I've got an electronic distributer from a truck with a reluctor and a pick-up coil and I seem to be getting intermittent output from it. A static test gives the proper resistance, now I want to test it running.

The question is, what kind of signal does it produce and how best is it tested with the engine running? Is it a very low voltage pulsed DC? And how best to test it while the engine is running so as not to interfere with the signal/running of the engine? I plan on back probing the plug at the ECU.

I've got a Simpson 260 analog VOM (20K Ohms/V), is this sensitive enough to measure what I am guessing is a fairly small signal without interfering with it?

I also have a fairly decent DMM but I don't know if the DMM is quick enough to lock on to this kind of signal?

 

You really need an oscilloscope to see the waveform.

Simpson 260s are great analog meters for general purpose use; but not really suitable for what you want to measure. Neither is the DMM unless it has an O'scope function.

The output will be a low-level signal that traverses ground when a reluctor tooth passes the sensor; ie: a brief excursion above and below ground. It will be at a low level. If you measure it with your meters, you will probably just read a constant average 0v.

 

Thanks for the quick response!

You really need an oscilloscope to see the waveform.

Simpson 260s are great analog meters for general purpose use; but not really suitable for what you want to measure. Neither is the DMM unless it has an O'scope function.

Yeah, I kind of suspected that it might take an O'scope

The output will be a low-level signal that traverses ground when a reluctor tooth passes the sensor; ie: a brief excursion above and below ground. It will be at a low level. If you measure it with your meters, you will probably just read a constant average 0v.

A signal above and below ground, so it is an AC signal?* My DMM has a Frequency function**, might that work? I'm not trying to read a value, but rather just observe the signal to see if it is causing the intermittent problem.

On my initial static test, I got 0 ohms resistance across the pick-up coil but observed that the plug was all gunked up and pretty corroded, so after I cleaned it up I got 314 ohms (within spec) and it ran fine for a day. Now it is back at it and I need to confirm that the intermittent problem is indeed with the pick-up coil, or in the wiring from it to the ECU, rather than in the ECU itself.


* I initially thought it might be a pulsed DC signal (can you say "points", D'oh!), but now that you point it out, I see that the magnetic tooth raises a small voltage in the coil as it passes and then it collapses, as the tooth passes, causing a reverse EMF ,,,, is that right?

**(500mV p-p sensitivity with an impedance of 1Mohm in parallel with 50pF, with a range of 1Hz to 2K MHz)

 

Well, you might be able to read it with your DMM. It's a mighty small signal, though.

If you can't read it with your DMM, and if you don't mind building a circuit, could probably whip something up from parts that you can buy at your local Radio Shack - if you have one nearby.

By the way, we have no clue where you are. That helps us help you a great deal when you need parts or supplies.

Click on the "User CP" link near the top, and then click "Edit your details". Scroll down to "Location", and at least fill in your country. State/province/etc and a nearby major city are also good things to put in there, but not absolutely necessary if you don't feel like sharing; time zone would be nice though.

 

If you have a soldering gun, place it near the reluctor and squeeze the trigger. That will induce enough voltage to light up your ignition system.