Alternator - How many amps should I get?

[MuddinBo]

Wat ampage alternator do I need for my 98 4.0 xj? The 90 or the 140?

[prerunner1982]

Depends on what other accessories you plan to add or already have.

It probably came stock with the 90, but if you can afford the 140 it won't hurt anything and will give you the extra output to power aux lights, air compressor, winch, etc..

[prerunner1982]

I didn't even see it said WATT... but I am sure he meant AMPs. Don't go confusing the guy Tony.

[Mudderoy]

Wat ampage alternator do I need for my 98 4.0 xj? The 90 or the 140?

140 watts is the capacity. It's really a part of ohms law P = I x V. The voltage (V) is pretty well fixed at 12 volts, so the current (I) is what must change to increase the Power (P) (in watts).

For a 13.8 volt system to achieve a capacity of 140 watts the alternator would need to handle a current load of 10 amps.

You could have a billion watt alternator and it would work fine on your Jeep, as long as it was putting out 12 volts DC, you just would be using a tiny fraction of it's capacity.

[Mudderoy]

I didn't even see it said WATT... but I am sure he meant AMPs. Don't go confusing the guy Tony.

Yeah 90 watts is the standard alternator that came with the 1998, I think there was a 120 watt available, not sure.

I believe the Grand Cherokee alternator that can be made to fit is a 130, or 135 watt alternator.

Basically you need to know what the current load is for all your electrical devices to determine if a 90 watt alternator or 140 watt alternator is what you need.

[NW99XJ]

An option that many people dont consider, is getting the stock alternator "re-wound"...this involves having a shop take one apart, then re-wind" the coils in there with more wire, this creates a higher output and in some cases will cost you less than what a high output alternator will run you. The limitations are only what the case will fit, and the skill of the tech doing the work.
I have also read places that you can also cram a ZJ alternator into our XJ's but it requires some basic modifications of the case, and/or mount and sometimes the plug... (i'm just going off of memory here)....

In any case, what the other posts mentioned are correct... you need to figure out what your specific needs are. Taking into consideration all of the components that you have now, and what you may be adding/replacing in the future... such as high wattage headlights, offroad lights, any other auxiliary lights, winch, audio amp, etc....
The more you have the more you need.
Here is a good online "Ohm's Law Calculator" http://www.onlineconversion.com/ohms_law.htm

Add up all you're wattage, plug in 12 volts, and click calculate... this will give you a good idea of the ballpark you want to shoot for. YES I KNOW alternators will put out more than 12 volts, but plugging in the actual voltage will result in lower amperage, and at 12 volts this will give you a more reasonable bit of headroom in your system draw.

[prerunner1982]

Yeah 90 AMP is the standard alternator that came with the 1998, I think there was a 120 AMP available, not sure.

I believe the Grand Cherokee alternator that can be made to fit is a 130, or 135 AMP alternator.

Basically you need to know what the current load is for all your electrical devices to determine if a 90 AMP alternator or 140 AMP alternator is what you need.

Fixed it for you.

[Mudderoy]

Fixed it for you.

Wow I really thought it was watts. Had to go look. Maybe I knew at one time and forgot. Thanks.

[denverd1]

Just so ya don't go to smokin belts, get the highest you can find. Winch, lights and big BASS systems like Tony's all draw juice off the batt and will need to recharge. :smiley-whacky119:

Add another battery if you're running a bunch of stuff and find its hard to start after a big drawdown. Put an isolator between the two and you're good to go.

[4.3LXJ]

My opinion for what is worth is that if you have a winch or plan to get on, get the highest amperage you can.

[MuddinBo]

Right now the only accessories I have is an after market pioneer radio, CB radio, and a 350 Watt amp with a 12 in sub

[4.3LXJ]

That will pull just under 30 amps. Might be better to get the 130 amp alternator so you have a little cushion if the AC and lights are running

[oderdene]

ZJ have 136A alternator

http://1.bp.blogspot.com/-mt_VkWNKqKM/T6ct0pm12jI/AAAAAAAAByw/l27EhR8SvLk/s320/IMG_4114.JPG

you have to shave a both bracket
http://4.bp.blogspot.com/-Y5Wmc4I5jJc/T6ciWSmaoII/AAAAAAAABx8/8kLIuPNKEfI/s320/IMG_4110+-+Copy.JPG

http://1.bp.blogspot.com/-Yvncr10hsto/T6ctRRRHFAI/AAAAAAAAByo/qrxObN-2yXk/s320/IMG_4109.JPG