Speedometer jumping, and inaccurate

[mpgFanatic]

Note: this topic came up in Member's Restorations, but I decided to address it here instead.

The car: a 65 Dart wagon with an unknown 69 4-speed transmission.

I have put 800 miles on it since then acording to the incredibly inaccurate odometer. The speedo rattles and the needle jumps around. I went by a sign that tells you how fast you are going and it said 24 while the guage said 40.

The speedometer and odometer do not necessarily share the same accuracy. If you had a factory transmission and rear gears and tire size, the odometer would likely be just about perfect (mine is off by less than 0.5% as measured against 100 miles worth of highway markers.) With your unknown 4-speed, you may need to calculate how far off the calibration is, then find out how many gear teeth are on the speedometer pinion, then figure out how many teeth you ought to have, and get a new speedo pinion. The odometer rollover is simply a function of gear reduction (rear end ratio, transmission tailshaft, and pinion gear). When the cable rotates 1000 revolutions per mile, the odometer is accurate, and it stays accurate despite the age of the vehicle.

(The pinion gear is just inside the transmission, at the end of the speedo cable-- watch out you'll probably need a bucket to catch tranny fluid when you pull it out. The 1965-older style gear is made of unobtanium, but the 1966-newer style is still fairly common.)

Once the odometer is accurate, THEN worry about the speedometer needle. If the magnets have lost some of their power (very likely) then you may need to have someone recalibrate (and clean and lubricate) the speedo head unit.

It might be tempting to do it in the other order (compensating for an inaccurate needle by changing the pinion gear), but you'll find it unsatisfactory because the speedometer is typically off by different percentages at different speeds. Mine showed more than 10-15mph error at city speeds, down to 5mph error at legal highway speeds, then back to 10mph error when even faster.

A jumping needle is often the sign of a poorly lubricated speedo cable, or a cable which is asked to make a sharp bend. The cable insert is actually speeding up and slowing down as it whips around inside the sheath, and the needle is giving an accurate indication of what it's being fed.

Please do fix the speedo head unit before it self-destructs. There aren't many replacements in the junkyard anymore. Don't ask me how I know...

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

 

[timbolia]

Thanks for starting this thread for me and my Wagon!

So there will be trany fluid where the cable attaches on a 4 speed?
And this twitchiness can hurt the speedo head? Would I be better off detaching it til I have time to sort this out?

I am hoping to spend as much of the holiday break as I can working on the car. I have the usual issues to address. Fuel gauge, light switch and seat belts.

Congrats on moving to SF, I lived in the Bay Area for 10 years or so and I still miss it.

 

[2 Darts]

My issues in this area come from: 1. Cable not fully seated in transmission pinion and in the speedometer head. 2. Lubrication of speedometer cable.

1. Tough to solve with the plastic clip on the back of the speedometer. It will connect without having the cable completely seated in the speedometer. If you can rotate the cable with your fingers and the car is stopped, suspect a broken speedo cable, or cable not seated firmly in transmission pinion.

2. I get powdered graphite at a locksmith for lubing the cable. It can be messy and don't sneeze. It avoids the problems of moisture and liquid lubricants. For maximum effectiveness, puff into cable at speedometer end.

 

[70Duster440]

FWIW, while the usual suspects have already been mentioned (kinked or dry cable), I had one that would not settle down till I lubed the connection at the speedo head. The cable end fits into a square shaped socket. That socket rotates inside an outer bushing where the cable connects. If the bushing has gone dry it will be noisy and you'll have a bouncy needle too.

 

[dusterdon]

A couple of years ago, at the end of the cruising season, I got on my duster really hard. After I slowed down, I noticed the speedo jumping around. Figured that I broke something inside. Put car away for winter. Over the winter bought new dash insert with speedo and guages. Of course, come the spring, the speedo was now fine. It was very accurate before acting up and now its still as accurate. Go figure.

 

[mpgFanatic]

So there will be trany fluid where the cable attaches on a 4 speed?
And this twitchiness can hurt the speedo head? Would I be better off detaching it til I have time to sort this out?

Sure, there's fluid pretty nearby-- after all, the plastic pinion gear that drives the speedometer cable is inside the transmission, meshes with a spiral gear cut on the output shaft. That entire gear mesh is lubricated by trans fluid. Concentric seals near the pinion gear keep fluid from exiting the transmission: one keeps it from dribbling on the ground, one keeps it from heading up the speedo cable sheath.

The way I figure it, the tiny little bearings holding the speedo needle were designed for however many million cycles of 0-60-0. No point in arriving at that wear point prematurely. And yes, I've bought two cars with speedo heads that had failed in such a catastrophic way that the needle had totally fallen out, sitting at the bottom of the instrument cluster, and the little spring was stretched out and wrapped around the mechanism a dozen times. (Perhaps a dirty/gummy/worn mechanism grabbed the needle and played a game of tetherball? That brief event would have been pretty exciting to watch.) The speedo repair guy said it was beyond hope, and he'd have to find another used one to get parts. That's obviously getting harder every year, so I try to take care of the parts I have to keep 'em running smoothly.

