Th350 shifting up and down repeatedly on hills.

CopperNick

Comic Book Super Hero
Supporting Member
Feb 20, 2018
3,318
2,971
113
Canada
Have to agree on the possibility of vacuum being an issue. Your description of the spider web of vacuum lines into which the t-box is plugged is one clue. TH350's like their vacuum to be as clean and direct as possible. For that you might have to to the original source on the intake manifold and change the fitting for a multiport one that can feed the box plus your other suckers. The while you're there thing to do is check the rubber hoses that make the connections between the primary steel line and the vacuum port and module to make sure they aren't split or torn or rotten.

The other thing is that in the lockup 350 the lockup only occurs in third at a certain speed or rpm. The lower gears don't get that. I pulled an E/C version out of my Non G-Body--G-Body and plugged in a 700R4 so the lockup thing is a familiar issue for me.

You can actually visit the transmission and disconnect the electrical connection that is plugged into the driver's side of the box and the unit will continue to function quite happily. That actually might also be a test for you to try to see if the lockup is popping in and out during hill climbs. Auto-sticks will mostly always try to put themselves into a comfortable range or gear when dealing with hills so it downshifting under climb or if the up-hill speed slows down to a certain mph is to be expected.


Nick
 

78Delta88

Royal Smart Person
Supporting Member
May 23, 2022
1,175
1
888
113
SW Arizona
Could be you have wrong filter. The other one works so not sure why it won't work if it's correct. The TH200 looks all most identical, but will not mount up correctly. Mounting holes are different. Could be you just got wrong one.

Did you ever get vacuum measurement at idle? If not check manifold vacuum at idle in park and in drive with foot on brake. If your running 16 to 22. You need red stripe modulator. If running less than 16 you have vacuum leak somewhere or running fairly open cam.
If your cam is 240 duration or more or has an LSA around 104, low vacuum all the way down to 12 can be normal. Just depends on build specs. If lower vacuum is normal for your build, then you need red and black stripe modulator.

A stock build with a stock cam should be around 18 to 20 at idle in park with engine running around 700 to 1000 RPM.

Up to about table spoon of material in pan is "normal" after 30 to 50k miles.

TH350 is noted for breaking the sprag and issues with direct drum. Part of the purpose for the sprag is hill holding.

Sounds like you have a spaghetti bowl of vacuum lines... That needs figured out.

At the back of the Q-Jet is a fitting for the line that supplies the brake booster. This should be a 3/8 steel line. Behind that there should be a fitting screwed directly into the intake manifold. This fitting should have a 3/16 or 1/4 inch hose nipple sticking straight up.

This is where the vacuum line needs to connect for the transmission modulator. If modulator is not working properly of if not connected properly the trans will have odd shifts.

I have a th200 and th350 in process of getting built, I can post pics... Show the difference.
 
Last edited:

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
Welp, I didn't have much time on sunday to really tackle anything on the car. I can't remember what vacuum my motor pulls the last tiem I checked it but I can check it possibly tomorrow, i got a gauge for it. I don't have a steel line running to the transmission, someone just put a braided rubber hose all the wayh down. It doesn't have holes in it, seems intact and where there was some light cracking ont he ends I was able to cut it off and still have hose left over. Yall think the hose could be collapsing on itself causing this issue? I looked for some steel line to use for the 350 but any of the pre bent lines are for different motors because obviously my 4.3 didn't come stock with a 350. I'll have to bend up some tubing myself I suppose but I'm not sure what line to use. 1/4"? I'll look out for a multiport for the intake manifold I can plug in. Not sure what vacuum is normal for a 4.3 TBI at idle but I'll get that tomorrow. I do have a steel line going to the brake booster from the manifold. and on th eopposite side of the manifold on the passenger side is the fitting with a 90 degree nipple thats going into that spider web of lines I have. The modulator on the transmission is red stripe adjustable. As for my lockup, I was actually able to hook up an ALDL reader that tells me when the lockup is engaged and it happens at 45 mph after gettign into third and it isn't engaging when I'm having the up and down shifting issue. I still unplugged it on one of my test drives and still the same issue. I took another look at my trans filter that I tried to install previously and it seems that there is a bit of expansion that happened betweenthe top and bottom of the metal pieces clamping the filter together and I may be able to install it if I can press it up into there, just seems the screws were a bit short for it. So for tomorrow I'll have some info on vacuum at idle, brake on and in drive. I'll try to get a vacuum gauge hooked up that I can watch while driving when this stuff happens. Sorry if this post seems like a bunch of spaghetti, just got so much going on. Thank you guys so far for all the help. Also Delta, If you'd like to post the pics of both trans I'd be happy to look and point out what I have, just to be 100% sure. Couldn't hurt.
 

78Delta88

Royal Smart Person
Supporting Member
May 23, 2022
1,175
1
888
113
SW Arizona
On the line you can use 3/16 or 1/4 steel fuel or brake line. Just use tubing bender, they run about $25 for a good one.

Run a piece of string from fitting to modulator and add about a foot to get length of line you need. If the line you have has cracking on ends, just replace line with new vacuum line or make the steel line and connect with 2 short pieces of hose to modulator and fittings.

Vacuum line should be 7/32 inch there or about. But you can just take with you and match up.
 

78Delta88

Royal Smart Person
Supporting Member
May 23, 2022
1,175
1
888
113
SW Arizona
TH200 is toward front, TH350 is toward rear.
20230406_113608.jpg

TH200 is toward front, TH350 is toward rear. Fairly easy to mistake them.
20230406_113704.jpg

Pic of non lock up Trans input shaft.
20230406_113712.jpg

Pic of lock up style trans
20230406_113716.jpg

Pic of filters... TH350 on Left, TH200 on right. Note how similar they look, but bolt holes will not interchange.
20230406_233200.jpg
 

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
I actually have some 7/32 hose, I can put that on there and have something to test with for now while I get some steel line ordered up. Thanks for the string tip, I'll try that.
 

78Delta88

Royal Smart Person
Supporting Member
May 23, 2022
1,175
1
888
113
SW Arizona
Stock TBI new should be around 20 to 22 inches mercury, normal used engine 16 to 20, a heavy used or worn out engine could be anywhere from. 14 to 18 inches mercury. Just won't know until tested.
 

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
Good to know. Also I'm in the So Cal area. would be a bit of a stretch to make my way to tuscon at the moment, but I really appreciate the offer.
 
  • Like
Reactions: 1 user

GBodyVillain

Not-quite-so-new-guy
Jan 4, 2021
28
16
3
I just saw your photos, I for sure have a th350. Stubby lookin tail, dipstick is closer to the bell housing unlike the 200 and the input shaft is long like the 350 in the photo. Also the filter on the left is what im used to seeing.
 
  • Like
Reactions: 1 user

GBodyForum is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com. Amazon, the Amazon logo, AmazonSupply, and the AmazonSupply logo are trademarks of Amazon.com, Inc. or its affiliates.

Please support GBodyForum Sponsors

Classic Truck Consoles Dixie Restoration Depot UMI Performance

Contact [email protected] for info on becoming a sponsor