Interior and Upholstery Jittery 88 GT Speedometer

ragtop88 · Jun 15, 2019

Here’s the short of it... i recently purchased an 88 GT with a jumpy speedometer. I replaced the speedometer cable not once, not twice, but yes - three times with the same results. I removed the cluster and checked the speedometer mechanism & do not see anything evident with it (moves freely with no excessive play). Stretched the cable out straight and rotated it with a drill — still jumpy. Anyone experience this??? SOOOOO tired of pulling the cluster.

Mustang5L5 · Jun 15, 2019

Usually it's the bushings in the speedo head that cause the binding

jrichker · Jun 15, 2019

Mustang5L5 said:
Usually it's the bushings in the speedo head that cause the binding

Do you have any way of testing the speedometer to prove your theory? I have replaced the cable and lubed it up good with graphite and still have jitters below 40 MPH.

Mustang5L5 · Jun 15, 2019

Unfortunately no, not an easy way to diagnose. Usually it's a process of elimination.

General karthief · Jun 15, 2019

I don't pay any attention to my speedometer below 80 mph
Ok, so I don't pay any attention to my speedometer at all

Jhp84 · Jun 16, 2019

We will be following this thread. The speedometer on our 91 GT works well until we get to 40 mph and then it jumps past the top speed to the bottom and dances back and forth like crazy. Thought about ignoring it, but since my son is 18, thought about putting a bright warning light on the cluster if he tries to go past 70 mph.

Mustang5L5 · Jun 16, 2019

Unfortunately there isn't a kit to rebuild the speedo head itself. I have my old 85mph speedo kicking around that worked perfectly. I should just swap the bushings to my current one as I get the speedo bounce as well. I've tried a few cables, and it done it on my AOD and t5 so I'm sure it's not trans related.

Btw, if you decided to change clusters or speedo heads, it's very easy to match the mileage up. Almost too easy

If I get a chance to dissect mine and find they are standard parts I'll update

Mustang5L5 · Jun 16, 2019

Ok went and grabbed my 85mph speedo.

Here's where I think the issue is. The speedo cable turns a cylinder that has two fingers that "grab" a cylinder connected to the speedo needle with spring resistance. Unsure if the gripping force is magnetism or friction, but the cylinder also turns two sets of gears that operate the odometer and tripometer. If there's any binding in those gears it will show as a bouncing speedo. Lubricstion may help.

If you decide to try a different speedo, all you need to do is tap out one side of the top bar and spin the digits to match the miles. Normally I hesitate to post things like this but it's been 30 years an most of these cars have messed up miles anyway and low mileage cars usually have a paper trail.

DemonGT · Jun 16, 2019

My 91 gt has always had a jumpy speedo. Iv replaced everything but the speedo and still dose it. I gave up and just use a gps speedo app on my phone lol

Dose the odometer still record correct if speedo is jumpy?

General karthief · Jun 16, 2019

It should register correct miles, the speed indicator needle I believe is driven magnetically.

Mustang5L5 · Jun 16, 2019

Yes, he odometer is physically connected to the speedo cable and turns by gearing directky as seen in above pic. You could physically hold the needle down and the odometer will still work.

I just remembered one method I used to see if the speedo head was bad was to hook a drill with adjustable chuck and variable speed up to the speedo head directly and turn it.

ragtop88 · Jun 17, 2019

Thanks for the replies... I removed the cluster, AGAIN, and disassembled the speedo. After checking it more carefully this time, I noticed the clock spring was crooked and binding on itself when the cup rotates. I repositioned it on the shaft, lubricated all pivot points (props to Mustang5L5 - excellent photos!!! ) and put it back together. I plan on reinstalling it tomorrow after work and test it. I’ll keep you posted... BTW- I lubed the speedo cable with white grease. Not sure if is the best lubricant or not but it’s what I had on hand.

ragtop88 · Jun 22, 2019

Well... back together again, test drive — NOPE!!! Jumps around like a $@&!? kangaroo. HAS to be the cable... it is aftermarket after all (Pioneer makes it). Could it be the VSS??? I didn’t change that; however it spins smoothly so I figured it was OK. Any additional suggestions, outside of pushing it into the Atlantic, would be warmly welcomed.

DemonGT · Jun 22, 2019

Buy Dakota digital cluster?

Mustang5L5 · Jun 22, 2019

Attach a drill to the cable end or VSS and spin it up and see if it still jumps

George7 · Jun 23, 2019

Looking at this thread brings me back the thread starting in Dec., 2017. Look at "My rebuilt speedo took a sht" between me and Blakeus Maximus. Same problems and probably there are earlier threads with the same problem.
Your jumping needle is unlikely to be the new cable, but caused by the interference in the speedo head between the fork and magnet wheel. Run a drill at the head on the bench and see if it runs OK. Even running smoothly on the bench may not help as attaching the cable after the head is installed on the panel will change the geometry in the head and create interference. After a few speedo replacements in several LXs and more than a few cables, I believe the problem is the worn bushings in the speedo head. They allow the cable to wobble and slide.
I have not found a kit or rebuilder that can replace all those little bushings. I know Blakeus had a rebuild but that didn't help either.
Eventually, I found a used speedo on Ebay that jumps below 40 mph but smooths out OK above . Good enough for me and there are GPS apps for your smart phone that work well. Beyond that are the pricey electronic speedos. Good luck. Let us know what you do.

EBW83 · Jun 23, 2019

Agree totally with George7 above. My original 85MPH head read low by 15-20MPH and jumped. Head behaved better using a drill but when hooked to cable would still jump. I bought a rebuilt head (140MPH) from Fox Mustang Resto and it works beautifully and is spot on accurate. Unfortunately, I don't find it on their website anymore but was under the impression that they did the 85MPH-140MPH conversion and rebuild in house. Might be worth a phone call to FMR. New head cost ~$160 end of 2017.

Mustang5L5 · Jun 24, 2019

BTW, I replaced a speedo cable and found the metal inner spring was a touch too long and caused the cable to bind and skip. I ended up trimming about 1/4" off the inner cable and carefully reshaped it back to a square. That did eliminate some of the bounce.

My cable bounces lightly up to 40MPH and is smooth after that. Good enough for me, but I do suspect it to be the speedo head.

jrichker · Jun 24, 2019

Potomus Pete said:
I am not good with electric

Mustang5L5 said:
BTW, I replaced a speedo cable and found the metal inner spring was a touch too long and caused the cable to bind and skip. I ended up trimming about 1/4" off the inner cable and carefully reshaped it back to a square. That did eliminate some of the bounce.

My cable bounces lightly up to 40MPH and is smooth after that. Good enough for me, but I do suspect it to be the speedo head.

That sounds exactly like what mine is doing....

ragtop88 · Jun 26, 2019

Mustang5L5 - interesting you mention the excessive inner cable length... both cables I tried from Pioneer were too long as well. I also trimmed the second one back, lubricated with a dry graphite powder and crossed my fingers. Improved but still slightly jittery under 40. Speedo rotates smoothly with a drill directly on the input bushing. I’m just going to leave it.... it’s the best I can get it.

Interior and Upholstery Jittery 88 GT Speedometer

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