Hi all, my speedometer is running wrong, it starts working only when I am in high RPMs and then stops at lower revs.
I suppose the gear inside the transmission has been installed wrongly or the gear is worn.
As I do not trust what is installed, can you please let me know what is the right color / number of teeths for a L82 1974 manual with stock real axel ratio and stock wheels diameter?

Thanks

 

A question first.
Which stock diff ratio?

 

3.70

 

Hi

I don't get this, what does the speedometer have to do with engine RPM ? It measures the car speed .
If it only works at higher speeds, something is broken. Can be the plasric gears, can be a sheared flex shaft and can also be a faulty speedo instrument.
Pull the flex cable from the gearbox and try with an electric drillmashine to spinn it. This will proof if the gears are broken or if the fault is flex cable or gauge.

Brgds

 

Of course speedometer is not connected with the engine RPM, however I am describing what it is happening in my car. I suppose higher RPM trigger the speedo gears and it starts running correctly (so this is an indication it is not a gauge fault).
Will try to simulate the rotation with an electric drill... thanks

 

Had the same issue, it had to be re-magnetized. There is a magnet inside that spins from the speedometer cable and makes the cup where the needle is attached too move. It is non contact and somehow creates a field to move the cup. The gauge repair guy said the magnets to get weak over time.
Works perfect now.

 

^^^THIS^^^
And, like any fine instrument, needs an occasional clean & lube
speedo, tach, all other gage repair shop.
D&M at Greenville SC USA
https://dandmrestoration.com/

 

Hi, I was hoping I didn't have to remove the speedometer... do you have any good guide in order to do it without disassembling all the car???

 

Related question - can a magnetization issue contribute to needle bounce not properly 'damping' needle movement?

 

I find this relationship between RPM and speedometer display counter-intuitive and curious. So let me get this straight: You can be loafing around your neighborhood at a low RPM and the speedometer doesn't function? But if you downshift to a higher gear or simply disengage the clutch and rev the engine - the speedo THEN and only THEN works fine? And the tachometer? That does work OK?

You can spin the speedo cable once disconnected from under the car - but not sure what that would prove unless you can also run the engine at RPM simultaneously (don't get under there w/ engine running)

I would think a worn driven gear might produce intermittent or catchy speed display but probably not RPM related. You can visually check the speedo driven gear by removing it at the tranny tail shaft. It's accessible from underneath from underneath. The cable itself may also benefit from lubricating if old/dry. Anyway I believe the 3.70 gear was silver but don't take my word for it - Google this up - plenty of transmission parts suppliers with websites have tables showing this (and can probably sell you a replacement gear if needed). There's also a seal that should probably be replaced if replacing the gear.

If you need to service the speedo itself the left side dash needs to come out and as memory serves that means dropping the steering column down, pulling the left dash pad forward enough to access the headlight switch and so on. If you don't have one - an Assembly Instruction Manual for your 73 is a great aid.

I'll be curious what your root cause issue turns out to be given the RPM connection to the speedo issue... Good luck

 

No simple, certain answer.
But, I can well understand a fifty year old gage might have more than one issue; perhaps weak magnet And a dirty, balky mechanism With dried-up, spent lube.
heck, sounds way too much like me.

I don't want to go into dash either. But if I had to and send a gage out, I might think real hard about sending the other out for service as well.

 

First my speedometer quit, so I figured no problem, just use the tach for speed estimates. Of course the tachometer quits a day latter. Be smart and have both restored by a competent shop. I would also buy new a new cable and transmission gear for the speedometer (two if you have cruse control).

 

Hi all, some updates here. Disconnected the cable, made some tests with a drill (counterclock rotation), luckily everything is working! So I suppose I need to chase the issue elsewere, probably a worn speedo driven gear or some issue in that area.
In fact, I was trying to reach the 90 degrees adapter that is connected to the sppedo cable and found it is wobbling around. Now the issue is: how do I remove it as I don't get a straight access to this area of the transmission???
This is the adapter installed that I am trying to remove:

 

This is what is suggesting the assembly manual:

 

My '71 has the standard wide-ratio Muncie M20 4-speed and the standard 3.36 axle ratio, and the speedo cable goes straight into the tailshaft WITHOUT any right angle adapter. My plastic driven gear was completely worn and non-functional. New gear (along w/ o-ring and seal) and it's good again. I had also previously installed a new speedo cable.

Note that your 3.70 is not the standard axle ratio - it was an optional ratio: Would you know if the 3.70 was a factory installed option? Or retrofitted by a PO or shop? I ask because it's 'possible' that the adapter may have been installed by the factory and also changes the ratio. Certain years with certain (higher) axle ratios sometimes got non 1:1 adapters to address speedometer accuracy. I don't have any specifics beyond that to offer or if that would be the case on a '74 w/ a 3.70 axle.

It's a little tight on space but I was able to access from underneath from passenger side with vehicle jacked up. I did have to use channel locks to get the knurled speedo cable retainer to loosen but nothing special beyond that. Those speedo retainers should only be finger tight when re-installing. Thinking more on it - such an adapter may be super tight to access given the crossmember and snug tunnel spacing... I don't know if, in your case, it may also be easier to also remove the crossmember or not.

Here's a YT video that clearly shows it... He seems to have a lot more space than I had, probably not a C3, but you get the idea...