Alternator ampage

Ive got to get my alternator replaced and evidently the stock model is 120 amp. All the parts stores sell 115 amp and 140 amp but of course the dealer has 120amp. The price difference is enourmous. Can i get away with the 140 amp or will I have to change other things to accommodate the increase in ampage

Any of the three will be just fine if they are made to fit your vehicle. If there is a great difference in price, it is likely that the dealer is selling a new one and the other two are rebuilt. That should not be a concern.

I agree.

 [i] Ive got to get my alternator replaced[/i]  

Why?  How old is your Jeep?  How long have you had it? 

Model year how many miles on it?

Has there been any prior problems with the alternator?  What and when? 

How big a difference is there? 

Are the part store parts, rebuilt, or really new?  If rebuilt, consider that the quality of rebuilt parts vary greatly.

Its an 07 Compass. Bought new. its got 65k miles on it.
Never had a problem with it before, but recently it started having problems turning on and occassionally the radio would just turn off while driving. Had a mechanic look at it but he swears it will take 3 hrs of labor while even the dealer says labor is 1.1hours.

The price diff is pretty big. I can get a 140amp (remanufactured) for 255 at the auto part store while the dealer charges 349 for the 120 amp new.

The rebuilt part is from O’reilly’s auto. Brand Ultima.

Thanks for all your help folks

I would get a second opinion of the health of the alternator.

Have the required starting/charging system tests complete with battery load test (by both carbon pile and conductance meter) and a parasitic draw test been performed? they need to be. Could you better describe what you mean when you say you have trouble with the car “turning on”? The radio issue is most lkely a simple (but expensive) problem contained within the radio itself.

Alternators typically are still considered “good” if they can produce 80% of rated capacity. The reason to have an alternator with a higher output capacity is if you have additional devices that consume electricity on your car. These devices can either be added by the manufacture when the car is made or added by the car owner after time of purchase or added by the Dealer before the car is delvered.

Can i get away with the 140 amp or will I have to change other things to accommodate the increase in ampage

The increase in current rating is an increase in POTENTIAL. The alternator only puts out what is being demanded of it. As long as you don’t add any additional loads, the alternator will only put out the same current as the older, lower current rated, one did.

Think of it like a bigger bucket at the top of a water tower. There’s more capacity but unless you change the pipes leading out of it, the flow is essentially the same. Not a perfect example but close enough for illustration.

Regardless, the wiring size (ampacity) of the conductors in your vehicle are chosen with a significant margin for safety. 20 additional amps on the main conductor is unlikely to cause any significant temperature rise or voltage drop even if you added more load to draw that extra power.

I tend to agree with cigroller about another opinion on the health of the alternator.
An '07 with only 60ish k miles is a bit young for an alt. failure unless there was something behind that failure such as a jump start, chronic dead battery, etc.

There are a number of reasons why an alt. may not charge so it would be interesting to know how the diagnosis was arrived at.

Getting a second opinion on whether you need an alternator or not is a good idea. It could be a weak battery instead. If you have a voltage meter, check the voltage across the battery with the engine off and then again with the engine running. With the engine off, the voltage should be 12.3 or more. If it is under 12, like it was recently on my wife’s car, then the battery may be the problem. With the engine running, the voltage should be over 14. There my be more sophisticated ways to check the problem, but this has worked for me more than once. I fully expoect the alternator in my truck to go any day now, as it is original equipment on a ten-year-old truck, with 197,000 miles on it. I went and bought a new (not rebuilt) one made by Delco at Advance auto for under $200.00 with only a $10.00 core charge. They are easy to install as long as you know how to take off and replace the serpentine belt and if you (THIS IS A MUST) unhook the negative terminal of the battery before you do it. This prevents you from arcing the electronic controls built into the alternator. I am carrying the new one around in the truck along with the tools I will need, and plan to replace it as soon as the weather here in upstate New York allows me to do it without freezing my hands.

The amazing thing about Advance Auto is that they offered me a $25.00 discount for ordering over the Internet and picking it up at the local store. The local store had it in stock, so I could have purchased it there for the higher price as I wouldn’t have received the $25.00 off. There was no shipping delay.

Get a second opinion, as others have said. That said, I’d go for the 140-amp if it will fit your car. If you can’t afford it, the 115 will be fine–5 amps difference isn’t going to matter much.