Jumpimg TACH

[Doctor J]

As far as I know few of us who have successfully swapped clusters with the tach on the 2nd generation Camry are noticing erratic tach operation.
On my car the pointer used to oscillate during acceleration, during the cruise condition under light throttle or after upshift creating impression of slipping tranny. These spikes caused pointer to go up to 3000 rpm, jump there like crazy or just fall back to the normal rpm after second or two
However the engine itself seems to run normally and Sears motor tester tach hooked up to the tach connector on the distributor, was pretty steady while cluster tach was misbehaving.
I was able to restore normal operation of the tach by moving signal wire to it from igniter to the distributor test connector, however after more then 3 years the tach started “to jump” sometimes.
So I was trying to investigate if this problem is dash wiring/cluster related or ignition system related [the textbook says that the high reading of tach can be due to voltage spikes in alternator, or high resistance in secondary circuit, or electrical noise].
My first step was to hook up same tach as on the car [see pictures] to the igniter, and get power to it from lighter adaptor. My 3 mile test drive indicated that the igniter signal caused “test” tach to jump, but the ‘cluster” tach was steady. That means the problem is ignition related. I will repeat the same test later as the cluster tach was not jumping during the first test. If they will jump in “unison “ the same reason causes them to do that.
The igniters were swapped before; the ignition coil is replaced with newer one and has no arching to the distributor shaft –very commonly seen; the rotor is new. The noise capacitor that sits atop the distributor came with the ign. Coil but is not new.
The wire from negative terminal of the ignition coil to the igniter has no breaks.
After second test I would swap used cap with wires [one wire is “open” and check if it changes the tach indication in theory this should give more spikes].
I could probably use “tachometer filter” used on first generation Supra and install it between igniter and tach on the signal wiring. Thanks for reading.



Band-aid hook up to distributor terminal ; my car's tach reads from here

the test tach hookup at igniter


Ready to go ;magic tape holds the tach


Test tach powered from cig. lighter socket; the gray wire goes to igniter


Another view of the hook-up

[atobe]

interesting... i never had jumpy tach after my swap

[JMSports]

Originally Posted by atobe

interesting... i never had jumpy tach after my swap

Mine is fine.

[71Corolla]

Mine has had the jumping problem for about 3 years, it is slowly getting worse.

I have never replaced any ignition parts (besides plugs) in over 400,000 kilometers I guess I am due?

[Doctor J]

I swapped distributor cap and wires and there were no changes.
However on the road test doing ‘street start’- rolling, then accelerating to 15 mph I noticed a flat spot at the beginning of acceleration [that commonly caused by vacuum leak, TPS problem etc, and it caused 800 rpm jump on the test [igniter hooked] tach but not on the distributor hooked cluster tach meaning it IS ignition related.
The rpm is calculated by the tach by measuring time between the impulses and instantaneously converting these into magnetic field that moves pointer.
So, the magnitude of impulses or average number of impulses per second is not considered by the tach.
The good idea can be to use scope while riding on the ‘treadmill’, as most of the times the tach misbehaving when car’s engine is under load.
Interestingly enough, that on my 79 Supra the ignition coil was arching from its tower to its negative terminal; this destroyed igniter , but never showed up on the tach , due to inline noise filter.

[71Corolla]

I did some close observations today. I noticed that in a fairly narrow vehicle speed range, I could make the tach jump from about 1500 to 3000 by only a slight change in throttle. By slight I mean just enough to slowly accelerate the car. The slight change in throttle causing the large jump in the tach probably rules out alternator noise or electrical noise from the high voltage.

That says to me that something is wrong with the throttle position sensor, or how it is interacting with the rest of the system perhaps?

[71Corolla]

Originally Posted by Doctor J

...as most of the times the tach misbehaving when car’s engine is under load.

Next time I'm driving I'll pay attention to the tach while the car is heading towards and then on an incline. I won't move the throttle at all and see if under load it does make the tach jump.

[JMSports]

Is it weird that my speedo wavers a little bit between like 10-40mph? Anyone else notice this. My old cluster didnt, or maybe i didnt notice it.

[Doctor J]

Update on the "jumping tach"
2 different igniters are causing different jumping habits for the pointer,
disconnecting of TP sensor does not affect it.
I would probably use noise filter from older cars between the igniter and tach to clean the signal.
As I mentioned before the Sears tune up tach is not jumping and is indicating correct RPM while hooked up instead of the factory tach.

[viktord1]

Maybe your car wants you to drive a bit faster
I've seen this same problem on a few Mercedes 190e's.

[Doctor J]

It looks like adding the tachometer filter from 1978 Toyota celica to the igniter solved this problem