Is there a crank position sensor for the 4AGE, or does the pickup coil in the distributor do that instead?
Trying to diagnose a sudden tach randomly flopping/pegging while engine is struggling to keep running.

On the AE86 the tach signal is output by the igniter which takes the voltage spike
generated by the coil negative and passes it on There are two trigger wheels
in the base of the distributor. The one with more teeth generates the rpm input to
the ECU and the one with less teeth generates the crank position signal. Neither of
those trigger wheels has any direct connection to what the tach is doing.

To make it clear... the trigger wheels rell the ECU what the engine is doing, and
the ECU uses that information to tell the igniter when to fire the coil. A signal to the
tach happens every time the coil fires. So if the engine is struggling, the speed will
be erratic, the spark timing will be erratic and the signal to the tach will be erratic.

Cheers... jondee86

Clear.
Big thanks Jondee!

I've always said the igniter is electronic points, so that sounds pretty close.
But if the coil lies to the igniter, the tach WILL freak. And the engine won't be too happy either.
Over the past 5 years, 3 of my old Toyota's have had the pickup coil or igniter go bad. In every case, the tach floppiness was the smoking gun.

I seem to be having this same issue on my Silvertop. Wondering if it is possible to replace the pick up coils in the dizzy or do you have to buy a new distributor?

If the problem is indeed the pickup coil and not the igniter, I believe replacing it should work.
However, I have not done that. I have just swapped out the whole thing.

Well my intermittent miss and tach floppieness turned out to be a bad ignition coil. It tested out of spec when hot and that did fit with the symptoms which only presented when the engine bay temps got hot enough. I put in a new coil today and have had two long drives with no issues.

Rick Did you replace the ignition coil with a stock Toyota coil or did you find a replacement coil with about the same primary resistance. I've been thinking about replacing my 30+ yr old coil, with something that will fire a .040-.044" plug gap.
DaveW7

Dave I used the Toyota part number (90919-02197) at Rockauto and came up with a Delphi coil (GN 10175) for $45. Amazon has a big selection also. The Toyota dealer wanted $140. I've got almost a hundred miles on it now, seems to be doing fine.

Rick
Thanx for the info, Dave