Help with troubleshooting variable reluctor/pickup coil

[hodgepodge1983]

Hello everyone, here's my info and problem:

Vehicle: 1974 Jeep Cherokee 401 with MS-II v3 and GM HEI distributor (4-pin module). The distributor was purchased brand new, is about 5 years old, but only has about 7,000 miles on it.

Everything was working properly. I then did a tune-up by replacing the cap, rotor, and wires (the plugs were done a few months earlier). The truck run brilliantly that same day. The next day, I took it for a drive and again it was performing flawlessly. Then, while idling at a red light, the engine turned off by itself. Trying to restart once it gave a few sputters but it would not start. Trying a second time yielded consistent cranking with no audible detection of combustion.

Here is what I have so far:

0) MegaSquirt seems fine - all sensors give their correct values and it's counting tach pulses on the stim (this is important...)

1) The distributor is getting 12 volts in run and crank

2) The old coil tested below spec for secondary resistance, so I replaced it with a new HEI Brute Thunder HEI coil, and their high performance $136 module to control it (I eventually want MegaSquirt controlling the coil). After replacing those two items, the Jeep started but was misfiring severely and died. Restarted a couple more times (again missing badly), and get the same as before - cranking but no sounds of combustion.

The datalog shows that the tach count keeps dropping randomly. Eventually, no tach signal is received at all. I have attached a datalog of this event.

9-9-2010_new_accel_coil_and_module.msl

3) The distributor did not jump time and the cam/distributor gears are in perfect shape. Not chewed up and rotor is in the correct location.

4) The pickup-coil/variable reluctor is currently reading 818 ohms as I sit typing this, and shows infinite resistance when connecting each wire to ground. These are both within spec, but I do not know if they read consistently good while cranking.

5) I read that spinning the distributor should produce over 1 volt AC at the pickup coil wires. Spinning it by hand produces fluctuating voltages (of course), but I can get a steady .6 volts and a high of 1 volt if I spin it fast enough.

My hypothesis: The pickup coil or reluctor is bad. It is sending out a weak reading and after cranking a bit, too low of a reading for the 4-pin HEI module to pick up. Because of this, the module is not firing the coil or firing it randomly. How can I further test this without replacing the darn thing? Does this make sense? What are your thoughts? How does the pickup coil/VR go bad in the first place? I have not removed it yet but it does not seem to be cracked. Any feedback you can give would be greatly appreciated. For further analysis, I have attached a datalog of MegaSquirt connected to the stimulator, so you can see it is getting a steady tach count.

[8974Ollie]

You are on the correct path with the reluctor coil. Inspect the coil wires real close where they exit the coil housing. the movement of the vacuum advance will cause the wires to brake right there.
To check the coil for continuity, use a Multimeter. Connect the two test leads to the two reluctor wires, then move the two wires around. If there are is a brake in the wires watching for loss of continuity.

[hodgepodge1983]

Thanks for the reply, Ollie. I have since replaced the pick-up coil (NOT the reluctor on the distributor shaft), and it started right up! So, it seems that pick-up coils can fail even though they test "GOOD". I was able to get up to 1.2volts AC when spinning the new one by hand.

However, I now have a new problem I've never had before. I let the engine warm up a bit and about 5 minutes into it, the engine cut off. It restarted immediately, but after I revved it a bit, it cut out again. It will randomly turn off now, very cleanly, as if I were turning the key off. It always starts right back up.

I've attached a datalog.... it seems the tach signal is still dropping randomly, or there is new noise that was never there before. I have to say I am now using the Accel Brute Thunder coil and Accel hi-performance module. Should I swap them with the original coil and module to see if it goes away? Any ideas?

Here is the link to the datalog, since it is too large to upload here: http://socalfsj.servebbs.net/storage052 ... ndomly.msl

[8974Ollie]

Reluctor coil, Pick up coil, same thing.
From the symptoms you are describing there is still a loose electrical connection some where. What are the chances the new pick up coil is bad?,
Wouldn't be the first time.
Check for cold solder connections too. Check the grounds.

[hodgepodge1983]

Hi Ollie. From what I understand, the reluctor (or reluctor wheel) is actually the toothed-wheel on the distributor shaft, and the pick-up coil is the piece that it spins inside. Anyway, I'm pretty sure it's good because the engine does start and run....it just seems that the signal is getting out of sync due to noise or missed triggering events. I have read about the gap being crucial in this type of pickup, but can't see any way to adjust it? Any ideas?

The wires are good, all my connections are good, and grounds are good.

Here is some new info:

If I reduce the next pulse tolerance from 40% to 30% or less, it does not seem to die. It will, however, occasionally stumble and ALMOST die, then pick back up. This happens for a split second every 3 to 5 minutes....in the datalog, it looks like a rapid rise in tach count before returning to a linear rise.

If I increase the next pulse tolerance to 50%, the engine dies immediately.

Is there a tach-signal analyzer that is not part of Tuner Pro? I simply can't afford to purchase that program at this time.

Either way, I think my next step will be to return the NPT to my old value (40%) and swap the old coil and old module back in to see whether or not the new ones are causing the problem. No one else has any feedback?

[trakkies]

Have you got a strobe timing light? Or a spark tester? If so, check if there is a spark when cranking in the fault mode. A neon across the coil negative to ground will check the LT side.

IMHO, many of these sort of faults are caused by a problem in the wiring loom, so I'd check the feed from the coil to the MS very carefully.

[hodgepodge1983]

It seems the random stalls were being caused by the $136 Accel 4-pin ignition module I was using (Part number 35373). I put in the original module and that seems to have fixed it. However, I am getting tach spikes/dropouts at high RPM, which makes me think that this new Brute Thunder coil is too noisy. Will update as I progress. Any input is always appreciated.