tachometer question

[jrusso07]

1970 SRL.

when car is in neutral, the tach shows accurate rpms as engine rpm changes. It matches the external digital tach I use to tune up the car. When I am driving, (load on engine). The tach doesn't go above 3000, but the engine rpms are higher than 3000 ( like 5000 if I am shifting from 2nd to third).

How can this happen...? Something amiss with the tach drive in the dizzy base?

[spl310]

Have you revved slowly while in neutral?

[jrusso07]

Have you revved slowly while in neutral?

Yes, the tach follows the engine rpms and I have an instrument tachometer connected and mechanical tach tracks digital tach.

very strange!

[spl310]

That is odd. I wonder if something is loose inside the tach itself stopping the needle.

[jrusso07]

Cable and tach run fine when driven from a power drill. I am thinking the drive in the dizzy slips or decouples when the engine is under load. The tach is accurate when cruising but if I down shift and accelerate it won't run to red line...which is why I like to have a working tach

[msampsel]

Cable and tach run fine when driven from a power drill. I am thinking the drive in the dizzy slips or decouples when the engine is under load. The tach is accurate when cruising but if I down shift and accelerate it won't run to red line...which is why I like to have a working tach

For a cutoff from before red line, MSD control helps me there. Cost about 200 (including shipping and tax from summit coil and control). It allows for 100 rpm steps. too. Set mine at 7700 rpm. Street fire is the cheep one.
I Am pleased with it so far. It's a back up for the pristine mechanical Nissan tach (what could possibly go wrong). And you get multiple spark too. Just a thought.

[C.Costine]

The pinion gear in the drive is, in my opinion, quite robust for driving a speedo and a cable. I would suspect the female that your cable plugs into. It is on the opposite end of the 2.5 inch long shaft that the gear is on, and unfortunately very difficult to see with the drive installed. Maybe you could get a mirror in place to check it. Perhaps even more likely is that the cable housing is stretched so that the cable doesn't insert as far as it is supposed to into the pinion shaft.

[yusef]

Did this ever get resolved? I'm having the same issue.

Thanks,
Joseph

[jrusso07]

I could not find anything obvious. Cable and drive seem to be in good order although I didn't remove the distributor to look at the drive gear or the output pinion gear as C.Costine notes above. I suspect this is worn and under load, there is enough slop in their meshing that I only get a partial reading on RPMs. Some day I will dig into it more. Cars are tucked in the barn for the winter and I mean tucked where this is no room to work on them.

[Jimmers74]

Had a similar problem with the speedo in my Ford truck. It seems the cable turns a magnet or a metal body inside a stationary magnet, that induces flux on the needle assembly. A mechanic took the speedo apart, cleaned and lubed and good as new. I had to reset the needle at the proper position, but it worked fine. Not sure how Datsun did it, but it is mechanical, and I would suspect worn/dirty grease being the culprit.