Assessing a used 3500 8.1

onebad8.1 · February 6, 2017

Theres a drain plug at the bottom of the diff its a 3/8in or 1/2in ratchet square,the trans is just drain and fill and the spin on filter, there is an internal filter to replace but you have to drop the pan to do so good luck and nice truck

maxpower29 · February 6, 2017

Theres a drain plug at the bottom of the diff its a 3/8in or 1/2in ratchet square,the trans is just drain and fill and the spin on filter, there is an internal filter to replace but you have to drop the pan to do so good luck and nice truck

Theres a drain plug at the bottom of the diff its a 3/8in or 1/2in ratchet square,the trans is just drain and fill and the spin on filter, there is an internal filter to replace but you have to drop the pan to do so good luck and nice truck

I thought there would be a drain plug too but when I looked it over there was none. Then when I searched it apparently they got rid of the drain plug in 2003 year models and newer for some reason. At least thats what I was reading. I never removed a diff cover before so hopefully its easy

Raptordude · February 7, 2017

It's easy

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maxpower29 · February 7, 2017

also how much fluid do you lose when you take the filter off and how much should you add for it? I know the drain is 7.4 I read

jusbecuz · February 7, 2017

I have a 2004 3500 8.1 and the rear diff has a drain plug. You can't see it because it is recessed in the bottom of the rear diff. I would think yours would be similar to mine? I think it's a 3/8 plug.

maxpower29 · February 7, 2017

Ill look again tomorrow but I'm sure there was none. Mine is the 11.5 rear too I thi k, if that matters

davester · February 7, 2017

My '04 Sierra 3500 with 11.5 rear has a drain plug. I missed seeing it before I pulled off the rear cover, noticed the plug on the inside when I was wiping out crud from the bottom of the diff...just as well, I found out the P.O. had replaced the G80 with a plain open diff...

Of course, I don't know if they swapped the whole axle or just the guts of the differential (but they probably just did the guts, as it's a cab&chassis, so the axle is narrower than a regular dually or a srw, and more rare/difficult/expensive to find...

maxpower29 · February 7, 2017

YES! Looked when I got home and bam there it was, missed it the first time. So happy, Im doing the fluid change this weekend!

jusbecuz · February 7, 2017

Cool, hope the pic helped. I just did mine a while back, front and rear diff with Mobil 1 75/90, tranny with BP Auto Tran Syn TES 295 tranny fluid, transfer case with gm transfer case fluid, and coolant, brake and power steering/hydro-boost flush.

maxpower29 · February 7, 2017

Nice, this is 2wd so Im just doing the rear diff, transmission and filter, just did the oil and filter, bought a new fuel filter, and then after that I'll look at the cooling and brake fluid. Was underneath it the last few weeks coating it with fluid film too to make sure it stays rust free as long as possible.

Im wondering why you guys dont bother dropping the transmission pan and dumping all the fluid though? Too much hassle? Just seems like such a waste to have to do two separate changes only to hit about 85% new fluid. $50 a jug here too

jusbecuz · February 7, 2017

For me, Allison doesn't recommend replacing the internal filter until a rebuild. So I just change the fluid every 50k and the spin on filter every 25k. I got my fluid for $37 per gallon on line w/ free shipping. I'm not dropping the new fluid out right away though, that seems like such a waste, so I'll do another change at another 50k, then have mostly 295 fluid, then go to the extended drain. I think the spin on filters are good for 50k as well, but I change mine at 25k, as the Allison spin ons are only around $9. Your truck has an in-line fuel filter? That's nice, mine is in the tank.

maxpower29 · February 7, 2017

ya from what I read 2003 was the last year. Looks simple and was only a couple bucks so it sounds like a good idea. I think i'll just swap filters and a couple gallons then change a couple more gallons next year and be good with the tranny. Runs great though man do i ever love this truck

jusbecuz · February 7, 2017

I love mine as well, engine and tranny have been great for 105k, I only had to change the water pump at 100k, and oil pan gasket around 103k, that's it for power train. The suspension has been another situation though. I've replaced both front wheel hub assemblies and left side inner/outer tie rod, pitman and idler arm and ball joints. Mine is a 4x4 so hopefully you won't have those issues. On the interior I've changed the automatic climate control module, the climate control fan sensor, and had the dash cluster repaired. Thank goodness for YouTube lol.

mike221 · February 8, 2017

Yeah, no flushing with the Allison 1k. Just drain, fill, and replace the spin-on filter. If you do decide to replace the internal filter, keep the gasket. Those pan gaskets are reusable. I run the Amsoil stuff in my truck (diff fluid and engine oil) with Transynd in the trans, and have had great success with it.

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