THM 350 issue with 85 C10 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

peresich · 02-03-2015, 05:47 PM

Alright, I got my first major issue to deal with on my new toy. Friday afternoon, as I was driving my daughter home from school, I lost 2nd and 3rd gear. 1st and R are still working. I tried to manually shift out of first but got nothing. It was cute watching my 10 year old start to panic as I drove down the highway at 20 MPH with the blinkers on.

"Daddy, what are you going to do?", she asked.

"Well honey, I am going to drive it in first gear back to work. Get your brother to come pick you up. Then drive it in first gear home tonight and take the lay on the floor in the garage and work on the transmission. If I didn't expect to spend some time getting dirty, then I wouldn't have bought a truck this old."

Anyway, I dropped the pan off of the transmission after draining most of the fluid out. The fluid was still bright red, so nothing seems to be burned up. Unfortunately, I got the wrong filter at the parts store and got sidelined a good chunk of the weekend with a stomach bug so the truck is still up on the ramps today. I am planning to put the new filter on and fill her back up with fluid this afternoon, but I figured I would ask if anyone had any advice on other things I should check out first.

I didn't see any other obvious signs of leakage besides the pretty rotten old gasket that was on there. It has to be the factory original because it looks ancient. There was a hard line running into the side of the transmission body that caught my eye as a potential weak spot, but I didn't see any fluid around the connection and it felt tight to me. Any tricks the old pros want to share with the rookie before he puts the pan back on for the first time?

greg64 · 02-03-2015, 09:31 PM

Okay, I can't claim to be an expert on auto trannies, but I had a similar thing happen with a th350 once. I'm pretty sure the problem was that the vacuum line wasn't connected. So the transmission figured I was floored all the time, and wouldn't shift until the governor kicked in. And idea how high you got the revs, while in first?

peresich · 02-04-2015, 12:36 AM

Not sure how high I got the revs. Not high enough to kick in the governor though.

My son and I got everything put back together tonight and did the eyeball inspection for any leaks. There was some fresh fluid on the floor after we refilled, but I spotted where it was coming from on the fill/dipstick tube and am not too concerned about that spot. I ran it for a few minutes in the garage and shifted through the gears on the tree before we took it around the block a few times. The first pass around the block it didn't shift, but as we were getting back to the house and both feeling disgusted it shifted. We took it around the block two more times and it shifted perfectly every time it should have. Hopefully, we took care of the issue. Time will tell.

peresich · 02-04-2015, 10:15 AM

Quote:

Originally Posted by peresich

Time will tell.

Famous last words. I didn't expect time to tell quite this fast. I cranked her up this morning and let her warm up long enough to defrost the windows and threw her into reverse to back out of the driveway and got nothing. I now have no gear change at all. I have Park handled pretty well but RND21 don't seem to be cooperating.

donut · 02-04-2015, 12:38 PM

I started typing a response early this am to tell you that when 2-3 went in my truck, it was only a week before r-1 went...

Can you pull the transmission? A bench build is a lot cheaper than having your truck towed to a transmission shop. Downside is you have to do the RR. Up to you and your wallet.
I was going to point you to clinebarger (sp), he's the resident trans expert. I find him hanging out in the engine/drivetrain section.

Chaplain · 02-04-2015, 03:30 PM

Updates as you go please.

peresich · 02-04-2015, 10:10 PM

Quote:

Originally Posted by Chaplain

Updates as you go please.

Limped her home today and put her back up on crutches. I rush ordered a new transmission fluid fill tube/dipstick since the dipstick is missing from mine. I figured with this much trouble I better make darn sure I am working with a proper amount of fluid before I start dragging her around anymore before I burn anything up. I should have that in Friday.

While I have her laid up, I went ahead and yanked the vacuum modulator. I figure that is easy enough to replace and it seems like a relatively common culprit for people complaining of the non shifting transmission. If nothing else, I will rule out one more possible issue.

I noticed a bead of trans fluid on a connection directly across the transmission from the vacuum modulator. It is on a hard line coming in to the driver side of the casing. The only exploded diagram I could find online makes it look like it is where the speedometer gear assembly would attach to the transmission. My speedometer hasn't worked since I bought the truck, so that was on my list of immediate concerns, but I haven't had a chance to get to it. Is that where the speedo gear is on the transmission housing? If so, should I consider replacing that while I am in there, or would just re-seating and tightening everything be enough to straighten out the transmission side of a speedo issue?

Drizzle · 02-04-2015, 11:15 PM

While replacing you vacuum modulator, replace the hose that goes from it to your intake. They sometimes collapse from vacuum and cause the issues you are experiencing with second and third. As for losing all gears, you may want to pull the tranny, flip it upside down and pull the trans adaptor out and either replace it or tune it up. This is the device that shifts all the gears and sends the fluid to the proper passages in the transmission. Make absolutely sure you read up on how to do it. It is easy to put a shift kit in but you need to know what your doing. There are videos in YouTube on how to do it. Also shift kits come with instructions But I wouldn't rely solely on them. YouTube helped me the most. Good luck.

