How to Convert a P’up Gas Tach for use with a C223 Diesel

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How to Convert a P’up Gas Tach for use with a C223 Diesel

Post by Paul »

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I think there are lots of diesel P’ups out there that would like to have a nice Isuzu tachometer type instrument cluster in the dash. Unfortunately there are too few diesel tach panels to go around. But there are also tach panels out there that were installed in gas P’ups. These can be converted for diesel use. I have just completed one and it is working well. Also the gas cluster that the tach resides in can be used to make a nice installation. Both the tach and the cluster will require some moderate modifications.

Before undertaking this project you need to be sure that your injection pump has a tach sensor in it, many do not. The tach sensor screws into the side of the pump and has two wires 6 inches long to a connector.

I will start by posting a revised schematic for the tach circuit with the needed changes highlighted in red and green. I also have photos of both the top and bottom of the circuit card showing the changes. These photos may not be good enough for a non electronics guy to get the job done. I hope to make more detailed instructions to add as an edit later.

Electronics guys can probably make the conversion from what I have now. The parts needed for conversion are readily available from an electronics store, or on line. The OEM chip on the card seems to be proprietary and I have not been able to source a data sheet or find any replacements. If anyone knows where to get the data sheet or parts please let me know. The p/n is NEC C1006C which is an 8 pin dip tachometer integrated circuit. The bright side here is that the chips seem to be tough and one should not be needed for this conversion.

You may wonder about the 6000 RPM red line on the gas tach dial. There are some choices here:

1 - - Use the dial as is and calibrate the circuit to it.

2 - - Use the dial as is and calibrate the circuit to read 2000 when 1000 is actually being measured.

3 - - Redo the art on the dial to match the diesel dial.

I chose option 2 because it will read higher on the dial and thus give some better measurement accuracy. Also, I am a low revver and certainly will not ever rev over 4000 RPM. And it will give a certain small amount of mental exercise to decode the actual RPM and it will be a conversation starter when your friends see it.

Here is a schematic with the parts to be removed shown in red:

P'up Gas Tach Schematic w Red.jpg

Here is a schematic with the parts to be added shown in green:

P'up Gas Tach converted for Diesel w Green.jpg

Here is a photo of the top of the circuit board before conversion:

PC Board.jpg

And here is a photo of the back of the board while still attached to the meter. Note that I have previously removed the board from the meter and replaced it using screws. You will need to cut the top off of some aluminum rivets to remove the board the first time. Also, you will need to unsolder the black and red meter wires from the board.

Back View.jpg

And here is a photo of the top of the circuit board after modification:

PCB top.jpg

And now a photo of the back of the board after modification:

PCB back.jpg

And here is what my dial face looks like with choice # 2:

Dial  X 500.jpg

As you can see it is not possible to see exactly what I did on the circuit board from the photos. I will try to give detailed instructions later.

The parts required are:

Capacitor 1.5 uf , 25v, ceramic
Capacitor .01uf, 25v
Capacitor 220uf, 25v, electrolytic
Resistors: 1k, 1/4 w 2 ea
1.5k, 1/4 w
33k, 1/4 w

Timing components:
Ct .oo47uf or .01 uf, 25v depending on which dial option you choose. See chart on green schematic.

Rt 39k, 1/4w plus a small 50k potentiometer in series to allow calibration adjustment.

Q1 : The existing PNP transistor on the board can be reused if you are careful in removing it. If you need to buy one, any 25v or higher PNP small signal transistor with a beta (current gain) of 100 or more will be good. I use a 2N3638 because I have lots of them but the circuit will not be fussy.

If you use a different transistor watch out for the lead assignments as they are not all the same :cry:


Connect a power supply or battery to the + & - terminals on the tach.

THE C223T tach sensor will generate a sine wave at 308.33 Hz per 1000 RPM of engine speed. So, what you need to do to calibrate is connect a sine wave signal generator between the input terminal and ground. Then adjust the generator for an output level of about 1/2 volt. Set the frequency to 616 Hz. Your tach should now produce a reading. Adjust the potentiometer so that the tach reads 2000 RPM.

Reset the frequency to multiples of 308 Hz and check that the tach reads within 100 RPM or so of the right speed. Do not expect perfect accuracy as the meters in these tachs are not perfect. The circuit is probably better than the meter.

Also check to see that the tach will remain steady when you vary the input voltage from about 1/4 volt to say 5 volts rms.

If this all works you have finished.

If you cannot get the tach to work, better check over your wiring. And if you have an oscilloscope available use it to chase the signal through the circuit.

'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.
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