V6 vs 4cyl tachometer wiring (for 1988 OEM tach)

David85 · Jan 1, 2013

Hi guys, tried doing a search and couldn't come up with a difinative answer.

Can you use a tachometer that was originally in a V6 equipped ranger (late 1987) on a 4 cyl engine?

I'm guessing the V6 wiring will expect a 3 pulse per revolution signal, while the 4 popper wants to see 2PPM.

I know on similar era F-series trucks, there is a second ground that must be connected to distinguish between V8 and 6cyl gassers. In that case, ford labelled it "G8" to indicate the second ground required for 8cyl engines.

Do rangers have something similar? Or did ford make a separate tach assemblies for 6 and 4 cyl engines?

Crossthreader · Jan 3, 2013

I don't have detailed information specific to the 87 MY, but the 99 has a black/yellow wire from the cluster that was grounded for 6 cyl.

Paul

fastpakr · Jan 3, 2013

I'm not sure how this works on a TFI system. However, on Ford's later EDIS units (including my '94), there is a generic/standardized tach pulse emitted by the ignition system regardless of cylinder count. That allows the same cluster to be used behind multiple engines with no changes. I'm running an Explorer cluster on my 4 cylinder Ranger, just a plug and play swap.

David85 · Jan 3, 2013

Thanks guys. I did find plenty of info on newer rangers that use a standard frequency signal across different engines but there isn't much on the 1st gen tachs.

Eventually I'll take the cluster out and hook the tach up to a function generator to figure out what's possible.

The factory V6 is gone in favour of a 2.3L turbo/intercooled diesel. My plan is to run a magnetic pickup off the camshaft sprocket to get a 2 PPM signal, hence my dilema.

David85 · Jan 22, 2013

I think I figured it out.

Three of the post terminals are self explanatory:

SIG = Tachometer pulse signal
BATT = 12V power
GND = Ground

L2 = secondary ground for use with V6 engines (3ppr). Leave this disconnected for 4 cyl (2ppr). Grounding this will drop the tachometer output by 1/3.

Calibration of the tach is done with a precision resistor thats twisted on to terminals at one end of the circuit board. I'm guessing a technician did this at the factory. Once mine was up and running on the bench, I noticed it was high by 5%, so I added another parallel resistor to bring the reading down slightly. I was able to get it bang on 3000RPM for a 100Hz input signal;

3000RPM/60seconds = 50 revolution/second

50 x 2 pulse/revolution = 100Hz

So to indicate 3000 RPM, I need to generate 100Hz (1500RPM = 50Hz and 200Hz would be 6000RPM.

Higher resistance at this location means higher tachometer reading, while lower will bring the RPM down for the same frequency input. In theory these tachometers could be easily calibrated for a almost any number of pulses per revolution. Pretty simple actually.

I've ordered an automotive grade hall effect sensor from digikey and hope to get that running soon.

David85 · Feb 2, 2013

A little late, but here's some pictures of the calibration resistor and the tach running in the classroom lab.

The 3900 reading should actually be 3000, but the calibration resistor has been removed to show the effect (Higher resistance = high RPM). adding a resistor in the k-ohm range would bring the reading down into the hundreds for the exact same frequency. Its actually pretty easy to calibrate if you have access to the equipment.

I also included a photo of the four pins. They are pretty self explanatory with "L2" being grounded for 6 cyl engines and left unused for 4 cyl engines. Since this truck originally had a V6, I put some heat shrink on the L2 since its no longer needed.

David85 · Feb 2, 2013

Here's the setup as installed in the truck.

The hall sensor is installed in the timing cover and aimed at the camshaft sprocket (4 spokes at 0.5 crank speed = 2 pulse per revolution). Basically, its set up as a camshaft speed sensor.

The catch with this particular sensor is it only puts out a 0.4V pulse. To solve this, I sent that pulse into a Darlington Pair Transistor to act as a crude amplifier. That got the pulse up over 7V. The tach seems to run well on anything above 4V.

I considered adding the darlington inside the cluster assembly, but was concerned about added interference on the long wire leading to the cabin. If this location proves too harsh for the circuit, I may decide to move it later. Darlingtons in this size range are dirt cheap so its no big loss.

The 12V is parallelled into the fuel pump circuit and the ground is simply taken from one of the cam cover attaching bolts.

And so far so good! :headbang:

I already test drove it and the readings seem bang on and very steady.

If I had to do it all over again, I'd be tempted to simply use a signal from the alternator. Now that I know the tach has such a wide input frequency range, it wouldn't be a big deal to recalibrate it for the alternator shaft speed. Although this way if the tach ever does die on me, I can replace it with any other 1st gen tach from a wrecker.

Anyway, I hope someone finds the info useful. It was a fun little project that I kept putting off.

lorro · Nov 10, 2018

hi david, I want to recalibrate my 1gen bronco II tach but I did install an v8, just I was looking for in your post, but I only have an voltimeter and thats it, my actual v6 resistor attached betwen the two terminals its yellow green orange orange brown 453kohms, I read that higher the resistance higher the rpm I would say i'ts inverse but I don't know a lot about electronics, could you please tell me how can I calculate which resistor do I need, a friend suggest me to attach in the circuit a potenciometer to calibrate the most presise way possible

V6 vs 4cyl tachometer wiring (for 1988 OEM tach)

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