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Old January 6th, 2018, 22:40
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clydefrog clydefrog is offline
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NP231 Black Fluid.....

So after 23k on the slip yoke eliminator kit I checked my t-case fluid and its black. Or very dark....I checked it about 3k after the slip yoke install and it was still red. I am running Swepco 714 trans fluid, 1987 NP231. Just wondering if this was normal with new bearings etc. I changed it of course. The Jeep sees extreme conditions only (towing, expo and 4wd) that's all i really use it for, no DD. Obviously has the V8 pumping its torque through it but thought it to be a bit odd to cause that much wear after so few miles. 23k is not much in my book to cause the fluid to go dark. Thoughts?
'87 with over 300k miles, and a CA legal LS swap.
Expedition built:

When you want to be someone you drive a lifted JK, when you already know who you are you drive an old XJ.
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Old January 7th, 2018, 07:00
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Anak Anak is offline
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Re: NP231 Black Fluid.....

Have you confirmed that your vent breather is properly connected and no dust is getting in there?

I only suggest that because my fluid was also black and I found that the PO had split the rubber vent tubing and it came off at some point and I didn't know it. I connected those two conditions in my own mind, though I suppose it is possible the fluid went black for some other reason.
Project "Scope Creep": 1996 4wd 4.0L, AX15
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Old January 7th, 2018, 07:50
trippled trippled is offline
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Re: NP231 Black Fluid.....

If it's gets run that hard id say it's pretty normal. That stuff really needs changed often since it's such a small quantity that does so much work. Even running amsoil in mine it still gets dark pretty fast.
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Old January 7th, 2018, 07:50
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Tim_MN Tim_MN is offline
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Re: NP231 Black Fluid.....

My thought is you need to follow a severe usage preventative maintenance schedule and replace the t-case fluid more often. It's not like the job is difficult or time consuming.

Originally Posted by bimmerjeeper

I'm starting to realize people on the Jeep forums are highly expert mechanics .....
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Old January 7th, 2018, 07:57
srb531 srb531 is offline
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Re: NP231 Black Fluid.....

i recently bought a used 241 tc w/ 85 k miles on it , the fluid was black , probably never changed since first use i ran cheap fluid for 200 miles then changed it to quality fluid which i change it every 6 months now . it stays red that way.
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Old January 7th, 2018, 19:12
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ehall ehall is offline
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Re: NP231 Black Fluid.....

Its not normal. Given that there isn't any friction material in a 231, the other likely sources are contamination or temperature. I would check the breather line and vent as suggested. If that's not problematic I would probably try to figure out a way to measure temperature, especially relative to the transmission. Its a big chunk of aluminum draws a lot of heat from the trans. If its localized heat, might have a defective oil pump or a bad bearing inside.
my '91 beater rebuild
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Old January 7th, 2018, 20:35
burntkat burntkat is offline
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Re: NP231 Black Fluid.....

It could be related to the chemistry of the fluid used, too.
-89 S-10: SAS, D44s front/rear. 5.7L V8. 700R4 with 4L60E upgrades. Built in my driveway.
-90 XJ 4.0, AW4, 231J: Clayton's longarms, RE 5.5" coils, hybrid 6" leaf pack, 33/12.50/15" MTs, Rancho 99012s and 99624s, Tera HD SYE, custom sliders, BW flares. Rehabbing with D44s, RE spring packs, RE 4.5" coils, and ACOS.
-94 ZJ: Budget boost, 31s. The wife's DD.
-97 XJ: Son's DD. 4" lift, ACOS, 4" spring pack, Upcountry coils, 31s
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