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Speedo problem....?

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kodanja

kodanja

Obi Wan
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Hello to everyone
I'm having a problem with my 71'TR6 speedometer,

I first replaced the speedo cable to the tranny (making sure that it was tightened up to the tranny), that did not seem to make a difference, so then I replaced the speedo.
It worked for a bit now it does not. Ocassionaly when i'm driving it will kick in the just stop, I'm not sure what to do next... /ubbthreads/images/graemlins/nonod.gif /ubbthreads/images/graemlins/england.gif /ubbthreads/images/graemlins/nopity.gif
 
First thing I would do is discconnect the cable from the speedo head and observe the end of the cable while driving. You should be able to feel (or see) it turning. Do not let the spinning cable engage your leg hair (though this would make the event memorable).

If the cable is spinning then look to see if it projects far enough out of the cable housing to engage the speedo.

My guess is that both of these checks will be okay but should be verified before pulling the speedo out of the dash.

If the speedo needle was never moving I would suspect a broken cable or badly damaged speedo mechanism (or both) but as it works intermittently I suspect the mechanism inside the speedo is binding, specifically the drag cup.

You might be able to recreate the condition by using a square item of appropriate size in your hand drill and inserted into the speedo head. I have used a beheaded finishing nail, small phillips screwdriver or (best of all) a piece of old speedo cable. Think about which way this needs to spin. OTOH, many problems involving drag or binding cannot be recreated on the work bench and only appear in actual use.

This site offers considerable detail on the care and feeding of the TR speedos:

https://mywebpages.comcast.net/rhodes/speedo.html
 
If your car does not have an overdrive, and if you have a bind in the speedo, it could also make the plastic gear drive in the transmission to go bad. If bad, I wouldn't replace it until your problem is solved. The drives are probably about $10 by now and you won't get far before you wear it out. Easy to replace.
A-type overdrives have a metal gear drive which should be pretty much bullet proof.
 
My thoughts were also that the problem is in the gearbox based on what you've already replaced.

As mentioned above, pull the speedo and use a drill with something like a sharpened square nail to drive the gauge. Also as mentioned above, think about the direction the needle needs to move, put the drill in reverse. If you find the gauge works, perhaps go for a drive with a passenger who can watch or try to grab the free end of the cable. Do this around town at low speed in high-gear to keep the cable rpm down. Your passenger should not be able to make the cable stop turning. If they can, and you confirm the cable's core isn't broken, the problem is inside the gearbox/overdrive.

If the TR6 speedo drive is like what was used on my GT6, there is a nylon worm gear on the output of the box that drives the speedo cable pinion. On my car the nylon gear had split and would no longer drive the speedo.
 
Thanx for the advise, I'll try the bench method first and ill see if the cable is actually turning or not .
I'll let ya know how i made out

(this car does not have O/D)
thanx again
 
Sometimes just by loosening the threaded fitting going into the speedo can eliminate a problem. If the square end is just a hair too long, it'll bottom out in the female end in the speedo.
 
[ QUOTE ]
...If the square end is just a hair too long, it'll bottom out in the female end in the speedo.

[/ QUOTE ]

If it is just a tiny bit too long a washer can be fitted over the end of the cable to keep it from binding when the knurled bit is tightened. But this problem (inner cable too long) usually causes the needle to not return to zero when stopped rather than having a needle that does not move when driving.
 
I have a different problem with my speedo in that it seems to be telling me I am going at a much greater speed than reality. It appears to be showing at least a 25 - 33% higher than the actual speed. Any ideas as to what this could be?

Cheers,

Jim.
 
I have a similar problem Jim, but I attribute it to having slightly smaller tires. Mine could be the differential though as Alan mentioned...I'll have to investigate further...
 
[ QUOTE ]
...It appears to be showing at least a 25 - 33% higher than the actual speed. Any ideas as to what this could be?

[/ QUOTE ]

I don't think a change in diff or tire size would introduce that much error. More likely the needle has loosened on the shaft or the spring has weakened.

Pretty easy to recalibrate these... that website again:

https://mywebpages.comcast.net/rhodes/speedo.html

...has the skinny on how to do this.
 
A speedo reading 23-30% higher can be due to significant changes in the final drive ratio, particularly in combination with changes in tire size. However, if the needle is bouncing while this happens, and in particular if it has happened without major mechanical changes to the car (i.e. It just started happening), I'd be prepared to have the gauge serviced. It's not unheard of for the needle bearing supporting the pointer to start to "drag" when it gets dirty or galls. If this is a new development that you can't attribute to mechanical changes you've made, get the gauge serviced now before it pegs the pointer and destroys the return hair-spring.
 