**MikeB** · 02-04-2015, 11:24 PM

Quote:

Originally Posted by peresich

I noticed a bead of trans fluid on a connection directly across the transmission from the vacuum modulator. It is on a hard line coming in to the driver side of the casing. The only exploded diagram I could find online makes it look like it is where the speedometer gear assembly would attach to the transmission. My speedometer hasn't worked since I bought the truck, so that was on my list of immediate concerns, but I haven't had a chance to get to it. Is that where the speedo gear is on the transmission housing? If so, should I consider replacing that while I am in there, or would just re-seating and tightening everything be enough to straighten out the transmission side of a speedo issue?

It's very common for the speedometer "bullet" o-ring to leak. I'd replace the bullet, which comes with a new o-ring. It's very easy to do. Any transmission supply store will have them. The driven gear just slides into it and will probably be OK. If it's toast, the drive gear is probably bad too. You'll have to remove the tailshaft to replace that, but it's no big deal.

http://www.transmissionpartsusa.com/...-00047338a.htm

Driven gear:

peresich · 02-09-2015, 09:39 AM

Quote:

Originally Posted by Chaplain

Updates as you go please.

Spent a few hours under her yesterday finally and got the vacuum modulator replaced and replaced the modulator hose. I confirmed that the majority of my leak is coming from my dipstick/fill tube which did not come in on Friday like it was supposed to. However, I still do not have 2nd and 3rd gear.

Trying to decide if it is time to limp over to a transmission shop and let them take a look at it or if I want to try pulling the governor gear out on my own at home. I really don't have the equipment at home to drop the transmission easily, but I didn't want to have other people doing the work for me on this rebuild. I think I am about to have to swallow my pride and let the pros handle this one though.

Drizzle · 02-09-2015, 10:37 AM

Quote:

Originally Posted by peresich

Spent a few hours under her yesterday finally and got the vacuum modulator replaced and replaced the modulator hose. I confirmed that the majority of my leak is coming from my dipstick/fill tube which did not come in on Friday like it was supposed to. However, I still do not have 2nd and 3rd gear.

Trying to decide if it is time to limp over to a transmission shop and let them take a look at it or if I want to try pulling the governor gear out on my own at home. I really don't have the equipment at home to drop the transmission easily, but I didn't want to have other people doing the work for me on this rebuild. I think I am about to have to swallow my pride and let the pros handle this one though.

Tough decisions for sure. I think it has something to do with your valve body which is easy to pull with the tranny still in. They aren't exactly cheap to replace or better yet usually can be repaired but there is no guarantee that's the problem Without a pro looking at it. You could roll the dice and work on that part yourself or take it somewhere. Good luck

peresich · 02-16-2015, 01:52 PM

Quote:

Originally Posted by Chaplain

Updates as you go please.

Old governor gear vs new governor gear. I think we found the problem. I am hopefully going to put the new one in tonight, but we have some very crap weather heading our way.

Drizzle · 02-16-2015, 02:47 PM

Quote:

Originally Posted by peresich

Old governor gear vs new governor gear. I think we found the problem. I am hopefully going to put the new one in tonight, but we have some very crap weather heading our way.

That doesn't look to good lol. I knew it was something around the adaptor plate.

GRADYS Performance · 02-17-2015, 01:06 PM

That is your shifting problem.

**rs74** · 02-17-2015, 01:18 PM

I was just getting ready to tell you to pull the governor and check the gear. Had the same problem on an old 350 turbo and just went through this last week on my 700r4. That is definitely the cause of a no shift. Here is what I pulled out of the 700r4 the other day. You may have to drill the hole out in the new gear in order to put the roll pin back through it. Not sure if your new dip stick tube takes the o-ring or boot but if it doesn't come with a new one then get one.

peresich · 02-04-2015, 03:33 PM

I put some more fluid in it and got it rolling again. I am going to get back under it tonight and see if I can find any other potential leak culprits. The pan was bone dry, so it has to be coming from somewhere up top.

I started a thread on the engine/drivetrain section too because I found a guy selling a turbo 400 for pretty cheap near me.

peresich · 02-05-2015, 01:33 PM

I didn't have a chance to get under it again and pull out the speedo gears yet. Should I expect it to be a 20 or 22 tooth driven gear?

**MikeB** · 02-05-2015, 07:57 PM

Quote:

Originally Posted by peresich

I didn't have a chance to get under it again and pull out the speedo gears yet. Should I expect it to be a 20 or 22 tooth driven gear?

