I bought an complete instrument cluster, because it had a KPH speedometer. So there was a spare tachometer to work with, without having fear to destroy it.
I removed it from the instrument cluster. The complete tachometer consists of three main parts:
- circuit board (which the 4 wire plug from the harness goes to); this is a frequency voltage converter, which means that the signal from the coil is transformed to a DC voltage which shows in dependency from cylinder number the RPM on the tachometer
- the tachometer itself (which is connected by two wires [red and black] with the circuit board)
- the white plastic housing which all parts are bolted to
First step: jack up your car... wait that is not needed.
Remove 2 bolts from the front side (they connect the tachometer with the plastic housing).
Remove 2 bolts at the back of the plastic housing, that is near the plug (see circle in picture); the two bolts hold the circuit board tight at the housing.
Now you can remove the housing (be patient, perhaps you need to push from the backside onto the four pins of the plug to get the circuit board removed from the housing; remember that the circuit board is connected with two wires to the tachometer, so the board can not be separated together with the housing)
If the housing is removed this is what it looks like (two arrows show the red and black wire; circles show the nuts on the board, where the screws on the backside bolt to):
The conversion works only from lower cylinder number to higher cylinder number. The V6 tacho shows with a V8 following RPM: RPMv6tacho=8/6*RPMreal. Means if your V8 is running at idle with 800RPM the V6 shows 8/6*800=1066 RPM. To get a lower reading on the tacho, you need to reduce DC voltage (red and black wire) at the tachometer. This can be done, with an addition resistor that is put into red wire (this is why you need to step up cylinder count, because you cannot increase the voltage easily, but reduce it with a resistor).
I cut up the red wire and elongated both ends so that the are outside of the housing to be able to put in a resistor or in my case to use a adjustable resistor like this:
I removed the dash cover and you can unplug the harness that goes to the installed original tacho, when everything is installed: The harness has enough wire length to move it over the whole assembly and put in the modified tachometer. Than it looks like this:
With a friend on the timing light (which has a tachometer function) and some engine running at different speeds, some yelling and some turning on the adjustable resistor it came out, that you need around 650 Ohms to have the right reading (V6 tacho with V8 engine; will be different if you use R4 tacho).
We tested at idle and at speeds of 2000/3000/4000RPM and it is not more off than 100 RPMs.
Now I need to decide if I will remove the elongated wires and solder in a normal/fixed 650 Ohm resistor or leave the elongated wires outside, that I'm able to adjust if needed without removal of the whole instrument cluster.
I also found out, that you can reach the pin (which you need to push to remove the light switch) from above when the dash is removed.
I will make some more pictures and write how to install the modified tachometer into the instrument cluster.
If something is unclear please ask, you know that I'm not a native speaker.
I removed it from the instrument cluster. The complete tachometer consists of three main parts:
- circuit board (which the 4 wire plug from the harness goes to); this is a frequency voltage converter, which means that the signal from the coil is transformed to a DC voltage which shows in dependency from cylinder number the RPM on the tachometer
- the tachometer itself (which is connected by two wires [red and black] with the circuit board)
- the white plastic housing which all parts are bolted to
First step: jack up your car... wait that is not needed.
Remove 2 bolts from the front side (they connect the tachometer with the plastic housing).
Remove 2 bolts at the back of the plastic housing, that is near the plug (see circle in picture); the two bolts hold the circuit board tight at the housing.
Now you can remove the housing (be patient, perhaps you need to push from the backside onto the four pins of the plug to get the circuit board removed from the housing; remember that the circuit board is connected with two wires to the tachometer, so the board can not be separated together with the housing)
If the housing is removed this is what it looks like (two arrows show the red and black wire; circles show the nuts on the board, where the screws on the backside bolt to):
The conversion works only from lower cylinder number to higher cylinder number. The V6 tacho shows with a V8 following RPM: RPMv6tacho=8/6*RPMreal. Means if your V8 is running at idle with 800RPM the V6 shows 8/6*800=1066 RPM. To get a lower reading on the tacho, you need to reduce DC voltage (red and black wire) at the tachometer. This can be done, with an addition resistor that is put into red wire (this is why you need to step up cylinder count, because you cannot increase the voltage easily, but reduce it with a resistor).
I cut up the red wire and elongated both ends so that the are outside of the housing to be able to put in a resistor or in my case to use a adjustable resistor like this:
I removed the dash cover and you can unplug the harness that goes to the installed original tacho, when everything is installed: The harness has enough wire length to move it over the whole assembly and put in the modified tachometer. Than it looks like this:
With a friend on the timing light (which has a tachometer function) and some engine running at different speeds, some yelling and some turning on the adjustable resistor it came out, that you need around 650 Ohms to have the right reading (V6 tacho with V8 engine; will be different if you use R4 tacho).
We tested at idle and at speeds of 2000/3000/4000RPM and it is not more off than 100 RPMs.
Now I need to decide if I will remove the elongated wires and solder in a normal/fixed 650 Ohm resistor or leave the elongated wires outside, that I'm able to adjust if needed without removal of the whole instrument cluster.
I also found out, that you can reach the pin (which you need to push to remove the light switch) from above when the dash is removed.
I will make some more pictures and write how to install the modified tachometer into the instrument cluster.
If something is unclear please ask, you know that I'm not a native speaker.