Fox 4 Cylinder to V8 Tach

[Mustang5L5]

The pots are for calibration. One calibrates the high range, the other calibrates the low range. The 89-later tachs have two pots, and the 87-88 tachs only have a single adjustment pot.

Calibration method using the pots is in the video up above.


What caught my eye is the traces that are bridges marked “cyl”. My tach doesn’t have that. I wonder if desoldering it changes it to a v8 version. Would need to see a 90-93 v8 tach and see if that is soldered together.

 

[Noobz347]

It actually looks pretty good. Turn it upside down and look closely to see if [any] of the solder joints have cracks --or-- Just go over each joint with a soldering iron and flux and reflow them. I don't see any burned components in the images. Just the heat discoloration on the board.

You may want to check the diodes in that heated area to make sure they're still only going one-way but you'd need to remove them.

If that's a voltage regulator (the square peg with the round hole in it) it's probably shot. the center lead is ground, the one on the left should be input and the one leading to the discolored portion of the board should be output. Pretty sure that thing is running wide open and not regulating anything ATM.

That's all I got with pikcherz. Overall, the thing looks pretty good

 

[Mustang5L5]

Hers the ‘88 v8 tach I have which the board layout is a bit different. There is a resistor marked with an “8” that is offset by itself


I should have an ‘88 4cyl tach Monday so I will compare. Will need to see how that translates to a 90-93 tach however since they appear to be a little different

 

[Mustang5L5]

This is an interesting thread showing add an adjustable pot to adjust the tach reading. Different generation but this method might also work depending on what we find.

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

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...

stangnet.com

 

[Noobz347]

This is an interesting thread showing add an adjustable pot to adjust the tach reading. Different generation but this method might also work depending on what we find.

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

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...

stangnet.com

Did you find that in the feed beneath this thread? Would be cool if you did. Means its working-ish

 

[93CalypsoConvert]

Desoldering the “Cyl” resistor makes it not work. The original resistor is 10 K ohms. I swapped out with a 51k, but it had no effect. It’s still reads double. Interestingly, increasing the resistance on the pot actually makes it read higher. Decreasing resistance reads lower.

 

[Mustang5L5]

Did you find that in the feed beneath this thread? Would be cool if you did. Means its working-ish

I did. Was at Thea top of recommended threads

 

[93CalypsoConvert]

I found that varying the input +12v resistance has no effect, nor does varying the resistance on the signal wire. I am going through and swapping the resistors out. Some of them are only a few ohms so I'm going to save those for last. Any other ideas on how I should approach this?

 

[limp]

And I are not one of them.
But I flew a toy helicopter once. Didn't end well

Were you the guy in " flight of the Phoenix"?

 

[93CalypsoConvert]

Success! I figured it out.

Here is how you do it for 1993 tach:

The blue resistor in the middle needs to be replaced to a 20.0K ohm resistor. My original one reads 43.2K Ohm.
I soldered a 50k ohm potentiometer into it and varied until tach reading was correct.

I found that the proper resistance was 20k ohms exactly.
I tested this with the tach out of the car running off of battery power:

I adjusted the two pots already on the board to 84.8 ohms and 9.64K ohms. This gave me results very close to a digital reading.
Here is a video of it working:


View: https://youtube.com/shorts/wxZXNY-y3wU?feature=share

I'm going to purchase a normal 20k ohm resistor and solder it in place of the original and reinstall.
After some more testing, it is ±100 rpm between 0-4000 rpm. I haven't tested past that.

 

[Mustang5L5]

nice work.


So what’s the existing value of the resistor for the 4cyl engine and what value resistor did you swap to?

The 87-89 tachs are different, so I’ll see if what you discovered holds true to those tachs as well when I get my hands on one next week. Good info to pass along.

 

[93CalypsoConvert]

43.2K original 20.0K swapped

 

[Mustang5L5]

43.2K original 20.0K swapped

Let me see if the same hood true with the older tachs.

 

[Mustang5L5]

Finally got a chance to mess with the 87-89 tach. First I hooked up the frequency generator to make sure they both output what I expected them to.

As you can see, 4 cyl tach is 2x.

Broke them down and started measuring/recording resistances. Not much is different. All the readings were pretty similar except for one resistor.

V8 tach was 210k ohms
4cyl was 188k ohms.

So roughly a 22k ohm difference…same as the newer tach it seem.

Need to grab a potentiometer to try adjusting the value.

It’s the small resistor on top right side that is standing vertically next to a larger resistor. Not confirmed yet, just suspected.

 

[93CalypsoConvert]

The resistor that I changed was the next component on the circuit after the potentiometer.

 

[Mustang5L5]

The resistor that I changed was the next component on the circuit after the potentiometer.

Thanks. I just checked the next resistor in-line and not much of a difference between the two. 56k ohms vs 58k ohms. Not sure if that’s enough to do the trick but I’ll test it.

 

[Mustang5L5]

Ok. First resistor after the potentiometer it is. What’s strange is the values are not that much different. 64.9k ohms on the 4 cyl and 60.4k ohms on the v8 tach. That’s according to the bands on the resistor and not measured values

I did try a 61k ohm resistor in the 4 cyl tach and it didn’t work, so there is some other differences in how these tachs are setup other than a sole resistor.

What’s even more interesting is the values drifted a lot from their markings. More than 10% in both cases. Able to compensate with the onboard potentiometer but that probably explains why these tachs are so inaccurate of those resistors are that far out of tolerance.

I tested a few resistors I had and it looks like 15-25k ohms is the range. Rather than guess I ordered a potentiometer in the correct range to dial it in.

Here’s the location for everyone who might find this thread in a search for the 87-89 tach. It’s the spot currently missing a resistor marked with the two red arrows. I’ll chime in with the correct value once I determine it.

 

[Noobz347]

Add thread tags when you're all done. Gimme a holler if you need help.

 

[Mustang5L5]

24k ohms is the value. Actually 24.5k would be better but I didn’t have that actual resistor on hand.


What I did was set the 20k adjustment pot to 10k ohms and found the 24k resistor put me almost on the money. Had to tune to 10.5k ohms with the adjustment pot and it was spot on.

So, 87-89 tach. 24k or 24.5k ohms is the resistor you need to put in the above spot.

Actually, 89 tachs have two adjustment pots. So this one could be different. I have an 89 tach in my car, but not pulling it out to verify right now.

Frequency set to 200 hz, which is 3k rpm.

 

[93CalypsoConvert]

Good stuff. I wonder what a 4-6-8 switch actually does do the circuit on tachometers equipped with it.