Tach works but not accurate?

[86-blk442]

Hey guys the tach in my 86 442 works but is not accurate. It reads about 500 higher than it should across the board. I've already checked the jumper in back and its soldered. There was never a plastic piece in there its a wire and i know it is original to the car. The original 442 tach has the wire soldered across both terminals, but I also have a tach out of a 1986 salon with 307 that only has the jumper soldered across the 8 cyl. terminal. I've tried both tachs in the car and neither one reads different. They read the same, about 500 rpm high.

Next I consulted the service manual and noticed that terminal 14 on C1 for the gauge cluster is only supposed to have 10 volts on it. With key on engine off I am getting full battery voltage (about 12.6v) from this terminal. Could this be causing my problem? This is the white wire that comes off of the distributor. My car no longer has the computer controlled distributor in it. It has an accel performance replacement #59113 unit sitting on top of my 355 olds. A while back I had a Sunpro tach hooked up in the car and it read lower than the factory unit (just like my timing light is reading vs. the factory tach).

Any suggestions? Thanks guys!

 

[Peter]

i replaced a tach in my fathers trans am one time and it read about half of the actual rpm after it was installed so we returned the tach to the manufacturer and got another and it fixed the problem so it was the tach itself. as far as diagnosis for it i have no idea

 

[dudeface]

this needs to be a sticky thread....................always the tach question. took me and a friend years to find a valid answer pre internet. so here ya go, spread the knowlage far and wide...................peace


http://tech.oldsgmail.com/eint_tach_fix.php

 

[86-blk442]

In my first post I stated that both tachs ive tried have already had the wires soldered in place. As far as I know they have always been like that. They never had the plastic jumpers or they were removed in favor of the wire years ago... The jumper leads are not the issue.

 

[79cutlasssalon442]

If anyone has it apart. Whats the size on the tach? Looking to replace mine with an aftermarket right in the rallye cluster

 

[86-blk442]

If anyone has it apart. Whats the size on the tach? Looking to replace mine with an aftermarket right in the rallye cluster

That would be some work... Definitely easier said than done. The hole in the front plate of the cluster is approx 2 11/16".

Heres what a cluster looks like inside with tach removed: