Wacky Tach!

[billboe]

My tachometer is freaking out! It works ok (at least it seems to...) at low RPMs, but, it jumps wildly & suddenly past redline when I push the throttle.

I removed the cable and it seemed fine... I then removed the sending unit (where the cable attaches to the gear case). I definitely have a problem here! The ?key? that fits into the slot that spins (on the gear case) is mangled. So, I know I need to change this, but, I don't understand why the tach jumps so high... Logically, it would seem like it would under register as opposed to over register!?!?

Has anybody had a similar problem? I don't understand what would have caused it!?!?

Billboe

 

[DocHeb]

I had a 69 Mustang that had a similiar problem. Under 10 mph the speedo would be pretty stable, but any faster it would start to gyrate and then swing between the low and high pins. One of the fine wires in the cable had broken, and I figured it would not cause problems until higher speeds when it would bind, slow the speedo down, then spring free and cause the needle to hit the high limit. Replaced the entire cable and it was fine.

The wire was pretty fine - any chance one of your's is broken but still aligned, making it difficult to see on casual inspection?

 

[billboe]

Hello Docheb,

Thanks for the reply... I looked closely at the cable, so, I don't believe that is the problem. I'm hoping that the sending unit problem I mentioned in the previous post solves the problem (fingers crossed)... Now all I need to do is make it to the Kubuta dealership to pick up the part.

I just found it strange that it would spike up instead of down...


Billboe

 

[hayden]

If the sender key is mangled, then the cable will sometimes be rotating at engine speed and sometimes it will be rotationg slower or not at all due to slippage. My guess would be that when the cable is slipping, and then catches, it experiences a very rapid acceleration and that causes the jumps on the gauge.

 

[Bird]

Good description. I hadn't been able to think of how to word a good answer, but the spikes are nothing unusual in my opinion, because of exactly what you said.

 

[GlueGuy]

Hayden hit the nail on the head. With mechanical tachs, there can be some "wind-up" in the cable when the key is catching/slipping/catching. The cable will also do this sometimes when it's starting to fail.

You may need a whole new cable (ends are usually swaged on), and you may also need a new receiver in the gear case.

And I'm disappointed that you didn't entitle this thread "Wacky tachy". /w3tcompact/icons/shocked.gif /w3tcompact/icons/laugh.gif

The GlueGuy

 

[Bird]

The first car I ever owned with a tachometer (new '62 Austin Healey Sprite) had the tachometer start doing the same thing. Worked fine up to nearly 2000 rpm, then went wild. The dealer pulled the cable; it was OK, so he ordered a whole new "head" and they replaced the tach in the dash. The new one did the same thing as the old one./w3tcompact/icons/frown.gif Then they tightened the fan belt and everything was fine./w3tcompact/icons/laugh.gif The tach was driven off the back of the generator./w3tcompact/icons/laugh.gif

 

[Bob_Trevithick]

My question would be why did it get mangled in the first place? The explanations about "wind-up" make sense, but I wonder if you don't have a problem internally which caused the sender to over-rev in some way?

Just thinking out loud...

Bob Trevithick

 

[GlueGuy]

Bird

You had a Sprite!?! How fun. We had an MG for a while. I got tired of dealing with the electrical system from Lucas Flash and Flicker and the joke of a carburator.

Bob
That's a good point. With a mechanical tach, the lubrication (or lack there of) and routing of the cable housing are probably the two largest culprits when the tach starts going whacky. So if the cable wasn't lubed correctly, or the routing of the cable caused it to fray, or become kinked it could cause the key or slot (or both) to become damaged.

The GlueGuy

 

[Bird]

GlueGuy, I actually had very good luck with the Sprite as far as everything working right, although the dealer did, at one point, overhaul the two SU carburetors. And the only reason I traded it off when I did was because I got tired of getting run over./w3tcompact/icons/frown.gif It got hit, sitting still, 5 times; 3 when it was parked and twice with me in it./w3tcompact/icons/frown.gif