Tachometer

[CloudyB]

Tachometer was working great.Drove my car(70 Javelin SST 390 4sp.) today with the kids.Coming home noticed tach jump all the way to 8,000rpms while going 40mph. Let the car sit for an hour and started it up.At Idle the tach was reading 4,000 rpms. Also noticed when I turn the power on without starting the car I notice the tach jump a little and set back down to 0.Never have this happen before.Checked all wires under the hood look fine.
Is it shot???? Any suggestions??? Thanks for your time.
Chris

[javelinjeff]

call tony @ california classic amc,(619) 423-0364,if anyone would know how to save or have an original one to replace it with,he would be the one to call

[javelinjeff]

that tach issue has me scratching my head (and a few other places).are you sure no wires got fused when that horn lead got hot?try connecting an aftermarket tach to the leads at the back of the instrument cluster and see if the same thing happens...if so it is not the tach itself.if an aftermarket tach works proper WHEN connected exactly where the stock one connects then we can assume your tach is failing although i have NEVER heard of one doing what yours is up to

[CloudyB]

Thanks again Jeff.Gonna go under the dash again today and do what you said.Like to go to a car show today from 12-4 but I let a guy park his trailer in my driveway for a day and I call just answering machine.He's got the locks on it all over the place.lol.lol.
Thanks Jeff. I'll tell yea what I find.

[javelinjeff]

even a cheap tach/dwell meter would work as a test for this tach issue.the main thing is to do the connection at the instrument cluster(last point).this means you may have to make some temporary leads to connect to---aint you glad you have those wiring diagrams!!!i notice no one else is posting replies on this issue,you definitely have a GREMLIN in that JAVELIN somewhere.maybe that car needs to be in MY garage(lol)....i promise SOME tread would be left on the tires when i got done with it!!!!!!

[CloudyB]

LMFAO.lol.lol.Boy do I have Diagrams for that gremlin, I MEAN Javelin.lol.
I also know a ex AMC service manager that now lives in HAWAII.Sipping on cocktails under a palm tree with a harem full of women following him around .
HE has been helping me out between the cocktails and women.lol.

[donsjave]

I have a 70 AMX that did the same thing and it took me a while to figure it out. The tac itself was fine but what it was did somewhat surprise me a little. It was the yellow wire that was connected to the relay switch for the starter. AMC tech manual says that this wire is disconnected if you have a tac that is original from the factory. For some reason this wire was grounding out somewhere within the loom that it was in, fixed the grounding out problem and havent had a problem with it since. My tac was reading about double from what it actually was and would not do it all of the time. Just found it to be an inconviencent thing and it did take me a while to fix it.

[CloudyB]

Thanks for the info donsjave.Went to a car show yesterday and tach acting like you said .(showing double).After the show the tach was fine going home.
Going to check that yellow wire from the relay to the starter.Oh mine is a 70 also
Thanks.

[Pauls70AMX]

Cloudyb did you fix the problem? The yellow wire should be the answer to your tach, or ground wire.

[CloudyB]

Thanks Paul and Dave.Yea I got it.