Need some Inliners help

[Geektrucker]

81 c10, 250 inline 6. Block and 883 tranny from 67-72 car. I've been having a random "shut off" for months. Regardless of cold, warm, or hot engine, could be driving along or at a stop the engine will just turn off for no reason.
Had problems a couple months ago, cleaned up some wiring and re-worked dizzy bat connection, everything seemed fine.
Day before yesterday went on an errand and twice I ended up coasting to side of road. Get out and wiggle everything, fires right back up.
Through process of elimination ( re-did ALL grounds..again, check entire wire harness in engine bay for any breaks.. non found), I eventually unplugged the connector to the oil pressure switch. NOT the oil pressure sending wire (gauge still operates fine).. and that seems to have cured it.. At the moment.
So..could a faulty oil pressure switch create those symptons or am I chasing electrical gremlins here?
Any help or thoughts are appreciated. Pic is of 3 wire connector I unplugged...top wire goes to gauge. I have not been able to source the switch, just the sender which is available everywhere.

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[Jarhead79]

Not that this is your issue but something I had happen while temporarily installing a tach. I accidentally had the power and ground wires touch base metal and it shut the truck off....wondering if the wire you disconnected was doing the same thing, also I had this happen once while driving, my wire that goes into my dizzy, it wasn't connected hood and it caused my truck to stall while driving.

 

[Geektrucker]

Not that this is your issue but something I had happen while temporarily installing a tach. I accidentally had the power and ground wires touch base metal and it shut the truck off....wondering if the wire you disconnected was doing the same thing, also I had this happen once while driving, my wire that goes into my dizzy, it wasn't connected hood and it caused my truck to stall while driving.

I thought it may have been grounding, which is why I took the whole loom / harness apart to check for any chaffing or broken wires..but no luck. This acts EXACTLY like the dizzy wire falling off..right down to the "fast cranking" when it won't refire immediately. Which is why last time this acted up, I replaced the dizzy connector, but to no luck.
I'll be able to tell in the day or two going back and forth to work. Hoping someone with old school inliner experience can shed some light on diagnosing this switch.

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[chengny]

That switch is used to energize the heater coil in the electric choke assist. I can't really see in the image, but it looks like the same harness connector/pressure switch setup as is used on a V8.

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Even if the switch isn't exactly the same physically, it's function and closing pressure are identical to the switch used in the V8 setup.

In the early 80's the EPA was pushing hard to enact new regulations. One concern was that engines were being choked longer than necessary. They mandated that the choke begin to open immediately after the engine was started. Also, they wanted choke opening to be automatic (i.e. the driver/operator taken out of the loop). GM's method of complying with that requirement was to supply power to the electric choke through a pressure switch that closed as soon as oil pressure was established. So, when the oil pressure reaches about 10 psi, that switch closes and supplies power (coming in on the PNK/WHT lead) over to the choke heater coil (on the LT BLU lead).

Power is also supplied to the DK BLU lead which wired back to the ground leg of the CHOKE warning light on the dash. The lamp's positive terminal is always hot in RUN. When the oil pressure switch closes, the negative side is also hot. With power applied to both sides of the lamp, there is no net flow of electrons through the filament and the CHOKE goes out. The circuit can function like this because the choke heater is grounded through the carb/manifold/engine block.

These show the how the circuit operates - they show a V8 but the circuit is identical for a 250:

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As in an 81:

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Anyway, about your question:

So..could a faulty oil pressure switch create those symptons or am I chasing electrical gremlins here?

I am working on the assumption that you believe the problem is ignition related. More specifically, and I don't mean to put words in your mouth, you think there is an intermittent loss of power to the primary side of the coil.


I can't think of any problem - in that choke circuit - that would cause a loss of voltage on the pink lead to the coil's B+ terminal.

 

[Geektrucker]

I am working on the assumption that you believe the problem is ignition related. More specifically, and I don't mean to put words in your mouth, you think there is an intermittent loss of power to the primary side of the coil.


I can't think of any problem - in that choke circuit - that would cause a loss of voltage on the pink lead to the coil's B+ terminal.

@chengny thank you for the great technical explanation. Based on your response, I'm chasing a gremlin.
I say it's more like the B+ dizzy wire coming off as when it dies, it stops firing on the cylinder... Usually with a stall, another cylinder or two will attempted to fire. When this happens, it just plain stops the engine.
Any idea on where to look next for trouble shooting?



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[chengny]

I was thinking more about this last night.

You seem to feel confident that the issue can be attributed to a loss of ignition/spark - due to a power supply issue to the primary windings of the coil. I am assuming that the no-spark condition was verified - during one of the many prior stall incidents - by pulling/grounding a plug and cranking the engine. No spark was observed, correct?

