tach problem ugh
Thread Starter
Intermediate
Joined: Nov 2007
Posts: 36
Likes: 0
From: Canyon Country CA
tach problem ugh
79 hei, key off reads 1500, key on reads 0, hit the starter
tach goes up with the rpm about 1000 until the engine starts then it drops to 0 and sits.
pulled the dash out and the circut looks ok ohmed the traces and didn't find any opens,
same results with the filter in and out.
just for fun hit the starter with the motor running and the tach tried to work,
Stumped
tach goes up with the rpm about 1000 until the engine starts then it drops to 0 and sits.
pulled the dash out and the circut looks ok ohmed the traces and didn't find any opens,
same results with the filter in and out.
just for fun hit the starter with the motor running and the tach tried to work,
Stumped
Team Owner
Joined: Jul 2006
Posts: 22,141
Likes: 4,156
From: Kannapolis NC
Sounds like the ignition switch has a bad contact or connector, if you hold it to start and the tach works. The more likely culprit is the tach board mounted on the back of the tach by three nuts. Not the printed circuit flimsy thing behind both, but the small board mounted directly to the tach. Does that board have a black burned spot on it?
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
To eliminate other issues, (shorted wire, ground problem and any other issue) and you have the tach out of the car you can hard wire it.
Run 12 volts from the alternator to the power side, a ground to the engine, and then run a pulse wire from the distributor to the tach.
Doing all this while holding it in your hand you can fire up the car and see if it’s a working properly without any outside interference.
Run 12 volts from the alternator to the power side, a ground to the engine, and then run a pulse wire from the distributor to the tach.
Doing all this while holding it in your hand you can fire up the car and see if it’s a working properly without any outside interference.
Drifting
Joined: Nov 2005
Posts: 1,795
Likes: 1
you should have one of these mounted on the
back of the tach itself
http://hometown.aol.com/coronet69/page18.html
back of the tach itself
http://hometown.aol.com/coronet69/page18.html
Team Owner
Joined: Jul 2006
Posts: 22,141
Likes: 4,156
From: Kannapolis NC
Never saw a 78 or 79 without a board, is the hei different? How does the tach make contact with the voltage/ground/pulse? It should have three long screws that plug into the flexible printed circuit ***'y. The tach board should be mounted on those three long screw with nuts to make electrical connection. Does the fact that it is hei somehow eliminate that board? P.S. the board is not visible unless you remove the six torx screws from the black cover around the tach/speedo, then the three torx screws from the tach and pull the tach out of the ***'y from the front, the board is attached to the tach and comes out with it.
Last edited by 65GGvert; Apr 17, 2008 at 10:50 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I think all he is seeing is the three connectors for the tach! I don't think he has opened it up to see the circuit board.
W
W
Thread Starter
Intermediate
Joined: Nov 2007
Posts: 36
Likes: 0
From: Canyon Country CA
tried running separate tach, ground, and power = no go,
sorry didn't know the board was inside the housing,
learning experience, found out the tach reads while the engine is cranking even with the tach wire disconnected but once the engine fires the tach drops to zero connected or not.
I think everything appears good up to the gauge, I pulled the board and the component side looks good, the back side has a overheated spot near a resistor and a couple diodes but no burnt traces.
I would feel better about buying a new board if there was more damage, but I guess at this point I don't know what else to test.
I've cleaned all connections, for geewizz I even switched ignition moduals.
any suggestions or just place an order.
sorry didn't know the board was inside the housing,
learning experience, found out the tach reads while the engine is cranking even with the tach wire disconnected but once the engine fires the tach drops to zero connected or not.
I think everything appears good up to the gauge, I pulled the board and the component side looks good, the back side has a overheated spot near a resistor and a couple diodes but no burnt traces.
I would feel better about buying a new board if there was more damage, but I guess at this point I don't know what else to test.
I've cleaned all connections, for geewizz I even switched ignition moduals.
any suggestions or just place an order.
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Racer
Joined: Dec 2007
Posts: 374
Likes: 12
From: PHILADELPHIA PA
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
When the ignition key is turned in the on position with out starting the car the tachometer is powered up! When powered up the tach should go to zero instantly. This is the same as doing a hardwire test! If you run 12 volts to the positive side and a verified ground to the ground side, the tachometer should instantly go to zero!
Once you introduce the pulse wire from the distributor then the tachometer should start reading the signal. (and working).
With your burned spot on the board I have to agree the board is done! They are easy to install and are pre-calibrated!
Willcox
Once you introduce the pulse wire from the distributor then the tachometer should start reading the signal. (and working).
With your burned spot on the board I have to agree the board is done! They are easy to install and are pre-calibrated!
Willcox
Instructor
Joined: Aug 2007
Posts: 138
Likes: 0
From: san antonio Texas
i have a 79 and this is what i did. I removed the housing after removing the tach face and the board, I left the speedo in place after disengaging the cable. Bought a new board from corvette central...how ever you feel confident buying from...replaced the board and replaced the thin circuit board on back on the housing, replaced any burned out bulbs. I tested the tach board by touching a 9 volt battery to the 12v and ground screws...if the board is good it will zero out. put everything back and all was good as new.
Team Owner
Joined: Jul 2006
Posts: 22,141
Likes: 4,156
From: Kannapolis NC
The tach board can be bad and it will still zero out with voltage applied. Be sure you get a board that is precalibrated for the 78/79. Willcox sells them. I also have a new one in the box that I bought from Willcox that has never been installed. If you can't find one, or want to buy mine, pm me and we can talk. You don't have to remove the speedometer or the ***'y that holds the tach/speedo. You already have yours apart apparently, but if not, you remove the front plastic lens, the black cover around the tach/speedo as I mentioned before, then the three screws holding the tach, it pulls out separately with the board attached and you don't have to disconnect and reconnect the speedo cable. I can have mine out in about three minutes now that I have done it a couple of times.