optispark bench testing

Anyone have any good info on bench testing the distributor? I pulled the distributor out of the car (it ran OK) and I had previously picked up a customer return Accel distributor in like-new condition for less than the cap and rotor alone would cost. I built a test rig on my work bench and used a wiring harness to power up the sensor and monitor the hi and lo resolution outputs with an oscilloscope.

On the working distributor I pulled from the car, I get a nicely defined set of pulses at about 2V from the hi-res signal, and an extremely weak (100mV or so?) response from the lo-res output. On the accel I get pretty much nothing from either output, so I guess the sensor assembly there is dead.

The 20 year old GM distributor is in pretty good physical shape, I see no evidence of corrosion or leakage inside. The input shaft bearing is shot, I think. It wiggles around enough to trigger the hi-rest output without turning. The cap and rotor are of course in pretty rough shape.

On the other hand, the Accel distributor is in like-new shape. The bearing is new and a bit stiff, and I can feel the rotor actually hitting some of the posts in the cap.

So, question time:

Is the output I see from the GM sensor normal? I would expect the same amplitude from both outputs.
Is the rotor hitting the terminals in the cap normal for a new distributor?
If the optispark sensor is bad, what's a good replacement? I see online mixed results with different manufacturer's sensors.

From everything I've read, the delco optis are the only reliable replacement. The aftermarket ones can and will fail, and it's always the sensor to blame. The original sensor was made by Mitsubishi. I've messed with a couple aftermarket ones before and have had an issue with a clunking sound on one of them, probably the same issue you see with yours. The difference is mine didn't do it until it got hot. I sent it back and reused the old opti with a new cap and rotor.

Well, I can tell you the sensor in the stock distributor is Mitsubishi.

Just for the interests of science, I got an AIP distributor, since it was only a few bucks more than just getting one of their sensors alone. (they seem pretty proud of their sensor technology, we'll see if it's any good!)

So what I was able to determine is that the pulses output by the AIP are on the 100mV range, similar to the Mitsubishi's low-res output. The high-res output on AIP's is about the same, whereas on the Mitsubishi the hi-res output pulses are significantly higher. It's unclear to me without a third control to validate which one is right, but it makes me believe the Mitsubishi sensor is probably still good. Or, at least, good enough. It ran with it, after all.

It also gave me the opportunity to compare the AIP, stock, and Accel distributor's components. The AIP and Accel caps appear identical in pretty much every way I can detect visually. The font on the cap, etc. all appear the same, so I assume they're coming from the same manufacturer. Only difference is the Accel's big sticker on the front. The screws that hold the cap down are different as well. The AIP ones differ in diameter from the Accel and stock units, which use the same screws.

The rotor on the AIP unit, along with the plastic shield inside were white and much more crudely molded than the Accel distributor, which has those components molded in black.

The sensors on both the AIP and Accel units differ. The interruptor wheel appears the same in both. The way they mount is slightly different; the spacer goes on the top of the Accel unit, unlike the stock unit.

Using the two distributors, I took the Accel cap, rotor, and shield and assembled them with the AIP screws to the AIP sensor/backplate assembly. I put a thin film of dielectric grease on the rubber gaskets and used a little threadlocker on the rotor and cap screws. I'll give this frankendistro a try once I get the motor back together.

Incidentally the physical interference I detected with the Accel distributor together (rotor rubbing some of the cap's terminals) seems to be due to the Accel backplate not being square. My guess would be someone tried to seat the unit on the engine by cranking down its mounting bolts and bent it.

If I can get hold of another optispark sensor I'll try it on the bench again and see what its output pulsetrain looks like.

Forgot to update this. Since I wasn't able to determine where the vacuum leak was on my AIP/Accel distributor I went and got a Delphi one. It has a Matsushita sensor in it, as expected. I benched both side by side and their behavior (electrically anyway) was the same. I ran a bead of ultra black around the cap on the Delphi unit just for safety's sake and got that installed. On the bench I was able to determine that the vacuum leak is at the bearing. I'm not sure that's fixable.

Bench testing an opti spark?
first time i,ve ever heard of anybody doing this,.
maybe hook the scope up to your left and right ears simultaneously, give your head a spin,.
see what kinda pattern you get??


You have way too much time on your hands,.

go wax the car,..

don't scratch it,.

Yup Nick. Actually the wagon could stand a good waxing, good idea!

I'm not sure I could get the trace to go slowly enough to bench test my head on a swivel stool, but it's certainly worth investigating

Good work on your investigation - it's exactly what I'd do if I had the time.

The other thing you could do is put the opti back on the motor, reinstall the damper and belt, and then start it up with no water pump. It'll be fine for the 30 seconds it'd take to make sure all cylinders are firing.

Well, it was interesting to do it. Plus leaving the unit on the bench powered on for 24 hours seemed like a good idea. As is firing up the motor when the water pump is still off. You don't really even need the belt installed for that if you really want to be lazy.