Speedometer Puzzle

[geordie]

Hi guys, it's been a while since I've posted anything......but I have a puzzle for anyone who knows something about speedometers.
Background - I have a 65 Mustang Coupe, 302 V8, modern T5 manual transmission. Car was finished about 5 years ago, speedometer functioned fine for about 3 1/2 years, finally starting behaving erratically, did not behave normally, would read 60 mph standing still, stick at high speed etc. The speedometer appears to be the original one....stamped FORD.
This weekend I finally got around to doing something about it. I checked the cable, seemed ok, no apparent binding, plastic gear on transmission end looked like new, so I replaced the speedometer with an aftermarket one I had. Put it back together, speedometer now rises normally with increasing speed and falls as you slow the car down. The trouble is that is seems to indicate about double the speed that I am doing.....so if I an traveling at about 25-30 mph, the speedo shows about 60 mph.
The only thing I changed out was the speedo.......what is going on? Could an aftermarket speedo have different gears from the stock speedo? Is there some kind of adjustment on the speedo (don't think so)? It it something electrical (don't think so as operation seems to be purely mechanical)?
Thanks in advance.

 

[geordie]

I have satisfied myself that the speedometer is a purley mechanical operation....so nothing electrical is wrong here. Seems very straightforward, All I have changed is the speedometer, no tire change, no rear end change, no speedometer driven gear change. It would appear that I have a bad, new, aftermarket, speedometer. Has anyone ever experienced this? So I appear to have two choices (1) try to correct this speedometer by changing the driven gear, or (2) buy another new speedometer and try that. Option 1 is much cheaper....but should not be necessary if the speedometer is working correctly.

 

[tos]

Maybe you ordered the Kilometer version?

 

[broncojunkie]

I don't believe you can fix that big of a difference by changing speedo gears. I'm not familiar with how the speedometer works on the older cats, but I'm wondering if the older speedo is compatible with the t5. I'm sure someone else who has done the deal swap could tell you.

I have a 70 coupe, but been too busy working on Fox bodies to really get into this one. I very well may do a t5 swap at some point.

 

[tos]

I did the T5 swap and my original speedo was within 3 mph without changing anything

 

[broncojunkie]

A lot of variables that I would have to research. Did Ford use the same speedo for all early Mustangs? Are they the same for auto and manual tranny?

 

[tos]

The speedo itself would be the same from manual to auto. You would change the gear at the transmission end of the speedo cable depending on your differential gear ratio and tire size. I did end up changing mine when I went to 3.73 ratio rear. It was only a tooth or two difference though.

 

[geordie]

The previous speedometer was working with the T5 application for a number of years.....before it went south.
Does anyone know if there is any kind of adjustment on the speedo itself....I think not.
I think my only option is to try another speedometer....

 

[geordie]

Maybe I should try connecting both the speedometers to a Dremel or similar, constant speed, compare the readings on both speedos, see if the gearing is different.

 

[tos]

That would be a good idea. Run a drill at full speed and check both.
I don't know how this speedo is built but I've worked on some foreign cars that had a magnetic pickup style speedo gage which consisted of a drum inside another drum and there was no gearing for the needle itself. If one drum got cocked a little then it would read irratically. As I said though I'm not sure how this one works.

 

[geordie]

The other thing that I've noticed is that it's almost as if the cable is too long. It's jammed into the speedometer and if difficult to remove. If it is too long and I continue to tighten the nut them I am pre loading whatever bearings are in the speedometer....I may shave a little off the cable length.

 

[geordie]

Today I connected both the original speedometer and the new one to an electric drill.

1. The gearing for the original speedometer and the new one IS DIFFERENT. The original reads about 70 mph at max. drill revs. The new one reads 120+mph at the same drill speed. Clearly something is wrong with the new speedometer.
2. Looking at the box the new speedometer came it, it clear that it has suffered some form of impact. The connection point on the speedometer, for the cable, has been forced into the box, the box is damaged, almost punctured. The needle shaft is also bent. I'm fairly sure that this did not happen in my garage......but I do have kids (LOL). I've had this part for a couple of years....so no chance of returning it.
3. Looking at the original speedometer, there is a lot of axial play on the cable connect, and depending on the axial position of the shaft, the needle sticks. I think that this excessive axial pay has been caused by tightening the cable sleeve too much and forcing the end of the cable against the stop. Possibly need to shorten the cable a tad. But I think that this is the reason that the current speedometer is misbehaving.

So I am going to order a new Scott Drake speedo from CJ Pony Parts, as I believe that these are pre-tested for accuracy.
Than I will try rebuilding the binnacle again.

 

[geordie]

All is right with the world. My new Scott Drake speedo from CJ Pony Parts, arrived and I installed in last weekend. It works !! I can only conclude that the other speedometer had been dropped / damaged. I did very carefully file the end of the speedo cable, to ensure that it slid into the opening smoothly and that the stop seated correctly. I also did not over tighten the cable nut on the speedometer.