Alternator replacement max RPM ('69 CJ-5)

[infernalcolonel]

Hi, guys; technical question about an alternator replacement.

I'm looking to swap out my alternator in the 69 CJ5 I just bought (it's dead, something the PO didn't realize or let me know, grrr...). Anyway, I was looking around and the Powermaster option seems like a good upgrade for the cost (around $AMC 150 -160 for a new one), and it's a 70/100A output, as opposed to the ~35 stock. Anyway, the specs state it has a max of 18,000 RPM.

Now my tach (third-party) isn't the most stable (and likely not the most accurate), but this being a 3-speed, it's not unheard-of for my RPM to jump over 6000 RPM, especially when on the highway, at least as far as the gauge goes (it jumps around quite a bit). I'm not sure of the engine:alternator RPM ratio, but I read that 1:3 is common, and looking at the size of the belt wheels on the engine, the alternator is at least 1/3 the size (I'm assuming I go by the top large wheel, not the one below it going to the driveshaft, right?). So I'm assuming 1:3 is pretty close, which means that at 6000 RPM, I'll be jumping over 18,000 at the alternator.

I can't find max RPM on any other alternators, so is that a common limit? And is this something I should be worried about? I'm not running a winch or light bar or anything crazy, as I'm not much of an offroader (especially in this restored beauty), but I figure I might as well get a good alternator since I'm upgrading. What happens if I overshoot this?

And what can I do to troubleshoot why my tach has Tourets Syndrome? I thought about putting in another one ( http://www.jeep4x4center.com/tachometer-fits-cj-factory-location-5459418.html ), since it looks more like the other gauges, even though it's listed for a later model range; I'm just worried that it doesn't have enough range.

Or is my tach not that accurate; in other words, does my jeep likely even reach that high of an RPM range? I'm running [I believe] 31" tires.

Or should I just look at another alternator? I don't know anything about Powermaster as opposed to other brands.

Thanks for the help!

 

[Old Dog]

I have to believe your current tach is reading wrong, I don't think your going 6,000 rpms. At least not if your engine is still in one piece.
IMO that alt. would be fine.

 

[infernalcolonel]

Any suggestions for fixing the tach? I'm pretty n00bish as far as all of this goes, but I'm determined to do all the work myself and learn as I go.

 

[Old Dog]

Any suggestions for fixing the tach?

Buy a new one.

 

[infernalcolonel]

I already picked out the tach from jeep4x4; I'm just wondering how to troubleshoot to figure out if the problem is the gauge or something in the engine compartment.

 

[BusaDave9]

I wouldn't worry about over spinning the alternator.
As for the tach. It's very possible it's a wiring problem. Wiggle the wires to see if it jumps. The tach should go to the minus side of the coil. It could also be a grounding problem at the tach.

 

[elwood blues]

infernalcolonel, when you created your account (18 days ago), you agreed to terms and conditions of using this site. One of those terms was this;

Which you obviously have not done;

It makes it easier for the other members of this site to answer your questions when we know what kind of vehicle you have.

 

[CJ]

Unless you are driving in low range or in 2nd gear I highly doubt that you are running 6000 at highway speeds. The lower pully on the crank is the one you should calculate a ratio from, but don't worry about it - you will never over rev an alternator with a stock setup (engine).
Also, unless you have a many aftermarket accessories, a stock alt should work.
As for testing the tach, just run a hot wire, a ground and a tach sense lead to the coil bypassing the current wiring. That will let you check the tach (I suspect it is bad)
The tach you linked to is a dash tach for a late model CJ, you will have to cut your dash to mount it.
Since we have no idea what engine, ignition, Transmission , Transfer Case or gear ratio you are running it makes it very hard to point you in a good direction. Some background info on you CJ is better then none, keeps us from guessing.

 

[ken98k]

It's been a long time since I install a tach, but it seems there was something that had to be changed for different engines 4, 6, 8.?
Maybe yours is telling you 6000 when it's actually 3000.

 

[BrockGrimes]

Skip the powermaster, buy an alt for a older 90's i believe caddy they are like 120+AMPS. But if your not runnig extra lights, winch etc just get a stock alt.