Also, another important detail is whether any other electrical components are effected when these stalls occur. If nothing else loses power, that would effectively eliminate everything (including fusible links) from the battery, across the starter and firewall junction block, through the firewall block connector and right up to the BAT 3 terminal on the ignition switch. All that would remain to cause a power drop to the coil would be the ignition switch - and the single pink lead that connects the switch (terminal IGN 3) to B+ at the coil.


Another assumption - you are running an early style HEI distributor. It has only one external lead connected to the primary (the pink lead that runs from the ignition switch to the B+ terminal on the cap). There is no ESC or EST involved.

You have thoroughly inspected all the wiring associated with the primary ignition circuit. I know from experience how much work is involved to strip the wiring from the loom - and that it's even harder to stuff it back in. I'm sure it was the last thing you wanted to do.

So, knowing that, it's probably safe to assume that the easy stuff has been eliminated. For example:

1. Distributor internals have been cleaned/inspected and checked electrically or replaced. This includes the ignition module, pickup coil and the two wires that connect them. The coil is good. Also, the wires into the ignition coil shell are okay and the negative pin is properly grounded via the ground strap. If there is any doubt at all that the coil may be failing - change it.

2. The main components in the secondary, like the cap, rotor (and wires if in really bad shape) have all been replaced with known good parts.

3. The vehicle's primary charging system and battery are working properly. System voltage is always normal (about 13.5 volts with the engine running).

NOTE: In order to generate the 40,000 volts required at the spark plug gaps, the HEI system requires an uninterrupted power supply to the primary of at least a 10.5 volts. Below that, the quality of the spark diminishes quickly - it turns into a feeble yellow trickle instead of the bright blue arc that indicates a good spark.

So, if all that has been checked, you might consider trying this:

Rather than running with the harness connector pulled from from the oil pressure switch, it might be a better diagnostic method to supply the B+ terminal of the coil with an independent power source (independent from the ignition switch and associated wiring.

All it would take is two long leads and a cheap SPST snap switch. Wire the leads to the switch (in the cab) and run the other ends out through the firewall. Connect one lead to the battery positive. Pull the pink wire from the B+ terminal at the coil and connect the other temporary lead to that terminal. Use the switch to supply the primary when running the engine. This will have no effect on any other circuit:

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Good switch cheap:

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Radio Shack
Catalog #: 2750602
$3.49

But, before doing the test as noted above, you might want to just leave the choke heater circuit unplugged from the switch and drive around normally. Wait and see what happens.

If the stalling condition returns, then try the temporary power supply to the coil. You'll know it was just a coincidence and that the choke heater is not the problem.

On the other hand, if weeks/months go by and you never stall again...well I guess I was wrong and the choke heater circuit is somehow effecting the power to the ignition primary.

 

[Geektrucker]

So things just stranger... Went to make the donuts this morning, made all of half a mile when it died. Nothing, no start, no attempted to crank.
Hopped out to check wires, and notice this:

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Well THAT'S not supposed to look like that! Long story short.. My dad @round1 came the rescue and replaced the solenoid for me while I was at work. That enabled me to drive it back home, where we found the positive battery cable was finger loose... BUT, she died idling in the driveway..once. Then we couldn't get it to to do it again...

I'm stumped [emoji35] [emoji36] [emoji24]

 

[89Suburban]

So things just stranger... Went to make the donuts this morning, made all of half a mile when it died. Nothing, no start, no attempted to crank.
Hopped out to check wires, and notice this:

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Well THAT'S not supposed to look like that! Long story short.. My dad @round1 came the rescue and replaced the solenoid for me while I was at work. That enabled me to drive it back home, where we found the positive battery cable was finger loose... BUT, she died idling in the driveway..once. Then we couldn't get it to to do it again...

I'm stumped [emoji35] [emoji36] [emoji24]

Doh!!!

 

[Jarhead79]

The cable being loose could've been the culprit...seen that before at my part time job. Also curious if you have a bad coil or maybe the dizzy is bad?

 

[Geektrucker]

The cable being loose could've been the culprit...seen that before at my part time job. Also curious if you have a bad coil or maybe the dizzy is bad?

Our thoughts too.. I have a jumper wire rigged up ready to go direct from batt to dizzy the next time it does it.

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[Geektrucker]

Step-son showed up.. We tinkered a bit, tore into the dizzy. We did notice..and fixed, the ground spade crimp off the ignition control module was sort-of loose, and came apart when we removed it.

Quick test drive didn't show anything (ran fine).. So, I'm cautiously optimistic. We'll see how it goes in the morning.

Oh, and by the way...we're at a dew point of 71, temp in the mid-80s, so just breathing produces a profound sweat.. I don't know how you southern boys do it! [emoji91] [emoji91] [emoji97] [emoji97]

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[HotRodPC]

Sorry, I just seen your last post in the other thread asking me to check this out.

So do you think you have it figured out?

So when this intermittent stall happens, does it usually start right back up or does it have to cool down a bit before it starts up again? I've seen coils go bad like this where it'll just shut off, cool down and start right back up.