Risks of Using Lower Resistance Pickup Coil (a.k.a. "Signal Generator") in Distributor?

Thread Starter

honda.lioness

Joined Apr 10, 2018
5
Will a lower resistance distributor pickup coil damage the igniter to which the pickup coil sends a voltage signal?

Detail:
My 1992 Toyota Pickup's distributor pickup coil broke . The function of the pickup coil is to send a signal to the igniter (a.k.a. "ignition control module"), which then processes the signal to make the main coil fire the spark plugs at the correct time. I believe the signal from the pickup coil is about 1.5 volts. Today I replaced the shredded pickup coil with a pickup coil from a 1988 Toyota Pickup. The ohms ratings are different for the two pickup coils.The 1992 truck starts fine. The check engine light went away and the codes stayed clear. I drove the truck for half an hour, and all seems fine.

The factory service manual specifies that:
-- the 1988 pickup coil have a resistance of 140 to 180 ohms (neither hot nor cold conditions specified). At cold, it measures about 165 ohms.
-- the 1992 pickup coil have a resistance of 185 to 275 ohms when cold, and 240 to 325 ohms when hot. At cold, it measures about 200 ohms.

If out of spec, the manual says to replace the housing.

Below are photos of the pickup coil, igniter and schematics. More detail about how the igniter works appears at:
http://web.archive.org/web/20080827....com:80/hondafaq/igniter-operation/index.html

I am wondering if the lower resistance of the coil leads to higher amps, resulting in more power dissipation inside the igniter, perhaps overheating it or at least shortening the igniter's life?

Will the 1988 pickup coil damage the 1992 truck's igniter?


download (1).jpg igniter_close_2_sml.jpg igniter-operation-1.gif
 

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BR-549

Joined Sep 22, 2013
4,938
Your talking about a pick up coil....but showing the ignition coil.

Which one are we talking about?

Which coil did you measure?
 

MrChips

Joined Oct 2, 2009
24,232
The pickup coil is used for timing information only. Not a lot of current flows in the pickup coil.
You'll be fine.
 

Thread Starter

honda.lioness

Joined Apr 10, 2018
5
BR-549, the schematic I attached shows the pickup coil within the boundaries of the distributor. This matches the physical reality: The pickup coil is mounted inside and on the distributor housing. The schematic also shows the ignition coil. The coil that broke on my 1992 truck's distributor is the pickup coil. I measured the pickup coil's resistance.
 
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Thread Starter

honda.lioness

Joined Apr 10, 2018
5
MrChips, thank you. I wish I knew why Toyota, sometime between 1988 and 1991 or so, kept the same physical shape, dimensions et cetera for the pickup coil but then presumably added more loops to the coil, increasing its resistance. Maybe overall, the manufacturer wanted a higher voltage signal from the pickup coil to the igniter?

At the salvage yard, I could not find any other Toyota models that use the same shape and dimensions of pickup coil.

I ordered a new aftermarket distributor from eBay today. This was both because I think the old dizzy is showing signs of wear (bearing and electrically) and because I want an original design of pickup coil for the truck. I am a little leary about the aftermarket aspect but figure the truck may not have too many years left in it anyway. E.g. the compression in the cylinders may be diminishing.
 

ian field

Joined Oct 27, 2012
6,539
The pickup coil is used for timing information only. Not a lot of current flows in the pickup coil.
You'll be fine.
Generalisations can be dangerous. The current might be small, but it probably feeds an IC input. It should have adequate protection - but finding out it hasn't is probably expensive.

Motorcycle ignition black boxes usually have an input C/R network matched to the PU characteristics - its all or part of the automatic advancer.
 

Thread Starter

honda.lioness

Joined Apr 10, 2018
5
ian field, the pickup coil sends a signal to the igniter. From discussion in the factory service manual, I think it is a roughly 1.5 volt signal. The igniter includes a Darlington transistor and chip.

BR-549, I think the 1992 truck is a tad sluggish with the 1988, lower resistance pickup coil. I could be wrong about this, of course.

Alec_t, thank you for the theory.

Tomorrow I replace the ignition coil itself (since this all started with the engine misfiring after warmup).

I will minimize driving until the new aftermarket dizzy arrives.

I appreciate the discussion. It adds to my electronics understanding.
 

ian field

Joined Oct 27, 2012
6,539
ian field, the pickup coil sends a signal to the igniter. From discussion in the factory service manual, I think it is a roughly 1.5 volt signal. The igniter includes a Darlington transistor and chip.

BR-549, I think the 1992 truck is a tad sluggish with the 1988, lower resistance pickup coil. I could be wrong about this, of course.

Alec_t, thank you for the theory.

Tomorrow I replace the ignition coil itself (since this all started with the engine misfiring after warmup).

I will minimize driving until the new aftermarket dizzy arrives.

I appreciate the discussion. It adds to my electronics understanding.
If a PU mismatch causes too much advance - it increases the chances of holeing pistons under load. Not enough advance is inefficient - it will probably result in more unburnt gas in the exhaust. the Lamda probe detects this - what happens next depends on what the ECU does about it. If it commands more advance, it might partially correct the problem. If it makes the mixture leaner - that's another route to holed pistons.
 

shortbus

Joined Sep 30, 2009
8,817
If a PU mismatch causes too much advance
Just how does this mismatch change distributor advance? Advance in a distributor like this is done by moving the plate it is bolted to, not by the output of the pickup. With a lower resistance in the pickup coil if anything it will just put out a lower voltage to the "igniter"(ignition module).
 

BR-549

Joined Sep 22, 2013
4,938
I don't know that it could, that's why I asked. Does the unit still use a mechanical advance?

I am not an expert.....but I would think the PU coil signal is compared to crankshaft position. This would give a TDC reference. If the PU coil is involved with the timing compare reference....this might effect acceleration.

Along with the current environmental algorithm of course.
 

ian field

Joined Oct 27, 2012
6,539
Just how does this mismatch change distributor advance? Advance in a distributor like this is done by moving the plate it is bolted to, not by the output of the pickup. With a lower resistance in the pickup coil if anything it will just put out a lower voltage to the "igniter"(ignition module).
If the distributor has automatic advance weights on the shaft - it probably doesn't.

Purely electronic advance became commonplace where a network of resistors and capacitors selectively delayed the pulses by frequency. It could be curve mapped in ROM - I didn't bother trying to find the service manual.
 

shortbus

Joined Sep 30, 2009
8,817
If the distributor has automatic advance weights on the shaft - it probably doesn't.

Purely electronic advance became commonplace where a network of resistors and capacitors selectively delayed the pulses by frequency. It could be curve mapped in ROM - I didn't bother trying to find the service manual.
Any that do the advance with a computer/ECM that I'm aware of do it by comparing the crank and cam sensors to the throttle opening. And do it in the ECM.
 
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