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79 vette- 36,000 miles. My tach suddenly stopped working. It was working fine, not erratic and just stopped working and sits on 0.
I did some research and found issues are typically the tach filter or the circuit board on the back of the tach.
I tried bypassing the tach filer. no change. I then removed the tach and tested following procedures I found at http://repairs.willcoxcorvette.com/w...-1978-1982.pdf
With voltmeter on the Ground Clip and the 12V Clip (in the dash) I got no voltage reading with the key on.
With voltmeter on the Ground Clip and the Signal Clip with engine running I get no reading with tach filter connected. If I bypass the tach filter I get 12V with key in on position or with engine running (so the Ground Clip is good).
From that testing I would surmise an issue with the hot wire between fuse box and tach. I confirmed all fuses in the box are good.
I followed the procedures to test the tach - I jumpered the ground pins and the 12V pins to a spare battery. Smoke instantly poured off the circuit board. Why? Did I apply too much voltage going direct to the battery? I removed the circuit board from the back of the tach and did the same test connecting the Ground and 12V. This causes the tach to jump to 2500.
Looking at the fuse box I see an unconnected Pink wire hanging down. See attached picture. Could this be the reason I don't have voltage at the tach? I'm guessing not since all other gauges are working.
So I have two issues. Why did connecting the tach to the battery burn up the circuit board and why don't I have 12 V at the tach. The Wilcox site says the next step is to pull the dash housing for the tach/speedometer to get at the wiring. This looks like it could be a bit of a hassle to do with having to disconnect the speedometer cable so I'm not sure I want to go down that path as I was planning to sell the car.. But even if I get the wiring issue resolved, I'm hesitant to buy a new circuit board and install it without understanding how/why I burned up this one. I don't want to melt another board.
Suggestions?
Yes, those are the instructions I have followed that have led me to finding that I have no power to the tach.
Something I forgot - before I came across these instructions I was wondering if the posts on the back pf the tach were just not making good contact with the clips in the dash so I ran jumper wires between the clips and the posts on the back of the tach with alligator clips (holding the tach in my hand). When i moved to turn the key to test with this setup, one of the clips must have slipped and either the hot or signal line got grounded as there was a spark. I decided to not proceed with this route and that's when I hooked up the hot and ground directly to a spare battery and the circuit board smoked and one of the integrated circuit chips started to melt. So I'm presuming this is because of shorting it out? I still have to figure out why I do not have power to the tach?
1979 Tach stopped working
