Electrical tachometer conversion from 6 to 8 cylinder (will also work for 4 to 6 or 4 to 8 but with different resistor size)

extra_stout

Mustang Master
Aug 27, 2018
673
775
103
Germany
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.
IMG_20211120_113445.jpg
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.
IMG_20211120_113508.jpg

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):
IMG_20211120_113539.jpg
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:
10k-ohm-pot-rm065-package-800x800-1.jpg

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:
IMG_20221007_205713.jpg
IMG_20221007_205714.jpg
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.
 
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extra_stout

Mustang Master
Aug 27, 2018
673
775
103
Germany
Here some more pictures how to remove the tacho from the instrument cluster.
How you get the instrument cluster out, is discribed in this thread (see video):
https://stangnet.com/mustang-forums/threads/led-light-conversion-completed.910430/
You only need to pull the plug from the main harness and another plug directly at the back of the tachometer. Then the speedometer cable needs to be removed by pushing down on the white plastic bracket on the location on the top that is pointing to the driver.
It is better to remove it completely, than working in the car.
IMG_20221019_180910.jpg
Now you have to remove 6 bolts that hold the transparent plastic and the upper part of the housing to the lower part of the housing. After removal it looks like that:
IMG_20221019_180921.jpg

On the back you can now remove the 3 bolts (red circles in picture) that hold the tachometer to the housing. Now you can remove it.
IMG_20221019_180942.jpg

This are the two tachometers D4ZF-17360 (which was installed originally in my car) and the D5ZF-17360 unit which I modified as described in the first post. Both are 6 cylinder application. Both have the same pins at the 4-pin-plug (see letters in the plastic housing near the plug).
IMG_20221019_181112.jpg

Because two units are different, I investigated the D4ZF-17360 unit, how it looks inside and if it can also be modified. It is just enough to remove the two bolts on the front of the tachometer and you can pull it apart like shown here:
IMG_20221019_181247.jpg
Same working principal as in the D5ZF-17360 unit, but the board looks different so I can not tell, how the resistor needs to be sized, but it should work the same way.

I also installed my KPH speedometer to test it and soldered in the D5ZF-17360 two fixed resistors to get 645 Ohm. I also installed new 5w5 bulbs you get everywhere here in Germany, because I didn't get the LED bulbs that Enzio is using; normal LEDs will not be dimmable with the light switch. Hope that the glass bulbs, are bright enough...
 
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Here some more pictures how to remove the tacho from the instrument cluster.
How you get the instrument cluster out, is discribed in this thread (see video):
https://stangnet.com/mustang-forums/threads/led-light-conversion-completed.910430/
You only need to pull the plug from the main harness and another plug directly at the back of the tachometer. Then the speedometer cable needs to be removed by pushing down on the white plastic bracket on the location on the top that is pointing to the driver.
It is better to remove it completely, than working in the car.
IMG_20221019_180910.jpg
Now you have to remove 6 bolts that hold the transparent plastic and the upper part of the housing to the lower part of the housing. After removal it looks like that:
IMG_20221019_180921.jpg

On the back you can now remove the 3 bolts (red circles in picture) that hold the tachometer to the housing. Now you can remove it.
IMG_20221019_180942.jpg

This are the two tachometers D4ZF-17360 (which was installed originally in my car) and the D5ZF-17360 unit which I modified as described in the first post. Both are 6 cylinder application. Both have the same pins at the 4-pin-plug (see letters in the plastic housing near the plug).
IMG_20221019_181112.jpg

Because two units are different, I investigated the D4ZF-17360 unit, how it looks inside and if it can also be modified. It is just enough to remove the two bolts on the front of the tachometer and you can pull it apart like shown here:
IMG_20221019_181247.jpg
Same working principal as in the D5ZF-17360 unit, but the board looks different so I can not tell, how the resistor needs to be sized, but it should work the same way.

I also installed my KPH speedometer to test it and soldered in the D5ZF-17360 two fixed resistors to get 645 Ohm. I also installed new 5w5 bulbs you get everywhere here in Germany, because I didn't get the LED bulbs that Enzio is using; normal LEDs will not be dimmable with the light switch. Hope that the glass bulbs, are bright enough...
Great tachometer articles and photos. My 6 was converted to a V8 and I need to get the tach working. This was a huge help. Thank you Snakeoilii
 
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