Hello,
The Tach on my 2001 4-runner started reading erratically yesterday at low speed, even though the engine was not speeding up or slowing down with the tach. One time, it stayed at zero rpm at a stop light, with the car running. At another stop light, it was jumping between zero and 1500 rpm.
I will appreciate any help as to what may be causing this, or what to fix on the vehicle.

Thanks
OO

 

Does the needle always drop like it lost electrical power to the gauge and then it comes back up, or does it spike the gauge too?

 

It does both. I initially noticed the needle spikes, then noticed the drops later. The engine does not rev up or down with the needle movement.

 

that is connected to the igniter
http://www.ncttora.com/fsm/2000/Wiri...ith%20Tach.pdf
look for shorts or bad ground

 

I agree w/ Eddie. A bad hot connection usually results in the needle dropping (loosing electricity). If the needle spikes it's usually grounding out somewhere. I think yours sounds like a short or bad ground wire/connection to me too.

 

I have a similar problem in my 92 cab, just wondering where is the ground and sender unit for rpm gauge

 

as I said before the tach is connected to the igniter there is a wire from the igniter that goes directly to the tach. the ground would be the other wire to the tach. this is a basic design. in after market tachs there are two wires one goes to a spark plug the other to ground.
it will double the spark count and give that as the rpm since the spark on most vehicles only fire once for every two times the engine turns over our trucks however fire every time due to the waste exhaust spark design. you are looking for a broken grounding out wire between the igniter and the tach. or a bad ground or both, could be simply a bad connection at the back of the tach.
all of the fsm's are at
http://www.ncttora.com/fsm/index.html
find yours and do the run down.