TachOmeter not working

[Ridgerunner]

The tach quit working in my truck last fall right, before I put it away for the winter season. I assumed it was the tach drive cable. I'm just taking it out now, and getting it ready for the up coming Memorial day parade and wanted to fix it before then. I pulled the (assumed) broken cable to replace it with another, but once off the truck, I could see that it was still functional and not broken. I did have a little ware spot in the outer sheathing from rubbing on the compressor pulley (just the outer rubber), so I switched it anyway with the new one. Hooked everything up, and the tach still didn't work. changed the tack with a different one and still nothing. I can spin the tach (both of them) with my fingers and the needle moves like they should. And if I unscrew the drive cable from the back of the tach, while the truck is running, the drive cable is spinning also, just like it should. I'm at a loss here, I spent an entire night working on it, and got no where......

Dumb question (maybe) At the back of the tach, where the square drive cable goes in, the hole is round, but the drive cable is square. What am I missing here? Why is the (female end) on the tach hole round, but the drive cable (male end) is square? How does this spin the gauge properly to function?

 

[Ridgerunner]

Ok, here it is.... Both tachs that I used, have I'd guess 1/4" of the drive cable tip broke off in them. That's why I couldn't see the square (female) drive down in the round hole in back of the input. So when I used them, the square drive would just spin, never engaging the drive, because the cable is broke off in there. So I used yet another tachometer and this one works (third time the charm...for the most part) With this (3rd) tach, the needle jumps all over. At idle (800rpms) the needle jumps from 300 to 1300 rpms. From low to high about once every second continuously bouncing. Uggg...

Bad tach? Get yet another one?

 

[Ridgerunner]

Ok, here it is.... Both tachs that I used, I'd guess 1/4" of the drive cable tip broke off in them. That's why I couldn't see the square (female) drive down in the round hole in back of the input. So when I used them, the square drive would just spin, never engaging the drive, because the cable is broke off in there. So I used yet another tachometer and this one works (third time the charm...for the most part) With this (3rd) tach, the needle jumps all over. At idle (800rpms) the needle jumps from 300 to 1300 rpms. From low to high about once every second continuously bouncing. Uggg...

Bad tach? Get yet another one?

 

[gimpyrobb]

I would try a drop of oil in the back of the tach. Not sure if that would screw up anything, but it might keep your cable from breaking off in there.

 

[Ridgerunner]

What makes the needle nervous and jumpy though?

 

[gimpyrobb]

Not sure. I think there is a slip fitting in there, which is why I said one drop of oil. It might smooth it out, it might slip too much and screw things up. I'd lube it if it were mine.

 

[topo]

The drive cable in the housing may need to be lubed . it slould move freely if it bindes it will make the needle jump when the tourque over power the spot where it is binding . You might check the fit of the square cable in to the square in the tach drive some civilian trucks have a plastic adapter that is square to make the cable end bigger and tighter fit .

 

[DieselBob]

Yep, as Topo said it sounds like cable bind. I use 10 weight oil and disconnect both ends of the cable and put a catch container on the low end and pour the oil through the other until it's pouring out the low end. This almost always cures the bouncing needle problem with mechanical speedometers or tachometer.

 

[rustystud]

Yep, cable binding in the sheave. They make a dry lubricant for cables, it uses an evaporating liquid to get the lube into the cable. I just for the life of me cannot remember the name right now! I'm sure if you google "dry lube for cables" you'll find something.

 

[Woodsplinter]

On the front of the engine is a tachometer adaptor. It connects the cable to the engine. If nothing else works, you might want to look at that.

It is covered in TM 9-2320-361-20, section 4-15, pages 4-28 & 4-29.

 

[gimpyrobb]

Some of those tach adapters have a grease zerk. Might want to see if yours has one.

 

[m-35tom]

tach cable should come out one end of the housing and it should be greased with a non fibreous grease. back of tach has square hole for square end of cable, if old piece is broken off in it, it must be removed. it you got a used cable, maybe the end on it was already broken making it too short.

 

[Ridgerunner]

I lubed the drive cable and now no more bouncy needle. Thanks for all the replies/help....

 

[Ridgerunner]

What is the proper route for the tach cable to be placed on an M35a2? I have three trucks, and of course, all three cables/trucks are routed differently. At the front of the engine where the cable connects to the 90 deg. drive, the first truck (#1) has the cable going just straight up then bends at the valve cover to go to the cab. The second truck (#2) has the cable running up through the coiled line for the first injector then turns to go to the cab. The third truck (#3) goes up, and behind and in between the coolant hose and the front of the block, then turns to go to the cab. All three run along the valve covers with the two hold downs with clamps after that point. The reason I ask is.... truck #1 has the least amount of bend in it, but tends to rub the V-belt and compressor pulley. Truck #2 has more bend yet, and I don't think they would have ran it through the coiled injector line like that from the factory, but it does keep the cable clear of the belt/pulley. Truck #3 has the most bend, and is completely out of the way of the belt/pulley. I went with truck #3 for my cable routing, but it's a pretty tight radius where it turns to run along the valve covers. I wonder if this tight bend has/had any thing to do with the cable being stressed and made the needle bouncy and if it will prematurely ware the cable inside as time goes on.

Just thinking out loud here....

 

[Woodsplinter]

Mine is routed like your #1 but has a hold-down clamp down by the pulley to keep it away from the pulley.

 

[gimpyrobb]

Thats how mine is too.