Thanks for the replies. It has done this since I bought the car last year. The needle does not bounce or otherwise move about and is actually very stable, just not showing the right speed.

I was replacing the rear oil seal on the gearbox last w/end and whilst I had the car up on blocks noticed that the differential did not fit with what I had expected.

The differential has the separate bracket/casting for the rear mounts and they are not part of the rear half of the diff case as I had thought TR250 diffs were designed (I have a spare diff from a TR250 that has the mounts as part of the rear half of the case. Could this be the smoking gun?

Cheers,

Jim.
 
Is it possible that someone swapped out your rearend for a Supra conversion, with a high ratio and limited slip? Goodparts offers this conversion and shows a picture on his website, for comparison. After putting overdrive in my 6, I had to send the speedo off to Nisonger to have it calibrated (and cleaned up). There is a technique for measuring the number of turns that your speedo cable will make over a short measured distance that will give the repair shop the correct info to make the adjustments needed. It escapes me at the moment but someone on this forum will pop up with the info if you need it.


Bill
 
The typical method for handling what TR6Bill mentioned above is to record your odometer reading and/or reset your trip odometer and drive a measured distance. The distance on your odometer/trip-meter won't match the actual distance you drive. You provide the two figures to the speedo shop and they will know the ratio they need to correct for.

For severe mismatches it isn't always possible to handle this inside the gauge. The odometer is gear driven so major changes to it have to be handled by replacing parts that may not be available to match your gearing. Such problems are usually handled with the installation of a ratio gearbox inline with the speedo cable. Using the measured distance ratio mentioned above the speedo shop custom builds a gearbox (about the size of a pack of cigarettes). This will match the cable speed to what the instrument needs.

Since most of us like the look of our car's original gauges, the other option of a programmable speedometer probably won't appear as attractive to you. However, a programmable speedo and matching sending unit are a one-time investment that doesn't involve cables or ratio boxes.
 
[ QUOTE ]
Could this be the smoking gun?

[/ QUOTE ]

Hi Jim,

Could be... But that's a lot of error to be caused by a just a gear ratio swap in the diff. 3.45:1, 3.7:1 and 4.1:1 are the most common ratios in TRs. A change from 3.45 to 3.7 is roughly 7% difference. Going from 3.7 to 4.1 is about an 11% change. Swapping from 3.45 to 4.1 is around 19% difference.

(Note: To be certain what's installed, if you ever have the diff cover off, you can count the teeth on the pinion and crownwheel, divide the number on CW by the number on the P to determine the ratio in the car. Alternative to that minor disassembly, you can put some marks on a wheel and the driveshaft and with a helper turning one or the other, count revolutions of one as compared to the other, do some division and calculate the ratio.)

Any chance you have an overdrive installed? If so, was it installed at a later date and is it possibly mated up with a non-OD speedo? I'd expect about 22, 25 or 28% error, depending upon the particular OD, which were made with these amounts of reduction. Alternatively, if an OD-specific speedo were put into a car that *doesn't* have an OD, the same amount of error would occur.

Triumph used a wide number of speedos, each matched to the rest of any particular car's setup such OD or non-OD, various diff ratios with and without OD, and I think even tire diameter in some cases. All of these factors can skew the speedo, if mismatched. I don't have the details, but speedos were marked with a number on the face that indicates what drivetrain setup it's intended for use with.

There are several companies in the U.S. that can basically rebuild any Triumph speedo to match any car setup. I've not got the names in front of me, maybe someone else does, or you might be able to Google for websites.

/ubbthreads/images/graemlins/cheers.gif
 
[ QUOTE ]
...Alternative to that minor disassembly, you can put some marks on a wheel and the driveshaft and with a helper turning one or the other, count revolutions of one as compared to the other, do some division and calculate the ratio.)

[/ QUOTE ]

And divide your answer by 2 since you're only turning one wheel. (okay, I think that's true... these things make my head hurt).

[ QUOTE ]
I don't have the details, but speedos were marked with a number on the face that indicates what drivetrain setup it's intended for use with.

[/ QUOTE ]

There is a 4 digit number on the speedo face, just below the odometer... that is the number of times the cable turns to register one mile. Everything else is just math... tire size & diff ratio. Not sure that OD v non-OD matters as the measurement is at the output.
 
I had the same problem when I installed an overdrive on my TR6. Must've had a "non-overdrive" speedo. Way, way off. There is a much more accurate method of calibrating the speedo by counting the number of revolutions on the speedo cable by attaching a tape flag to the cockpit end (speedo removed) and push the car over a specific measured distance (about 50 ft.). The exact distance someone on the forum can tell you, I have forgotten. Mail this info to Nisonger (or any other good repair shop) and they can recalibrate it perfectly.


Bill
 
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