Looking to replace the vacuum modulator on my recently acquired, used TH350. The ID code on the driver side above the pan rail starts with 35M. Photo attached. If I am correct, the 3 stands for Olds, the 5 is either 1975 or 1985 and the M is for Lansing. Is there a definitive way to tell if it's a 1975 vs 85? The seller did not know. Or would it not even make a difference as far as the vacuum modulator goes. Like, they would be the same for both. I do see there may be different part listings based on the rear gear ratio?

Thanks.

 

You could likely determine if the 5 is 75 vs 85 examining the title & referencing the VIN plate w/ the VIN derivative stamp on the transmission. The alpha-numeric stamp you provided is a VIN derivative stamp. I think the easiest method is to examine the RH side of the transmission which should have the year stamped into the transmission servo cover. Possibility maybe a service transmission won't have a stamp on the servo cover; yet, take a look at the servo cover & see if you have stamp. Here's a representative example.

 

Examine the VIN plate on the dash. The alpha-numeric number stamped into the TH350 you provided is the VIN derivative number e.g. the number is derived from the VIN plate. Yes, the 1st number designates Oldsmobile. Five (5) would be the year of manufacture. The last six numbers are derived from the VIN plate on the dashboard. On your vehicle the VIN last six numbers would be 146843. If the vehicle is a 1975 w/ a VIN plate ending in 146843; and, in particular, you have a vested title which demonstrates same VIN number & the transmission VIN derivative demonstrates the same number (146843) - it is a 1975 TH350 transmission.

 

Methinks that may not be possible:

 

Valid point. Servo cover stamp (unless it's service replacement, maybe) should demonstrate year.

 

Just get an adjustable modulator from your preferred auto parts supplier. The same modulator will fit a T350, T400, or an ST300. You can also use the one off your old trans if you have one.

 

Thank you for all the replies. Yes, unfortunately the trans was already out of the vehicle so I don't have access to the VIN. The servo cover has nothing stamped on it. I know this trans was rebuilt with a shift kit. I tried to find a part number on the modulator thats in it (it is an adjustable red stripe), but nothing is legible even after trying to clean it up. I guess i ciuld just try to see if the old one works, but just trying to be proactive and i know they are not that expensive. I'm just going to go with an adjustable for a 1975.

Thanks again.

 

Why not just take the old one with you to the parts store and look at modulators for both 1975 and 1985 and see which one it matches?

 

The only RWD automatics that Olds used in the 1985 model year were TH200C and 200-4R, so this CAN'T be a 1985 TH350.

 

An '85 would have a lockup torque converter with a square electrical plug on the left side of the transmission case near the shift shaft. A '75 would not have this connection.

250's looked like 350's, 250's had an external band adjustment on the right side near the cooler line connection. The adjuster protruded from the case and was a short threaded rod with a lock nut.

Either way I'd prefer the adjustable version.

 

Just use an adjustable modulator and adjust it to your preference. Sounds like you already have one - just check it to ensure it holds vacuum.

 

Thanks. Joe and Sugar, based on your info, I'm confident it is a 1975. Thanks again.

 

If you get an adjustable modulator, keep in mind you only have a couple turns of adjustment either way. And at most, you can move shift points 2-4 mph.

 

Got it. Thanks.