Gosh, life was simpler when these toys were just old ratty beaters, and parts were junkyard-plentiful...

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

 

[timbolia]

I disconnected the speedo yesterday when I replaced a bad headlight switch.
I will get some graphite and check the speedo head next weekend (back to work)

I did notice on the drive in today that the noise was still coming from under the dash, so that makes me think it is the cable.

 

[mpgFanatic]

I did notice on the drive in today that the noise was still coming from under the dash, so that makes me think it is the cable.

Yes, cables tend to make the most noise of any of the components, but they're easy to fix/lube/whatever.

In contrast, an old speedo head needs cleaning/lube/calibration but it generally doesn't make a whole lot of warning noise before it self destructs. As automotive repair jobs go, this one is pretty finicky. I almost never farm things out, in favor of simply buying another expensive tool that I may use only once... but this one's easier to let someone else do.

I do wonder, though, how much longer the speedo repair guys will be in business before they all retire. Cable-operated machinery = dinosaur technology.

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

 

[archlab]

One thing that i did to solve a jumpy Speedo needle was to pull off the needle & re-coil the little spring. I did this a couple of times, as the first time totally stopped the needle. Th e second time, I loosened the coiled tension a tad, then the needle went 0-60 with no problems. However, I can't attest to the accuracy - I'll check that. BTW, I say '0-60' because I recall somewhere in the past someone telling me that the 'high-speed' on a cordless drill will get you up to 60mph. That has always seemed to be what I've found - so this 'method' may be OK for ball-parking a speed. But, accuracy's different. This usually keeps me from speeding (that + 8mpg).

 

[65 Dartman]

I do wonder, though, how much longer the speedo repair guys will be in business before they all retire. Cable-operated machinery = dinosaur technology.

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

Can't answer the longevity question for speedo repair shops but can pass on a great place in Denver (Deluxe Speedometer and Radio Service) for repairing our speedos. I've had 2 done by them where they calibrate the odometer and then the speedometer part. One was a regular MPH speedo; the other a KPH speedo going in my 65 Barracuda (it will be my oldest son's and going to the Netherlands where speed is in KPHs). Also had a Porsche VDO 160 MPH mechanical speedo done the same way plus some minor mods by a place in FL. This one is going in my 65 Dart with a customer 66 Barracuda dash.

 

[169dart4y]

I had the same issue with a jumping speedo and when I would go over 65 mph it would then go crazy and wander up in the 80+ range.
The cable was dry 40 years old 69 , popped the cable sprayed lube in the end of cable . Done haven't had an issue since

 

[timbolia]

That sound like what I need to do. Been runnin with the speedo diconected, just need to get in and do this this weekend.

 

[65 wagon]

That sound like what I need to do. Been runnin with the speedo diconected, just need to get in and do this this weekend.

=========================================
Not sure if you resolved this or not...but I have hasd the same issue on many a ride. The cable was the issue 98% of the time (Unless I didn't address it soon enough...then the head would also need attention Replacement cables are pretty cheap. I always replace a suspect one & lube the old (spare) & new one at the same time. Works 98% of the time!

 

[belmateo]

I had the same problem that I will share with you. It turned out to be a worn out gear that turns off of the Speedometer gear (90 degrees) for the Odometer. That gear was causing the speedo to jump as it was hitting the worn out places on that gear. Hope this helps.

 

[timbolia]

I still need to look at this, but that is a possibility. I think it is the cable though since I still get some noise with the speedo detached.

 

[Poppabear]

Sure, there's fluid pretty nearby-- after all, the plastic pinion gear that drives the speedometer cable is inside the transmission, meshes with a spiral gear cut on the output shaft. That entire gear mesh is lubricated by trans fluid. Concentric seals near the pinion gear keep fluid from exiting the transmission: one keeps it from dribbling on the ground, one keeps it from heading up the speedo cable sheath.

The way I figure it, the tiny little bearings holding the speedo needle were designed for however many million cycles of 0-60-0. No point in arriving at that wear point prematurely. And yes, I've bought two cars with speedo heads that had failed in such a catastrophic way that the needle had totally fallen out, sitting at the bottom of the instrument cluster, and the little spring was stretched out and wrapped around the mechanism a dozen times. (Perhaps a dirty/gummy/worn mechanism grabbed the needle and played a game of tetherball? That brief event would have been pretty exciting to watch.) The speedo repair guy said it was beyond hope, and he'd have to find another used one to get parts. That's obviously getting harder every year, so I try to take care of the parts I have to keep 'em running smoothly.

Gosh, life was simpler when these toys were just old ratty beaters, and parts were junkyard-plentiful...

- Erik

64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd

This is a dumb question but Do you run the drill in F or R ?

I have never tried that way to check.