Does anyone one know if it is possible to make a factory tacho in an 89 RRC work without the pickup coming from the alternator? I want to change alt and pulley ratio.
Does anyone one have any experience with 4.7 'series' R&P (10t pinion 47t ring), has anyone had a set cryo treated and did it/would it make a difference? (with 4.2/auto/maxi tcase/f&r locked/34" tyre)
Cheers
Daryl

Daryl:
I don't have any exact answers for you, however, I'm assuimng that you're concerned that when you change the speed of the alternator, you'll change the readout of the tach.

I have recently purchased a salvage instrument panel for my Disco for another project. Behind the tach, there is a lot of circuitry. While I haven't gotten into it very deeply yet, I would expect that there would be some form of calibration that would allow you to compensate for the differing speed of the alternator. If I learn anymore as I get into it deeper, I'll let you know. Also, I don't know if I'll be needing this tach yet.

If there is a provision for recalibrating the tachometers, it should be pretty straightforward to hook up an outboard tach to your coil and adjust the dash tach to match.

Good luck!

BTW-why are you changing the alternator speed? I've often thought about doing the same to insure higher voltage at idle.
Paul

Can't help you much on the changing the speedo, but I have run the gears you are referring to. I had them mated to an ARB (which in my opinion makes the set up much stronger over stock). I had no problems with the ring and pinion failures. I broke numerous axles with this set up and the ring and pinions always held up.

I would suggest that you purchase the GBR 4.75 gears if you want a stronger set-up. The cost of pulling the ring and pinion (or setting it up, is very high....I am assuming you cannot set up gears though???, not to mention paying for the cryo). I also think the strength of the ring and pinion set up has a GREAT deal with how well they are set up. I have heard of failures with the 4.70, but I remember that they were owner set up also. I wouldn't worry about it if it was me and I break a TON of stuff.

Way

Do not get series 4.70s they are trash and use a spacer ring that makes matters even worse. Get 4.75s from GBR or Ashcroft and have them professionally set-up.

I'm not for sure but the tach may have an adjustment, the d90 ones do anyway. If it has an adjustment use a photo tachometer to set the tach properly.

PT,
I want to turn the alt faster cause it doesnt put out enough at low revs to keep up with lights etc, and as an aside I also want the water pump turning faster and the only way to do this is increase the pulley size at the crank cause the pulley on the pump cant be made smaller easier.


Seems opinions are divided about the 4.7 gears!

I feel your pain over low alternator output at lower engine speeds! The increase in cooling water flow wouldn't hurt either! As sensitive as these vehicles are to low voltage, it doesn't make sense for the alternators to run so slow. By chance, have you put a voltmeter on your truck while you were driving it? I have, it was very enlightening to see how various ancillaries affected the voltage of the entire electrical system. With air conditioning on, lights, ham radio, etc. the charging voltage on my truck falls to slightly more than 13 volts. With all of those off, it approaches 14 volts.

As for the gears, I agree with Adam. The setup is as important as the gears. I blew up a set of gears because I didn't take the time to set the pinion preload (hey, I was 16 and far more stupid than now-the rear-end was off a '57 Chevy and the flange wouldn't mate to my '72 Nova). Since then, I have set up several sets of gears and not had any problems since-when I took the time to measure twice!

As for the series gears, if I had a trail rig only that I didn't need to drive to work every day and wasn't afraid of having to pull it out of the woods, I don't think I'd hesitate to take the chance. Lots of caveats there though!

Paul