Rev Counters (Tachometer)
I have had several enquiries regarding rev counters recently, so I think it may be worth a few words here as well.
The rev counters fitted to the MkI and MkII are different in style and operation, they are wired differently and this has caused some confusion.
On the MkI the rev counter is wired in series with the coil, i.e. the wiring from the ignition switch is connected to the induction loop at the back of the rev counter (there is no direct electrical connection to the gauge itself) on some later cars a dummy wire loop (grey or white) can be found behind the instrument panel, this is substituted for the induction loop.
From the rev counter the wiring then goes to the ballast resistor, a small pink wire wrapped into the loom, then to the starter solenoid (black/yellow) and finally to the coil itself, the solenoid connection being the ballast override used during engine cranking.
Incidentally, many people find the ballast wire is quite inaccurate as a resistor, perhaps in an effort to increase reliability; the resistance is on the high side. Originally when the car was built it didn’t prove a problem but as modern fuels can be more difficult to ignite a stronger spark is often used. A simple upgrade would be to replace the ballast wire with a new ceramic resistor, which is more accurate.
On the MkII the rev counter is wired the same as many aftermarket models, the sensing wire (green) being connected to the negative or points side of the coil.
Replacing a standard instrument panel with one from a sport or Ghia is a relatively straightforward job; both panels are the same size so one directly replaces the other, and the panel is built of smaller modules that can be interchanged. If you really wanted the speedo. could be on the left and the rev counter on the right!
To fit simply remove the steering column top and then the panel itself, disconnect the speedo. cable from under the dash and unplug the two wiring plugs by squeezing their ends.
Looking carefully at the printed circuit on the back of the instruments, you should see that the connections for the fuel and temp. gauges on the sport dash correspond to 2 unused connections on the standard wiring loom. By sliding a jewellers screwdriver down the backs of the brass inserts they can by removed from the small plug (the 2 at either end) and fitted to the larger plug, this way the fuel and temp gauges on your new sport dash will work, easy as that!
To get the rev counter to work you now need to remove the centre wire (backlight) from the small plug and insert it into the hole that the temp.sender wire came from. Take a spare brass insert from a scrap car and feed into the hole from which you removed the backlight wire previously, now connect this new wire to the negative side of the coil. If you wish to retain the original wiring fittings you will need a double connector, these can be found on original equipment suppressors (often fitted on the Sierra). Cut the suppressor off and connect your new wire using solder and heatshrink tubing to keep out the water, that’s it! Simply reassemble and test.
The author cannot accept responsibility for any claims arising from matter included in this text. As with all electrical matters, if you have any doubts contact an auto electrician BEFORE starting the job.