Datsun Roadster Forum-311s.org

So, I pull out of the driveway, head towards town on a back country road and I start hearing (as I come to stop) something rubbing - whose speed seems to correlate to tire rotation. I pull out across the divided hiway and I'm doing 40 keeping up with traffic. a soccer mom comes along side and now I'm doing 120. Dang! but, we're not killing it. i'm still only in 4th. wait, now i'm doing 160! I gas it a bit and now the gauge has gone past 160, nearly approaching 0 from the other side! I can hear a weird whirring sound which has the speedo really excited and I'm bouncing bewteen 80 and 130 as I cruise along keeping up with soccer mom. The miles are clicking by on the trip meter and I manage to get in about 4 miles on my .5 mile drive to the hardware store (where I was headed).
I make my purchases and head home. Speedo steady at 40 again for a bit. then it jumps to 60, 80, 120 and settles in at 20 as I approach my turn.

So, what's the diagnosis? Googling "wonky speedo" suggests that this is a phenomenon know to more than just roadsters.
Anyone out there successfully repair such an issue?
thanks,
Andy B - "sorry officer, I honestly have no clue how fast I'm going..." ya, didn't think that would work.

Andy, I knew what to look @ up until you said that the odometer was also affected. Generally when the speed reads high & erratically that either the speedometer cable is dry & binding or the speedometer head bearings are dry or worn. I can’t see how either of these will cause the odometer to be off also. Please try another trip & make sure the odometer is also off.

Peace, Pat

So my guess is that the slip rate between the cup and cone in the speedometer housing is changing as you drive. Rare but it can happen, what does not make sense is the odometer which should not be effected by this issue.
Dave

Check with a speedo shop if the cable adjusting doesn't help the erratic reading
It needs to go to a pro shop for anything to do with the head .... federal regs apply
I'd check the tail shaft bushing the drives haft yoke goes into if it's very loose the output shaft will move around / orbit which will cause the speedo gear to vary its output. Or the plastic gear could be cracked worn too a u joint rusted can cause it

My mistake. I was watching the tenths of miles clicking by on the trip meter. Odometer appears to not be affected.
It does sound like there's something loose in there. Forgot to mention the speedo cable was is new. This is happening on my 68 which I recently completed the restoration on. I now have 700 miles on it. "New" cable means I got it at some point over the last 4 or 5ish years and have only started using it since February.
Dried out cable?

Yes indeed a dry cable. I used a brand new Tach. cable which broke. New but un-lubed! My mistake!

What would be a good lube for such cable?

nismou20 wrote: Thu Aug 24, 2023 11:13 am What would be a good lube for such cable?

A separate thread said to use graphite powder.

Dr Google has spoken:-
"What do you lubricate speedometer cables with?
Use white Lithium grease on the flexible cable. Easiest is to extract the cable, put a liberal coat of grease over it and re-insert."

My tach does something similar once in a while and my solution has been to actually loosen the cable’s connection on the back of the tach a tiny bit and shifting the cable left/right/up/down until the noise disappears. I think the cable routing puts stress on the mechanism sometimes and causes the issue.

I had this problem with my '66 tach , I had to grease the "head" in the back of the unit. Remove the unit from the car, where the cable screws onto the head. If you examine the back you will see a cup plug (mine had one). I used an e z out or a small tap (don't remember which) and you can extract the cup plug. Once it is out, you will see that passage goes to the shaft and you can grease it. You can dig out the old grease with a small screwdriver, mine was waxy and hardened. Clean it out and put in new grease ( I used a product called Magalube (teflon grease}. Work it down in the hole and it will start to lubricate the shaft. You can probably use any light grease, don't overdo it. When you look thru the passage you will see the shaft turning. If the head has returned to zero, you should be done. Reinstall the cup plug and test it. It fixed mine years ago, it has worked good ever since. Good luck!

So I got the speedo out. I'm guessing it's not a good thing that I can hear small parts rattling around inside... will see what I see when I open it up

And there's a mystery part floating around. Really tiny. Measures 3.75mm at its widest point.

Errrrk!

Yep. Boxed it up and shipped it off to Huntington Beach for a rebuild at Global Tech Instruments. Bad bushings are a known culprit.
More to come.