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

 

[Jeff396]

I have a '66 Triumph motorcycle where Lucas, The Prince of Darkness, still reigns supreme. What's the joke, spit on the ground and it will still stop running due to moisture!/w3tcompact/icons/smile.gif My brother used to own a motercycle shop where he made a pretty good living totally rewiring British motorcycles.

Jeff

 

[billboe]

Hayden,

That makes sense to me...

/w3tcompact/icons/frown.gifOUCH! I just called the dealer... $83.04 for the sending unit!


Billboe

 

[billboe]

<font color=blue>And I'm disappointed that you didn't entitle this thread "Wacky tachy".</font color=blue>

I guess I missed that one... I also recently had to replace the air vent cock because the knob fell off. You can imagine the subject titles that I could have used there! /w3tcompact/icons/wink.gif

Billboe

 

[billboe]

Yeah Bob, I was wondering that also!?!? When I removed the sending unit, the bolts weren't totally tight, so, that could have been the culprit. However, the bolts weren't that loose!?!? It seemed that the "key" just didn't insert far enough... I just hope a new sending unit will not have the same problem...

Billboe

 

[billboe]

The sending unit I have mentioned before is referred to the "CASE,H/METER DRIVE" (part #: 38240-34660) in the parts manual. I know this cable is attached to the tach because the tach moves when I turn the cable by hand. I just think it is interesting that it seems to also drive the hour meter... I assume the hour meter operates at a constant speed when the cable is turning (no matter the engine RPMs) and that it doesn't turn faster when the engine is turning faster...

I would have assumed that the hour meter was an electrical clock and not driven mechanically...


Billboe

 

[hayden]

Nope, the hour meter actually counts engine rotations. It's calibrated to count hours at your 540 PTO RPM engine speed. Interesting way of measuring time, isn't it. But when you really think about it, engine wear is really a function of the number of times the thing goes around so it makes sense that there is less cumulative wear over the course of 1 hour at 1000 RPM as compared to 1 hour at 2600 RPM.

Either way, it seems there are lots of "hour" meters that work this way.

 

[Bob_Trevithick]

Just as an amusing aside here, when my first little tractor, a GT235, was delivered, it had no hour meter.. so I had the dealer install one. A few days later, after returning from a vacation, I was surprised to find that my tractor now had 216 or so hours on it. Seems the mechanic hooked it to a wire that was always hot. oopsey! /w3tcompact/icons/smile.gif

Bob Trevithick

 

[Bird]

<font color=blue>I would have assumed that the hour meter was an electrical clock and not driven mechanically</font color=blue>

Just depends on which tractor you buy./w3tcompact/icons/wink.gif My '95 B7100 was an electric clock, and while it was wired right; not like Bob's, on a wire that was always hot/w3tcompact/icons/laugh.gif, it sure would keep running if you left the key on, even with the engine off. But my B2710 is like Hayden described (and what your tractor has).

 

[billboe]

Ok... I'll give this thread a bit more life...

I ended up replacing the sending unit (carver's saved me a few buck and a drive to the dealer) and my tach still doesn't work properly /w3tcompact/icons/frown.gif! I did another close inspection of the cable and found no problems/w3tcompact/icons/crazy.gif. I removed the new sending uint to make sure it wasn't having the same problems as the old one (it wasn't, it was still looking good!)/w3tcompact/icons/crazy.gif... The only thing left is the tach itself!?!? Any thoughts!?!?


Thanks!

 

[Kubota3710]

Did you ever find the fix?

Mine on an L3710 just went wacky also. Turn key off and it settles above 3000 rpm. Start the tractor and it vibrates in the high 3000s. It has 750 hours and I recently got it so I don't know if the cable has ever been lubed.

Anyone else had this one or know what likely cause?

Thanks, Rich