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Thu Aug 07, 2014 9:37 am

Tach works fine , until headlight switch is activated. Then the tach reading drops to 200 RPM and almost stays there all the time with lights on. The light wire from the tach is hooked into the gray/blue wire. When you disconnect that wire altogether the tach stops working. Any ideas what would cause that?

Nova73
Akron, PA

Thu Aug 07, 2014 11:15 am

Light wire? The tach should be illuminated from above as is the speedo. Sounds like you have something crossed where by luck the tach is getting a ground through the instrument illumination rather than the chassis. When you turn the head lights on, the ground becomes power. Grounds are funny things, on small circuits they will actually work through the bulb filaments.

Sat Oct 11, 2014 7:28 pm

Quote: Tach works fine , until headlight switch is activated. Then the tach reading drops to 200 RPM and almost stays there all the time with lights on. The light wire from the tach is hooked into the gray/blue wire. When you disconnect that wire altogether the tach stops working. Any ideas what would cause that?

Any solutions nova73? I now have the exact same problem!

Fri Oct 17, 2014 4:24 pm

Quote: Grounds are funny things, on small circuits they will actually work through the bulb filaments.

Right on rsxsr. So I basically had this same problem. Headlights on and the tach (as well as other warning lights and gauges) went all wonky. Turns out the ground on the wiring harness into the back of the tachometer was bad. I re-secured the ground and all the problems went away.

Thanks for the advice!

Fri Oct 17, 2014 6:15 pm

Where is the ground for the tach? Everything in my cluster is working except the tach. Ive been thinking it could be a ground issue. The tach was working fine about 5 months ago when I last drove the van. Ive been working on it since. I have noticed my blue foil circuit to be falling apart up around the mounting screws for the tach housing. Any relation to the non-working tach?

Sat Oct 18, 2014 8:08 am

There is a brown ground wire in the harness that attaches to the back of the tach. But I assume that this ground is for the entire "blue foil" assembly.

Sat Oct 18, 2014 3:45 pm

Yea I think it is. I traced that one already. I was hoping there was something I was missing that would answer my non-working tach. Its my last bug to work out before I get my van back on the road. Maybe its just gone bad? Not sure how to test the tach directly. It is a smallcar modified tach that was working fine before I took the van apart 5 months ago. Smallcar said to send it in for testing but I am anxious and don't want to wait. I guess I can drive the van without a tach for awhile. Thanks for the feedback. GreggK

Sat Oct 18, 2014 9:34 pm

You can view a "blue foil" wiring diagram here.

http://www.kpcnsk.com/?p=138

Seems like you should at least be able to trace the continuity of the connections from the wiring harness connection to the tachometer. Once you get to the tachometer, I don't know how to trouble-shoot its components.

Fri Oct 31, 2014 6:44 pm

So I tried everything I could think of to troubleshoot the tach problem.
I finally sent the unit back to smallcar for testing. Unit tested fine but they found a cocoon of some kind inside that was interfering with the needle movement. Im sure its those spiders Ive been finding every time I take a part off my van. Well tach works and Im happy. I knew I didn't mess anything up when I took it all apart. Thanks for the help guys.