Electrical Speedometer way off

Andresquintana.mma

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Nov 28, 2014
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I bought a 95 gt with minor mods and a few small problems which are mostly fixed now. One of the problems was the speedometer didn't work. The guy said it wasn't the right transmission and the cable wasn't plugged in so today I get under there and it's plugged in but the vss isn't connected try to connect it but it doesn't fit. So I tie up the wires and start driving. Speedo works now but jumps all the way to almost maxed out almost makes a full rotation when I'm going like 30 mph.
 
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7991LXnSHO

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Since no one has sn95 specific help yet, I will offer general suggestions. You probably have deduced this is likely why it was disconnected. If you can ID the trans or get some pics or numbers, someone here can get you SN 95 specific advice, or we can at least Id what you might have.

Unless the design changed a lot between a Fox and SN95, I doubt the gear drive would be that incredibly off. Unless there is a major gear reduction in the right year VSS, I suspect the cable or speedometer head is having big troubles. Jumping, palsy, way too fast or way too slow readings are normal signs of cable and mechanical speedometer head unit problems. Smooth readings that are off by 10-20% are often the sign of wrong gears and tires. The error is more on BigFoot Clones and Donks, but I hope your Mustang does not fit either description.
Are you are on good enough terms with the PP to find out what the transmission is out of? Even if the speedometer needs rebuilt, fitting the right VSS will make your computer happier.
 

Chythar

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Gear ratios for the VSS should be the same between Foxes and SN95s. There is a plastic gear on the VSS, and it is spun by a gear inside the transmission. The VSS tells the computer tells the computer how fast it's rotating, and the computer sends the signal to the speedometer. You use a specific plastic gear on the VSS depending on the rear-end gear ratio in the rear axle. The gear inside the transmission is supposed to stay the same. Ford installs the right gear on the VSS when it comes from the factory, but if you change the gears in the rear axle you have to change the gear on the VSS to match.

In theory, the transmission shouldn't matter as long as the VSS gear inside the tranny is the same. You can have a different gear installed, but we'll ignore that possibility for the moment as it complicates things.

It all depends on what gears the previous owner has in the rear axle. You can figure out it yourself by jacking up the rear end and putting the tranny in neutral. Put a sharpie mark on the driveshaft, then count how many times the rear wheels rotate until the driveshaft rotates once. Examples are 2.73, 3.08, 3.27, 3.55, 3.73, 4.08. There are plastic gears for all but the last two. For our years, Ford didn't install 3.73's or 4.10's so there's no plastic gear that's right. The only gear you can use will leave your speedo off by several MPH and it breaks easily.

The VSS is awkward to slide in as the two gears have to mesh properly. And there should be a single bolt that holds the VSS in place.

I'll guess that the VSS gear you have is for 2.73's, but you've got 3.73's or 4.10's in the rear end. Another possibility is a bad VSS or fried wires leading to the VSS. Let's check the VSS gears and the rear end gear ratio first. So, go figure out the rear end ratio like I mentioned, then look at the VSS gear. Write down the color of the plastic gear and count the number of curved teeth on it. Then come on back.
 

General karthief

wonder how much it would cost to ship you a pair
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When I was doing research on my rear gear change there was a change in the tooth count in the gear inside the trans, (I bought a rebuilt trans and needed to know what drive gear was inside) I don't recall exactly when or why the change was made, all I recall is there was a 7 count and a 9 count (?) gear, not sure if this even factors in to this particular issue
 

7991LXnSHO

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Almost maxed out by 30 does not sound like the difference between 7 and 9 teeth gear. The percentage of change {~20%} does not seem enough. At least it is not on a TH-350. Checking out the rear end ratio is still a good idea.
 

General karthief

wonder how much it would cost to ship you a pair
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Almost maxed out by 30 does not sound like the difference between 7 and 9 teeth gear. The percentage of change {~20%} does not seem enough. At least it is not on a TH-350. Checking out the rear end ratio is still a good idea.
I thought it had no bearing, just wanted mention it to confuse the issue.
 

7991LXnSHO

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I thought it had no bearing, just wanted mention it to confuse the issue.
Your thought/knowledge might be quite important in the final adjustment of things. Who knows what hackery has been done in the last 25 years to this poor car?

I pulled of the Lincoln Highway in a little town to look at an SN-95 with a for sale sign in the window. It looked good on the first drive by earlier in the day. The up close inspection was a forensic nightmare, and I bet I’d owe the Car Fox a six pack for his report. It is like looking at at first gen Mustangs was back in high school, but with more wires and plastic pieces.