My '98 Jimmy is running me through all the most classic Blazer issues. First there was the LIM gaskets, which I replaced. Then there was the P1870 and the leaking TCC valve, which I replaced. Now I'm working on the HVAC system again. I replaced the control head in the summertime because it was leaking vacuum and wouldn't seal. Now I know why.

The new control head is leaking vacuum just like the old one and this time there's transmission fluid all over the switch. Lucky for me I caught it early. I tested the vacuum actuators and they hold vacuum well and will activate their doors. I know the system well enough to know that the tranny fluid is in the switch because it's getting sucked out of the transfer case. I also know that it's getting into the transfer case because of a faulty input shaft seal which means that fixing all of this means pulling the transfer case off the back of the transmission to replace the input shaft seal.

What I'd like to know is if I have to drop the entire transmission to get the transfer case off the back. My buddy Alex and I can handle the job. I just want to be prepared if I have to remove that transmission.

Bonus question: is there any hope for the vacuum switch in the heater control? Will it ever seal properly again if I somehow clean out all the transmission fluid?

 

Transmission does not have to be removed, just the transfer case. Replace the vacuum switch on the transfer case too, it is a contributing factor to the transmission fluid getting into the transfer case.

 

Possibly...clean it up well, see what happens. You can actually buy that switch separately from the control module, if you want.

 

I'm definitely going to replace the transfer case vacuum switch. I've heard that the new OEM switch was redesigned to better seal against sucking up fluid. I've also heard that the nipples on the Dorman switch don't seal up with the hoses as well as the factory switch so I'm going to avoid the Dorman solution and get a good one.

Working under the Jimmy in the cold has become an annual tradition. This time last year I was putting a new flex plate in my '93. That job inspired me to buy insulated overalls.

 

I have not done this myself but in here AintQik did a nice little write up. I thought it was worth saving. See what you think.
https://blazerforum.com/forum/2nd-ge...m-lines-74959/

 

That's an excellent write-up and a perfect example of why I love the Blazer Forum. All of the information I need in life is available right here.

If I feel motivated I'll get the Jimmy up on jack stands in the morning and see how much fluid is in the transfer case.

 

Today I pulled the fill plug on the transfer case and—surprise—lots of ATF came pouring out! I'm getting all the classic Blazer stuff out of the way on this one. I pulled the front and rear driveshafts and disconnected everything from the transfer case today. Next day I have free I'll drop the case and replace the seal and put it back together and then go on to cleaning the vacuum lines.

One question: a little bit of fluid, maybe an ounce, came out of the rear of the case when I pulled the drive shaft. Does that mean the rear seal is leaking also? Should I try and replace the rear output seal while it's out from under the truck?

 

i would spend a little bit of time and replace all the seals on the t-case. if you are gonna have it out of the truck, why not spend a few more dollars for some seals and make sure that you don't have to mess with it for 10 years.

 

The output shaft seals against the driveshaft yoke so some fluid loss is to be expected when you remove the driveshaft. As mentioned, probably not a bad idea to replace it while it's out, if for no other reason, preventative maintanance. Make sure you oil the sealing surface before you install the driveshaft.

 

That's about what I figured. I've pulled drive shafts out of transmissions before and lost a couple tablespoons of fluid.

Sounds like replacing the output shaft seal is a worthwhile idea. Hopefully I'll get the job done here in the next couple days.