Nothing - no joy.

I checked the pins (I think I followed a testing protocol from Ischmitz) at the DME and confirmed a bad sensor. As they are a PITA with the engine in, we replaced both. By "we" I mean that I diagnosed the problem and my long suffering husband did the hard part - LOL! ;)

angela

Ingo, not that I am trying to avoid sending my box out to you, LOL.

Is it hard to set the clearance for the sensors if the engine is in the car?

Hi Chris,

not really - one trick is to super-glue a penny to the sensor, then shove it against the FW. That's the correct spacing. Then remove the penny.

I've seen many replace these sensors and it should be entirely doable.

Cheers,
Ingo

Partial engine drop helps.

Check that all the plug connections off of the ignition switch into the firewall are fully seated. I replaced a turn signal switch and didn't get one of these seated properly (I had removed them for access). After driving over bumps for a couple of days one of them started to work itself loose. Car started cutting out on the highway but would start back up - first sign was the speedo or tach going dead before the engine cut out. Finally would not restart. Took about half an hour to figure it out. Probably not your issue but similar symptoms. Worth a quick look before buying parts.