Diagnosing The Tachometer

I planned to pull the drive-shaft and replace the universal joint today, as the weather prediction said it would be n the mid 50’s with a 0% chance of rain.  Unfortunately it was in the 40’s and raining when I had a chance to work on the car.  So I switched gears and decided to work on a problem I was having with the in dash tachometer.

The factory tachometer for the Mustang is a different design than the typical tachometer.  It is installed in series with the positive side of the ignition coil.  Most tachometers connect to the negative side of the coil.  Since this tachometer is in series with the primary ignition circuit, if it fails the coil will not receive any voltage.  It also requires a ballast resistor with a resistance of 1.5 ohms, which on the mustang is in the form of a nichrome resistance wire that is located between the tachometer and the primary side of the coil.  For a more detailed description and wiring diagram, there are several resources on the Internet. One I can suggest is The Tach Man who also repairs and calibrates the factory tachometers.  I haven’t used his services, but there is some good information on his page.

My car did not originally come with the factory tachometer.  I added it many years ago.  The under dash wiring harnesses were different for cars with the factory tachometer, and are very difficult to find.  When you do find one it will be very expensive.  So I chose to modify the wiring myself, as there really aren’t that many differences between the two.  I made the changes and everything worked for years.

Then I took the car into the last restoration shop, who saw that I had done my own wiring and were adamant that it could never work.  It didn’t seem to matter to them that I had been driving it that way for over 10 years and it was working fine.  They said it absolutely had to be rewired with a factory harness.  The clincher for them was that I had just installed a new engine at that time and when I started it for the first time the charging system didn’t work.  After they “looked at it” the alternator wiring harness melted.  They replaced it with a new harness and went to work from there, replacing the under dash harness, the engine compartment harness, and the voltage regulator to no avail.  Finally they replaced the alternator, which was most likely the original problem, and the charging system worked again.

With the charging system working I checked the tachometer and gauges and they all appeared to be working.  It wasn’t until much later that I observed that the tachometer wasn’t displaying the correct engine RPM.  It consistently read low, and seemed to be limited to about 2500 RPM maximum.  The car went into its’ 15 year garage vacation at that point and I never had a chance to look at the problem until now.  The fix turned out to be very simple.

When the new engine had been installed.I replaced the ignition coil with an aftermarket high performance coil.  What I hadn’t realized was that coils have different primary resistance values.  I had several factory coils including my original one.  They all measured 1.5 ohms primary resistance.  The new ignition coil measured 3.0 ohms resistance.  Since the tachometer is wired in series with the positive primary side of the coil and the ballast resistance wire the resistance affected the tachometer reading.  Factory stock would be 1.5 ohms for the ballast and 1.5 ohms for the coil.  Now I had 1.5 ohms for the ballast and 3.0 ohms for the coil.  I purchased a new Pertronix coil with 1.5 ohms primary resistance and installed it.  Lo and behold the tachometer now appears to read correctly.  When I have a chance I plan to hook up my tachometer/dwell meter and compare the readings with the factory tachometer.  But for now I’m happy that my tachometer is reading a lot closer to the correct engine RPM.  That and I have a nice new coil.

Maybe tomorrow I’ll get the opportunity to work on the universal joint, weather permitting.