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Discussion Starter · #1 · (Edited)
My alternator is not working and I need a replacement. However, when I go to all the autopart stores the alternator they give me does not match what I pulled from my 1990 4.2L Jeep Wrangler. They seem to be the same design in general, but the holes for the bolts are in the wrong places (they are turned 180 degrees). I just bought the jeep a month ago and I don't know if the alternator is stock or perhaps it's from another year Jeep. Any help would be appreciated. I added pics so you can see what I'm referring to.
 

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Welcome to the Forum VicYJ,

Does your YJ have A/C or a bracket for an A/C compressor? I seem to remember that non-A/C YJs had V-belts and YJs with A/C have serpentine belts. Yes, pics will help.

Good Luck, L.M.
 

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Discussion Starter · #6 ·
I also just found a part # for alternator replaced 4 years ago but still not sure what alternator it is. (Part # MPA 7273109)
 

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Are there any numbers on the old Alt? That type of pulley is for a serpentine belt, not a V belt. I swapped in an updated alt in my old 75 and had to change the pulley.
 

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'89 YJ 4.2 with MC-2150 Carb & HEI, 2-1/2" Ex. AX-15, NP231 SYE, Adams shafts, F&R ARB, 3
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Here is the one I bought from Napa, I had to take tools with me to switch Pulleys so I didnt have to change Serpentine Belt length.

1989 Jeep YJ 7273109 | NAPA Auto Parts (napaonline.com)

It may be a rebuilt engine, but it's still a 4.2, unless they changed brackets on you. with A/C you should be using the Serpentine belt that you have. Specify that when you ask for your alternator
 

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Discussion Starter · #11 ·
Here is the one I bought from Napa, I had to take tools with me to switch Pulleys so I didnt have to change Serpentine Belt length.

1989 Jeep YJ 7273109 | NAPA Auto Parts (napaonline.com)

It may be a rebuilt engine, but it's still a 4.2, unless they changed brackets on you. with A/C you should be using the Serpentine belt that you have. Specify that when you ask for your alternator
Thank you I will head over to Napa and see how it goes.
 

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When you went to the parts store did you tell them that your YJ has A/C? That may be the difference in alternator housings and even more likely the difference in pulleys. (see post #4).

Just below the space between #2 plug and #3 there's a machined boss. (see pic #1). There should be a series of numbers and letters stamped there. (see pic #2)

First digit = year (the pic is of a 1981 4.2 block, made Dec 14)

Second and
Third digit = month (1-12)

Fourth digit = C = 4.2

Fifth and
sixth digit = day of month (1-31)

I would guess that no matter when the block was made, that whomever installed it (assuming it was a re-man long block) would have used the original vehicle components. If it was a used motor, all bets are off.

Good Luck, L.M.
 

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Discussion Starter · #13 · (Edited)
When you went to the parts store did you tell them that your YJ has A/C? That may be the difference in alternator housings and even more likely the difference in pulleys. (see post #4).

Just below the space between #2 plug and #3 there's a machined boss. (see pic #1). There should be a series of numbers and letters stamped there. (see pic #2)

First digit = year (the pic is of a 1981 4.2 block, made Dec 14)

Second and
Third digit = month (1-12)

Fourth digit = C = 4.2

Fifth and
sixth digit = day of month (1-31)

I would guess that no matter when the block was made, that whomever installed it (assuming it was a re-man long block) would have used the original vehicle components. If it was a used motor, all bets are off.

Good Luck, L.M.
Great info! I think I'll have this sorted out by the end of the week with all the help and advise.
 

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as written before, late 90´ AC equipped YJ with 4.2 was with serpentine belt. The brackets for that are different. And if i recall correctly, it shared the alternator with 2,5engine and also an early XJ/MJ.If it helps, then the serp. belt pulley part number is J8134194 The alternator should be this one for example form Omix 17225.05. But please verify before buy, its a long time ago i had a 4.2 with serpentine belt here

Font Slope Circle Auto part Art
 

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'89 YJ 4.2 with MC-2150 Carb & HEI, 2-1/2" Ex. AX-15, NP231 SYE, Adams shafts, F&R ARB, 3
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There is a bolt and spacer, parts 20 and 22 in diagram above that serves to align pulley so clocking is singular. Unless you are willing to disassemble and reassemble the two halves, front and rear and not sure if that's possible.
 

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Can it just be clocked different?

My friend had a Chevy C10 years ago that ate alternators. He was always going to an autoparts store and buying a new one every 6 months or so. One of the alternators he ended up with was clocked wrong and I remember asking a GM guy i knew about it and he said you could take the bolts out and clock it correctly and put the bolts back in. So we did it and it worked, Jeeps up until the 1990 used GM alternators. There is a high output alternator off some GM car i dont remember the year make and model of that guys swap into their 258 powered jeeps.
 

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Discussion Starter · #18 ·
My friend had a Chevy C10 years ago that ate alternators. He was always going to an autoparts store and buying a new one every 6 months or so. One of the alternators he ended up with was clocked wrong and I remember asking a GM guy i knew about it and he said you could take the bolts out and clock it correctly and put the bolts back in. So we did it and it worked, Jeeps up until the 1990 used GM alternators. There is a high output alternator off some GM car i dont remember the year make and model of that guys swap into their 258 powered jeeps.
I think think it will be possible to clock it correctly. I took the bolts of the nonworking alternator and it was fairly simple to clock 180 degrees. I ordered alternator from Napa and will let you know how it goes.
 

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Keep your original pulley. In case the replacement it a different size
 

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I also just found a part # for alternator replaced 4 years ago but still not sure what alternator it is. (Part # MPA 7273109)
Here is an easy way----take the old one in with you and tell parts you want to match it up with another one.
 
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