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Discussion Starter #1
My '90 SPG has been experiencing an occasional loss of ignition power. The symptoms are jerky shifting and sometimes complete cutout and loss of tach signal.

I have found some issues in the wiring harness (poor connections) that I will address. I have already tested for voltage at the distributor connector and it was over 10 volts as per the manual. I wonder if this is really a definitive test of the harness as it doesn't take much current to get a reading on a DVM.

To make diagnosis simpler, is there any way to reliably test the sensor in the distributor? I'd like to be able to rule that out as a factor. How could that be done and can it be done without disassembling the distributor?

I also have several extra distributors that need testing. I imagine some sort of heat source may be required to simulate the engine-compartment environment?
 

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It is not difficult to rebuild the dizzy. Check my post from a couple of weeks ago and read through to the end. I had a learning curve, that mostly boiled down to getting actual pin punches, thanks to Jim's recommendation. The rest is very simple.
 

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You can test the Hall sensor with a scope, but it's a bit of a pain. You basically need to test while driving, as you won't account for heat and vibration if the car is just sitting around. Watch the waveform, look for anomalies.
 

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Discussion Starter #5
Today, I cleaned up the harness I made to adapt an earlier style distributor to the '90 connector. Replaced some butt connectors, cleaned the connections, re-taped with silicone stretch tape, cleaned the cap and rotor and replaced the spark plugs that were worn. I don't know if worn plugs put more stress on the sensor, maybe not if all it does is trigger. Have driven over sixty miles so far without incident. More miles will tell me if it is definitely fixed. It would probably be a good idea to rebuild one of my spare distributors and bring it on long trips. Thanks to all for the advice everyone.

BTW, this new format is better, as I am now able to bring up the Forum on my phone. Haven't checked it out yet, but I hope it is now possible to post bigger pictures without using a third-party site. If you research old threads, many of the pictures linked to these sites have disappeared.
 

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Hall sensors can indeed be tested but before that make sure nothing else is wrong or broken? Mark the position of the distributor on the head and take it off the engine to have a really good look at it. Does the rotating vane pass through the gate in the sensor easily? Are any of the wires damaged or maybe broken?

Section 340 pages 6/7/8 in the 16v bentley manual have the process for ignition system testing on a turbo motor with distributor-mounted hall sensor.

Specifically the top of page 7.

I've done a similar thing to test a crank-mounted hall sensor (which are much more of a PITA to access).
 

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You have a touching faith in that test, but it won't reveal the ones that fail when they're hot, or the ones that test good but aren't. After being fooled a few times the first year they came out, I never bothered with that "test" again.
 

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100% agreed. That test is not reliable - the only thing it's useful for is knowing whether the sensor is completely wrecked, and maybe not even that reliably. Equating a waveform to an AC voltage is at best a budget approximation, and adding in vibration and temperature variables... bleh... not useful. Several years ago I spent the $100 on a Picoscope (software oscilloscope) and never looked back. Great tool to have around - you can test the Hall sensor, injector pulses, even the APC solenoid.

FWIW, spark plugs have nothing to do with anything... the Hall sensor signals the ICM and the ICM triggers the coil... or it doesn't. :)
 

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Discussion Starter #9
I suspected as much about the spark plugs affecting to Hall sensor. But, I am glad I did replace them as they were way out of spec.

I put another fifty-three miles on the car today, so it is looking like I found the cause this time. But for future peace of mind, I will have to do some research on the Picoscope, I imagine it could also be used to test oxygen sensor response?
 

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Agree that no test is foolproof, but any test that someone can do without specialised gear is good 'in the field'.
 
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