Drivetrain Speedo problem

Matt88Vert

Member
Jun 27, 2022
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5
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Alright guys this speedo not working is driving me crazy. Only owned this car a a few months. It’s an 88 GT manual. When we bought the car the speedo was changed to a 140. 2 previous owners ago the speedo stopped working. They left all the factory stuff just unhooked it and replaced it with a GPS unit. It worked great when we bought it. And if we are driving in in town where it picks up good. I’m the mountains not so well. So I decided to fix the org one. Removed GPS and hooked up old cable. No good. Ordered new cable, gear, servo, and reinstalled. Nope, still not working. My only guess as to what’s wrong may be the inner gear in trans. Before I go so far to drop trans. I wanted to ask if there is something that you have to change when swapping to a 140 cluster? Being I didn’t do it. I’m not sure what was changed. It bolts up in the org spot. The gauge meant for in the cluster great.
Is there anything else that you have to change in this swap to make it work?
What’s y’all’s thoughts on what it could be?

Thanks in advance.
Matt
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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I second that. Hook a drill up and see if the speedo moves.

If you have a cheapo little handheld tachometer and can tell me what RPM your drill spins at, i can tell you what speed that should be on the speedometer.
 
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KRUISR

5 Year Member
Apr 16, 2015
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If it is indeed the gear inside the trans, i believe with 2 people it can be done with the trans in the car.
I would not want to even try pulling the tail shaft with the trans in the car, that does not sound fun...
I agree with rednotch, having pulled the tailshaft off a couple T5s, I would not want to try and get the tailshaft on properly while still in the car.

A T5 is not that heavy to remove from car. I have had a tricky time getting the tailshaft on while trying to get the shifter lined up with the detents, roll pin in and not knock the speedo gear out of place. Best to do it out of the car.
 

manicmechanic007

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Sep 26, 2017
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Usually just take two fingers and twist the cable both ways and see if the speedo moves
That way you can see if it is binding or the housing is worn (if the cable jumps around in your fingers)
88 GT T5 has a cruise speed sensor that the cable goes into right?
The bearings in those used to take a chit
I would not assume that the drive gear is worn
It is easy to check with your finger and or a flashlight and a mirror
The gear teeth should be flat not concave across the surface of the gear
If the gear is worn the middle teeth will look wider
 
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Matt88Vert

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Jun 27, 2022
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Ok, so I got to mess with it more this weekend. I have narrowed it down in my opinion to the transmission gear. I can get the needle to move by putting a drill on the end of the gear. I have replaced the cable, speedometer servo "sender" and gear on end of servo. Is there anything else that I am missing? All that's left is the gear in the tail shaft correct?
 

manicmechanic007

5 Year Member
Sep 26, 2017
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Correct
I want you to look in there and inspect the gear
Use a mirror and a flashlight
And use your finger
You can feel the teeth
If they feel concave with a dip in the middle, the gear needs to be replaced
They make the driven gears in left and right pitch
Having the wrong pitch tears up the drive gear quick
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Look inside the hole and see if you physucally see a drive gear.

It's VERY common for the small snap ring that secures the speedo gear to the mainshaft to fail, causing the speedo gear to walk down the mainshaft. You will need to pull the tailhousing off to install a new snap ring.

If you look in the hole, you should see a gear like this. 88 would be a yellow 7-tooth drive gear (this photo is not a mustang, but similar to what you’ll see)
1657557775149.png




If it's not there, then it walked. This is the clip that failed



It installs here to hold the gear in place

1657557928556.png




Here's an install article
 
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Matt88Vert

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Jun 27, 2022
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Look inside the hole and see if you physucally see a drive gear.

It's VERY common for the small snap ring that secures the speedo gear to the mainshaft to fail, causing the speedo gear to walk down the mainshaft. You will need to pull the tailhousing off to install a new snap ring.

If you look in the hole, you should see a gear like this. 88 would be a yellow 7-tooth drive gear
1657557775149.png




If it's not there, then it walked. This is the clip that failed



It installs here to hold the gear in place

1657557928556.png




Here's an install article


Looks like you have been where I am already. Thanks to you and

manicmechanic007 for the help.​

 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Looks like you have been where I am already.

Not me personally but I did a lot of T5 rebuilds for my friends and local Mustang guys in my younger days. I prob rebuilt a few dozen. I saw this a lot.

5th gear snap ring failing and losing 5th because the driven gear slips down the mainshaft a tad was also a common issue I saw
 

Matt88Vert

Member
Jun 27, 2022
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Alright so I got the speedo fixed. At least it’s working now. But now my problem is it’s 10mph high at 60 mph on a GPS. I went to LMR and ordered the drive gear I needed according to all the info I had. The color and teeth match up on the driver gear to the one that I removed. The cable gear was worn so I replaced with a new one as well earlier. I ordered it from LMR as well. Gear ratio, 5-Speed tranny, and correct tires. LMR said I needed a 17 tooth gear and it’s white. I did not count the teeth on the old cable gear before tossing. But I do know it was white as well. Any idea how I can determine what gear I need to fix problem?
Again Speedo n car reads 70 mph but gps say 60 mph.
Thanks again for the help guys.
 

KRUISR

5 Year Member
Apr 16, 2015
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You never said above, but what tire size and rear end gear ratio do you have?

Here is why I ask:

Here is a screen shot from LMR Speedometer Gear Calculator

gear calculator.jpg


This is what you would get if you plugged in the numbers for a stock 88 GT equipped with a T5 and 2.73 gears and stock sized tires.

You will see it recommends the white 17 tooth gear. BUT you will notice in the top right it says the GEAR YOU NEED is 15 tooth. Such a gear does not exist and they recommend the closest one available.

I haven't done the math, but the error you are seeing would be close for this scenario.
 
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Matt88Vert

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Jun 27, 2022
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Yellow 7 tooth drive gear came out and was replaced with same. 17 tooth cable gear came out and was replaced with same. Speedo has been upgraded to 140mph.

KRUISER, I guess I missed that when I placed to order. I did it again and got the same thing you did. I didn't read it the first time. All the specs are same as you have.
2.73
5 speed
225/60/15

So how do i correct my problem? Not finding a 15 tooth gear anywhere.

I am guessing this is why the previous owner had installed the GPS speedo. But the trip didn't work. And I hated the black bulge on the dash for the antenna.