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TR4/4A Wavering speedometer

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Marla

Marla

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DSCF5904 (1).jpgSo I do have a question about the speedometer on my 1964 TR4...the needle is rock steady at 40 mph and below,but starts wavering back and forth above 40 mph...I am thinking unhook the cable from the speedometer housing and maybe clean and do a bit of graphite?...can anyone clue me in on how the speedometer cable is attached to the housing?...Marla
 
If you undo it from the speedo head, the inner cable should just pull right out. It's only held by a bit of plastic that fits inside the housing and comes out with the cable.
Be careful not to bend the cable too tightly.

As a test, you can try swapping cables between the speedo and tach head. The fittings are the same, and they turn at almost the same speed (in 4th gear).
If the speedo wavers even when driven by the tach cable, the problem may be the drive mechanism for the trip and odometer. It turns only part of the time, so any binding can cause the needle to waver.
 
Refitting the cable is, as they say, reverse of the above - but you will probably need to twiddle it a bit while pushing it home for the square end to engage the speedo drive on the gearbox.

The cable will be very dirty when it comes out so be ready for that. Some use graphite, some use grease to lube the cable. Either way, doesn't take a lot and I usually do not lube the last foot or so nearest the speedo head. I think the lube is most needed in the curved sections and leaving a bit of a gap discourages the lube from migrating into the delicate works.
 
TR3driver said:
As a test, you can try swapping cables between the speedo and tach head. The fittings are the same, and they turn at almost the same speed (in 4th gear).
If the speedo wavers even when driven by the tach cable, the problem may be the drive mechanism for the trip and odometer. It turns only part of the time, so any binding can cause the needle to waver.
Click to expand...

My speedo in the TR3 has been wandering by as much as 10-20 miles per hour. I previously had lubed the speedo cable. So took Randall's advice and swapped the tach and speedo cables to try to decide if I had cable or speedo issue. Both speedo and tach were smooth after the swap. So my question is ... how come? I had expected one of the gauges to be bumpy. Any ideas? Cheers, Mike
 
mgedit said:
My speedo in the TR3 has been wandering by as much as 10-20 miles per hour. I previously had lubed the speedo cable. So took Randall's advice and swapped the tach and speedo cables to try to decide if I had cable or speedo issue. Both speedo and tach were smooth after the swap. So my question is ... how come? I had expected one of the gauges to be bumpy. Any ideas? Cheers, Mike
Click to expand...
My guess is that means the speedo head is binding slightly (actually the trip/odometer mechanism) as I suggested before. The effect is much less with the shorter tach cable, as there isn't as much cable length to wind up while the trip/odo wheels are moving, and unwind when they stop.

The mechanism is "interesting"; in effect there is a tiny slip clutch for every digit wheel (except the tenths wheel on the trip meter). Gears and a ratchet mechanism cause the shaft carrying the digit wheels to move by 1/10 turn every 1/10 mile; then a latch mechanism 'decides' which wheels are allowed to move each time. You can see some of it in the photos at https://s258.photobucket.com/user/TR3driver/library/Speedometer repair?sort=6&page=1
the brass flat washers are the clutches, while the shiny metal loops mounted to the frame are what hold the digit wheels from turning (except when depressed by the tab on the next lower wheel).

In my case, the digit wheels were actually rusty (!) causing a lot of extra friction to the brass washer. Even after all that cleaning, it still wavers more than it should; one of these days I'll revisit and polish the surfaces more carefully.
 
Interesting. When I redid my speedo, I don't think I took that mechanism apart, although I did reset mileage to zero. As a temporay fix could I use some silicon spay lubricate (or something else) to smooth out the operation of odometer? I am surprised that cable length has such a sigificant effect. Bouncing about 10-15 mph at 50 mph with speedo cable and smooth as silk with tach cable. Might speedo be more sensitive to bit longer end on cable? I had planned to measure to double check amount sticking out in both cables. If speedo continues to bounce, will leave disconnected in short term. Cheers, Mike
 
I'd be really careful with that, as the numbers seemed pretty fragile on mine. Even a dry camel hair artists paint brush was taking off the numbers. Any sort of spray might leave them permanently discolored.

Another possibility occurs to me (no idea if this actually makes sense or not) : Could the inner speedo cable have worked loose, so it is more flexible to rotation? It's just a bunch of wire strands twisted together, so if they were partly untwisted, they might wiggle back and forth easier.

I guess swapping the cables wasn't all that informative after all. Sorry about that!
 
I though swapping the cables was a great idea, just suprised at result. Yes, understand how fragile the lettering can be on old gauges ... thanks for the reminder/caution. The speedo cable appears to be undamaged as I had it out for lubrication. Mystery continues. If it still bounces when I switch cables back, I'm going to disconnect speedo to avoid possible damage to mechanism. Cheers, Mike
 
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mgedit said:
My speedo in the TR3 has been wandering by as much as 10-20 miles per hour. I previously had lubed the speedo cable. So took Randall's advice and swapped the tach and speedo cables to try to decide if I had cable or speedo issue. Both speedo and tach were smooth after the swap. So my question is ... how come? I had expected one of the gauges to be bumpy. Any ideas? Cheers, Mike
Click to expand...
Was it fairly easy to make this swap?...I mean I am up for attemtping this operation but really do not want to get in a situation where performing something like this will cause bigger problems...Marla
 
Yes, fairly easy to make swap on my TR3. Both speedo and tach have same screw on fittings that you should be able to undo by hand. You just have to get your hand(s) in there to unscrew them and swap them. My size and lack of flexibility were the biggest issues :smile:

Don't think it will cause other issues, just be careful. You can swap back after a short test drive. Would be interesting to see what results you obtain if you feel up to it.

Cheers, Mike
 
I like that swap over Randall that is the first time I have heard of it. I had a bouncing needle on a tr3 and changed the cable with a new one from one of the big 3 and had no effect. Then I purchased a NOS cable and the needle stayed steady.
 
Would be interesting to know how the NOS cable differed from the new ones. Cheers, Mike
 
Just swapped my cables back again. Here is a comparison between the two cables. No idea if one or both are original. The tach one has the nylon bushing. Maybe it runs smoother? Raining now so have not tried again since switching back. Both cables look to have very similar "extension" lengths and if anything the speedo one is a few thou shorter. Cheers, Mike

Comparison.jpg
 
Took TR3 out for a short drive today and back to speedo bounce. Disconnected for now till I can investigate further or borrow another speedo cable. Cheers, Mike
 
Just replaced the speedometer cable on my 1964 TR4...the speedometer reads rock steady...very pleased that I was able to do this on my own thanks to the good advice on this forum...Marla
 
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