I'm working on my co-workers 1988 GMC Sierra 1/2 Ton K1500 and I want to change the oil in the front 4wd differential.

Does anybody know what type of oil/fluid goes in these? 75W-90? 30W? ATF?
It seems that there's a lot of information out there but all different so far.

Also, I replaced the actuator motor but it still is extremely slow to engage.
Not sure if its because its low on fluid....or if there's some other reason it would engage so slow.
Anybody have experience with this?

Is it an amperage issue where a higher amp alternator could help this slow engagement?
Newer battery and replacement amperage alternator on the truck (I believe it to be a 95amp)

Any help is greatly appreciated.

 

Anyone???

Has no one changed their front diff fluid??

 

Use 75W90 or 80W90 gear oil in the front diff. It's no different than the 8.25" IFS under a 2012 for all intents and purposes.

Transfercase will take ATF.

Being it's an 88 it uses a thermal actuator and they are complete POS. Did you put in an electric one or another thermal style? You can get the newer style electric actuator and harness to replace it, or a Posi Lock cable system. Your choice. The thermals always were slow, right from day one. I guess make sure the collar moves freely in the front diff as well that connects and disconnects. Low on oil wouldn't help the situation at all.