I've just installed a SAAS 2" tacho in my ke20 5k. It seemed easy enough. Black to earth, Orange to (+) battery, Red to ignition, Green to (-) coil and White to dash lights but ran it to ignition instead so its always illuminated.

 

The tacho works fine when the engine is free revving . (ie: when the car is stationary). The problem is, when driving, it moves erratically and reads almost double the expected rpm.

I'm using a GT40 coil and an electronic dizzy.

 

Any takers?

 

PS: Yes, the tacho is switched to 4 cylinder....

http://www.rollaclub.com/board/topic/30183-old-corolla-tachos-and-electronic-distributors/

 

 

Not sure if this is your issue, but similar thread with a useful toymods thread attached.

It could be the issue indeed. I just don't understand how or why it's gone wrong.

Radio Frequency Interference.

Basically, the tach signal wire is picking up the electromagnetic discharge from the plug leads. At idle the voltage is average and doesn't effect the signal. But, at higher rpm and higher engine load the voltage output increases and each plug lead send an extra extra pulse through the signal wire to the tachometer effective doubling the reading....

 

Now. How to fix this?

Edited March 10, 201313 yr by GJM85

Go back to the stock coil & ballast resistor and see if it fixes it.

 

Shield the tacho wire?

 

New plug leads?

 

Not spikes from the alty by any chance?? Got a capacitor on there??

Yeah I don't have of the old coil and stuff anymore. Just read another thread on here. http://www.rollaclub.com/board/topic/44291-after-market-tacho-not-accurate-reads-double/

Apparently should have a rectifier diode on the signal wire.

I have rectified the issue....

 

I bought bulk diodes to help out fellow members, so if your having issues with your tacho there'll be a for sale thread shortly.

Edited March 12, 201313 yr by GJM85