Fox 4 Cylinder to V8 Tach

[93CalypsoConvert]

SOLVED: See post #30 and on for solution

My car was originally a 4cyl, now V8 swapped, and the factory tach reads double the rpm. Its a 93', so the switch on the back of the cluster is not present. After doing some reading I found that you can solder a 50k resistor on the tach's printed board to get it so work. Does anyone know exactly where this resistor would need to be soldered? I'm trying to make everything in the factory cluster work.

 

[AeroCoupe]

Do you want to keep the 85 mph speedo as well? I just swapped in a V8 cluster when I did mine but they are probably not as cheap now. From what I have read it will not be as accurate as the V8 tach.

 

[93CalypsoConvert]

That would be nice to get a 140mph, but I can't seem to find one under $150. Also, my current cluster reflects the true mileage on the body. And I'm cheap

 

[AeroCoupe]

They are out there you just have to hunt a little and catch it before someone else does. So I hate to say this but your car has a 4 cylinder VIN so its value to a collector that cares about mileage is zero as they are looking for V8 cars. Realize I am not saying this to be a dick as my Coupe is a swapped car. I have had offers for the car and those that know these cars and are not collectors really don't give a crap if its swapped. Another benefit of the V8 cluster is the tack is a 7000 rpm vs the 6000 rpm of the 4 cylinder one.

I just took a picture of the old cluster and wrote the mileage on the swap cluster and what the swap cluster was at mileage wise when they got swapped. You can "adjust" the mileage on the new cluster and I will leave that up to you to research.

 

[Mustang5L5]

It would be on the S (signal) connection to the tach. I do not know the effect of what a 50K resistor would do to the tach. You can use a signal generator to generate a frequency to verify that the tach operation is correct.

Fixing the mileage is pretty easy to do. Just need to to drive out the pin and adjust the wheels to the correct mileage. I think i have the Ford Racing instructions somewhere on how to do it with the 140/160MPH speedos they sold.

EDIT: Searching around a bit, it seems that the method involves desoldering the existing resistor and replacing it with a 50k resistor, and not putting it inline to the S wire. Looking at the tach I have here, there are 10 resistors on the board. Would need to identify which resistor is the one that needs to be replaced first. You'd then need to desolder it, and solder in the replacement.

If i had a 4cyl tach in hand, i can prob figure it out.

This video shows how you can bench-test the tach with a $10 frequency generator


View: https://youtu.be/UkHoSIdTpbc

 

[General karthief]

That would be nice to get a 140mph, but I can't seem to find one under $150. Also, my current cluster reflects the true mileage on the body. And I'm cheap

You have not discussed a 140 speedometer with me, I have several parts clusters.
PM me if interested.

 

[93CalypsoConvert]

I see. Good info. I'd still like to experiment with what I have. A $0.15 resistor is a lot cheaper than buying another cluster/tach. I will experiment around and see what I can find. If anyone knows which exact resistor needs to be replaced, that would be great. I'll keep everyone updated.

Side thought- What are the chances I could swap the guts of another 90s ford tach (Like from an F150) into the fox cluster? I wonder if they would use the same circuit board. I could probably pick up a truck cluster for much cheaper than a mustang's

 

[Mustang5L5]

The part number on the tach mechanism is Mustang specific. The body does mount by three bolts, so they can be swapped but no idea if a truck one is compatible.

I'll see if i can read the resistor values on this later.

 

[Mustang5L5]

I snagged a cheap 4cyl tach. I'm curious to see if this can be done.

 

[AeroCoupe]

Just could not help yourself could you? Be interesting to see what you find out.

 

[Mustang5L5]

Just could not help yourself could you?

I couldn't. It's the engineer in me. It will just eat at me

 

[AeroCoupe]

Double E or ME?

 

[Mustang5L5]

ME but if I could go back in time I would have gone EE as I am also a licensed electrician and always liked electronics.

 

[AeroCoupe]

I’m an ME in a family of civil’s…weirdos.

 

[Noobz347]

You have not discussed a 140 speedometer with me, I have several parts clusters.
PM me if interested.

 

[93CalypsoConvert]

I'm also an ME... Something about these cars must attract us engineers

 

[Noobz347]

We have more than a hand full of aircraft mechs here too.

 

[General karthief]

We have more than a hand full of aircraft mechs here too.

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

 

[Noobz347]

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

We both lived

 

[93CalypsoConvert]

Here’s what I’m working with. It’s all apart.
I have no idea where to start. I have my eye on that 20k ohm pot in the top right on the last picture.