Tach cluster issues

[BlueK20]

Hey guys, I bought a 1980 Tach Cluster for my 1978 K20, Hooked everything up and all gauges work except tach, at first when i turned on the truck, it zero'd itself (was at 1500) and while cranking over truck it said about 400 RPM, but once truck was running, the needle would point straight down (Literally at my legs) and it doesn't respond to revs at all.

Is the tach screwed? or do i have another issue, i ended up undoing ground and moving it to other areas, and the needle would wrap around and do the same thing.

any ideas? or am i stuck with buying one from classic industries or somewhere?

also does anyone reproduce the tach cluster housing?

 

[Green79Scottsdale]

Pretty sure something is messed up on the circuit board of the tach. (Not to be confused with the printed circuit for the other gauges.) There is a good write up somewhere on the web about what the problem might be. You could fix it yourself if you can solder on circuit boards. I have a link to it on my home computer. One of the Corvette parts stores makes a new one for around $90. I can't for the life of me remember which Corvette place however. The repoped ones from Classic fit like crap. I bought and sent it back because it was so bad. I do not believe anyone reproduces the tach housing, no.

edit: Willcox Corvette

edit again: Here's a fix-it link... http://howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm

 

[BlueK20]

I just realized. I don't have a "tach filter" on the tach wire. Would that fry the tach? It didn't come with a harness so I just used the wires from my autometer

 

[Green79Scottsdale]

Possibly. I have been thinking of putting a filter in the wire when/if I ever get my factory one installed. I am assuming you can use one from any GM vehicle. I thought I saw one cheap for the El Camino/Monte Carlo recently.

I have an aftermarket tach harness from Classic for our trucks, and it doesn't have a filter on it. Not sure what that means.

 

[daylight]

howdy i going to a swap myself from a 81 blazer to 78 c10. did you have to change the senders on the oil, water, guages? i only had idiot lights. on the tach harness what does the filter look like? was going to order one from classic parts.the guys at classic were not sure about the tach filter?

thanks gil

 

[Snoots]

GM tachs have a nasty habit of not working properly without the filter installed.
The filters also have a nasty habit of dying.

 

[hatzie]

GM tachs have a nasty habit of not working properly without the filter installed.
The filters also have a nasty habit of dying.

The GM tach filters are not complex. Resistors and an electrolytic capacitor. Forty five years is a long lifespan for electrolytic capacitors. GM likely they used carbon resistors too because they're cheap... those don't last any longer than the electrolytics.
The 1983 wiring diagrams have the schematic for the tach filter.

The tach circuit isn't incredibly complex either. It can be repaired if it's not one of the coated boards.