'72 jumping tach

[jmp]

My tach needle kinda jumps when I'm not idling -- while cruising and while accelerating. Well, actually, it's more of a sharp drop of a couple hundred rpm, followed by a quick recovery. I don't notice the engine doing anything, so I assume it's the tach itself.

Could it be a dry cable (saw a post 'bout a bouncing speedo and the solution was graphite on the cable)?

It only seems to do this after the car is warm.

Oh, and the tach also dropped to 0 once (and no, I didn't stall the car! ). I was at a light, and revving the engine to 2500 or so. When the light went green I released the clutch -- not too fast -- and the tach dropped to 0 for a couple of seconds before zipping back up to 3000+.

If it is the cable, I've noticed some people have had trouble replacing it (10+ hours with gf screaming in the background ); is it truly that difficult?


Thx.

 

[rwd]

IIRC, the most common problem with the tachs on that one are with the tach drive gear and the gear on the distributor shaft, not so much with cable lubrication, although the cable can fail. The cable goes in at a bad angle, and the tach drive gear often gets chewed up. I wouldn't let it go too long without diagnosing further, because if it's the gears, the worst case scenario would be the tach gear chewing up the one on the distributor shaft. That would not be nice.

I'm sure someone with more recent experience will post shortly.

If you don't have some kind of manual, I'd definitely recommend getting one, even a cheapie. The repair/replacement is so much hard as awkward. Web sites like Eckler's and M/A frequently have breakdown sheets and occasionally instructions.

 

[jmp]

OK, I guess that make sense (if the gear is slipping, then catching I would probably see what I'm seeing).

So, as I understand it, there are two gears that could fail? Well, I'll look it up when I get home -- I've got 4 different manuals there (Hayne's, Chilton, a 1972 Chevy book with entries for all Chevy's in that year, and a 72 Vette build manual).

Thx.

 

[rwd]

That should work manual-wise.

IIRC, there's one gear with a short shaft that drives the tach cable, and it's driven by a gear on the distributor shaft itself. They interface at a 90-degree angle. IIRC, there's a cup-like cover that screws on to the distributor housing through which the cable runs that comes off. It wasn't hard to get the drive gear out to inspect it, just awkward.

After I fixed mine, I saw a trick replacement piece that was claimed to prevent future failures, but I don't recall how it
worked. If it's still around, it should be on one of the websites.

Sorry I can't be more precise; mine was a big block, though, so the details may vary.

 

[Fred's Vettes]

Good morning, I had the same problem with the Tach in my 73 and 74 and the fix was replacing the tach gear. It's a 5 minute job. You can check you cable by turning it by hand and have someone look at the tach for you and if it moves then it,s more than likely the gear. They can be purchased through any of the Vette catalogs. The 90 degree adaptor is also an option or they make a cable with a splint to curve it natural from the distribitor if you want to make sure the cable doesn't break in the future. Good Luck ! Fred

 

[jmp]

Thanks!

Btw, it's amazing how useless those manuals can be -- Haynes doesn't mention the tach at all; Chilton mentions it, but only wrt to an HEI dist; and the 72 Chevy service manual mentions it in passing ("remove the tach cable...")!

Hmm... I didn't check my restoration guide -- I wonder if that says anything?

 

[rwd]

It's fairly self-evident. Here's one of the parts:

http://www.ecklers.com/product.asp?pf_id=A3503&dept_id=1877

From the look of it, the top end goes into the distributor and the cable screws on to the other fitting. Like I said, these weren't around when I fixed mine, however. One of the websites or catalogs likely has a breakdown if the manuals don't.