TexJJ

So for any of you that remember, my dad and I have been tracing this RANDOM engine jerk that occurs at a time of 200 miles to 5 miles apart. I can go a week in city driving with no jerk, or start it up and go a few miles and BAM! Normal symptoms will be running along the highway in 5th gear say at 65mph, all is well then a sudden jerk, the tach drops to say 500 (which can't be the case as there is no way the tires or clutch slip that much.

So we have replaced just about everything from ECU to plugs all to no avail. ALL THE ELECTRICS are new including the BIG 3 upgrade. It feels like an electrical issue, but we just weren't able to trace.

So this weekend we pulled out the factory service schematics where we found an interesting tie in point. Apparently the igniter sends a signal (which I think is a switched ground) to the tach AND the A/C amplifier. From what I can tell, the amplifier compares the engine speed and the compressor speed to insure that no lock up has occurred as a safety valve. We hypothesized that perhaps the A/C amplifier is grounding or sending a positive signal through the line which would 1) cause the tach to drop and 2) spike the igniter causing a momentary jerk in the ignition.

We disconnected the a/c amplifier taking advantage of the good weather and drove it on a 300 mile trip which would ALWAYS cause at least 1 or 2 jerks and NOTHING. The 4runner ran PERFECT! So I think we have isolated the problem, but does this make any sense? Does anyone have any schematics for the actual A/C amplifier circuit itself. It seems like a pretty simple pc board design.

What do you all think? Have we exorcised the demons or should I go get some Holy Water?


Thanks,

JJ

toyota_mdt_tech

The tach dropping tells you its a primary ignition problem. This will be revealed in the form of a trouble code. Fetch codes and report back!

TexJJ

Well never get error codes! We've tried that numerous times. But good try. New ignitor and coil nonetheless although original is all within spec. Wires check out.

HAVOC

what year runner?

91ToySR5

i had this problem with my 91 pickup when i first got it. usually after going up a grade in 5th gear on the highway - i'd let off the gas slightly as the road leveled out and i'd get that hesitation - the tach needle drops approx 500rpm and if u look real quick the CEL flashes real quick. after i set timing to approx 12 deg's it never happend again. after i replaced my motor a year later and had stock timing i get it once in awhile again. i'm sure if i turn back timing to 12 deg's it won't happen. from talking to a toyota parts manager, he had mentioned they had some problems with the inards of the distributor where they'd start to pull apart. try changing ur timing and see if it goes away.

toyota_mdt_tech

Well, a few items that wont set codes would be secondary ignition system, injector/injector circuits and anything fuel ie fuel pump, pressure etc.