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NeCrOmAnCeR · 02-08-2004, 02:27 PM

This is my first post so please bear with me. Having adapted many GM tachs to other vehicles i may have a solution to those which do not read correctly. When I placed an 86 pontiac tach into the custom dash for my wifes 67 mustang, I found this valuable site containing valuable information on adjusting the electronic components to make a six cylinder tach read correctly on a 8 cyl. for those tachs which progressively get worse with age, the most common fault is most likely the electrolytic capacitor. These devices eventually leak or suffer some other electrical failure so replacing the capacitor will most likely return proper operation. Since i have never recieved an inoperative tach i have not been able to verify this. To adjust the reading, simply follow the modifications shown on this site. effetively removing the calibration resistor and replacing it with a multiturn variable resistor allows you to adjust the accuracy, or the number of cylinders the tach operates on. Although this information is intended for fierros, I have see the same components used on the GM tachs (older) I have used. This same site also has the capacitor values needed if that is all you need.
When replacing the dash of my 74 GMC with the tach dash out of a 78 (I think it was), I traced all the circuits of the new dash to the connector location, and those of the old dash, and moved the contacts of the electrical connector to the proper location. I replaced the temp and oil pressure gauges with aftermarket gauges which fit into the stock location and gives me more confidence in the readings.
If you are creative with image editing software you could also generate a new tach face reading up to any desired RPM (8 grand) and place it over the stock tach face and then use the beforementioned modification to adjust the reading of the tack to match. If anyone would like to try this, I am interested because if done correctly, it would maintain the stock look and be very unique. With my research work I don't have much time to do this myself, but it is an idea i've had for a few years.

http://www.thefieroclub.com/osg/tachmod.html
Capacitor values

http://home.t-online.de/home/O_Scholz/tach.html
Modification

Hope this information helps those of you with ailing tachs, and gives valuable information.

02-08-2004, 02:27 PM	#17
NeCrOmAnCeR Registered User Join Date: Feb 2004 Location: Hodgenville, Ky Posts: 735	This is my first post so please bear with me. Having adapted many GM tachs to other vehicles i may have a solution to those which do not read correctly. When I placed an 86 pontiac tach into the custom dash for my wifes 67 mustang, I found this valuable site containing valuable information on adjusting the electronic components to make a six cylinder tach read correctly on a 8 cyl. for those tachs which progressively get worse with age, the most common fault is most likely the electrolytic capacitor. These devices eventually leak or suffer some other electrical failure so replacing the capacitor will most likely return proper operation. Since i have never recieved an inoperative tach i have not been able to verify this. To adjust the reading, simply follow the modifications shown on this site. effetively removing the calibration resistor and replacing it with a multiturn variable resistor allows you to adjust the accuracy, or the number of cylinders the tach operates on. Although this information is intended for fierros, I have see the same components used on the GM tachs (older) I have used. This same site also has the capacitor values needed if that is all you need. When replacing the dash of my 74 GMC with the tach dash out of a 78 (I think it was), I traced all the circuits of the new dash to the connector location, and those of the old dash, and moved the contacts of the electrical connector to the proper location. I replaced the temp and oil pressure gauges with aftermarket gauges which fit into the stock location and gives me more confidence in the readings. If you are creative with image editing software you could also generate a new tach face reading up to any desired RPM (8 grand) and place it over the stock tach face and then use the beforementioned modification to adjust the reading of the tack to match. If anyone would like to try this, I am interested because if done correctly, it would maintain the stock look and be very unique. With my research work I don't have much time to do this myself, but it is an idea i've had for a few years. http://www.thefieroclub.com/osg/tachmod.html Capacitor values http://home.t-online.de/home/O_Scholz/tach.html Modification Hope this information helps those of you with ailing tachs, and gives valuable information.