Depends on the tire OD and rear axle ratio from the factory, unless someone has changed it. Google "TH350 speedo calcs" to determine what you need for your current tires and axle ratio.

peresich · 02-18-2015, 12:52 PM

Alright, this is the single most infuriating thing that I have had to ask for help with. The stupid governor cover clip won't go back on. Is there a trick to getting that idiotic thing to click back into place that I am just not seeing? The gear slid right in and seated fine. The cover itself is seated all the way down. I just can't get the clip on.

This thing:

**rs74** · 02-18-2015, 01:07 PM

Quote:

Originally Posted by peresich

Alright, this is the single most infuriating thing that I have had to ask for help with. The stupid governor cover clip won't go back on. Is there a trick to getting that idiotic thing to click back into place that I am just not seeing? The gear slid right in and seated fine. The cover itself is seated all the way down. I just can't get the clip on.

This thing:

Not sure exactly what you are having an issue with here. There is a hole on the top side for the holding clip to seat down into. Just big enough for the end of the clip. The bottom seats in between two little notches built into the case preventing it from moving. I will say this though. When I did mine last weak the clip sprung open a little. I had to manually bend it back before putting it on.

peresich · 02-18-2015, 01:13 PM

Quote:

Originally Posted by rs74

Not sure exactly what you are having an issue with here. There is a hole on the top side for the holding clip to seat down into. Just big enough for the end of the clip. The bottom seats in between two little notches built into the case preventing it from moving. I will say this though. When I did mine last weak the clip sprung open a little. I had to manually bend it back before putting it on.

I am wondering if I bent mine out of shape in the removal process. The transmission shop that I got the replacement gear from charged me $10 for the gear, so I doubt they would charge me much for a replacement clip. I got the top side into the hole but I couldn't get the bottom to seat into the notches for the life of me.

Ever pop a blood vessel in your nose from straining too hard at something. Let me tell you, it is difficult to tell the difference between blood dripping on the floor and transmission fluid until your wife about has a fit and makes you get out from under the truck when she sees you.

peresich · 02-21-2015, 10:17 AM

So I came home from work Wednesday and my wife told me that she got under there and got the clip on. Something about vice grips and her small hammer. She keeps her own set of tools hidden from everyone else just to be able to do things like this. I'm not complaining though.

The transmission shifts like a dream. I am going to have to adjust the vacuum modulator I think because it is shifting very quickly through 1-2-3 on heavy acceleration, but that should be a minor thing. I have an appointment Monday to have a local shop replace my dipstick/fill tube because the bolt holding it on is in a very difficult to reach location and they are only going to charge me $45 to do it.

Feels good to finally have the truck running again.

Slammed84 · 02-21-2015, 10:27 AM

Can we trade wives? Mine doesnt do any wrenching, hahaha.

Chaplain · 02-21-2015, 10:56 AM

I'm saying...

peresich · 02-26-2015, 04:55 PM

When we were dating our first argument was because I was changing the spark plugs on my Mustang 5.0 and couldn't reach the last one very well because it was darn near under the driver seat. She walked into the garage and took the wrench from my hand and finished the job for me. I was so mad that I wanted to scream, but I also knew that I was never going to let her get away. Her stepdad at the time drove a stock car in local races and she helped him wrench on it and she is as country as they come. It's nice to have the help.

That said, I am going to have to drop the transmission and do a rebuild after all. It shifts like a dream now, but the clutches are going out. It is slipping pretty bad now.

02-03-2015, 05:47 PM	#1
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	THM 350 issue with 85 C10 Alright, I got my first major issue to deal with on my new toy. Friday afternoon, as I was driving my daughter home from school, I lost 2nd and 3rd gear. 1st and R are still working. I tried to manually shift out of first but got nothing. It was cute watching my 10 year old start to panic as I drove down the highway at 20 MPH with the blinkers on. "Daddy, what are you going to do?", she asked. "Well honey, I am going to drive it in first gear back to work. Get your brother to come pick you up. Then drive it in first gear home tonight and take the lay on the floor in the garage and work on the transmission. If I didn't expect to spend some time getting dirty, then I wouldn't have bought a truck this old." Anyway, I dropped the pan off of the transmission after draining most of the fluid out. The fluid was still bright red, so nothing seems to be burned up. Unfortunately, I got the wrong filter at the parts store and got sidelined a good chunk of the weekend with a stomach bug so the truck is still up on the ramps today. I am planning to put the new filter on and fill her back up with fluid this afternoon, but I figured I would ask if anyone had any advice on other things I should check out first. I didn't see any other obvious signs of leakage besides the pretty rotten old gasket that was on there. It has to be the factory original because it looks ancient. There was a hard line running into the side of the transmission body that caught my eye as a potential weak spot, but I didn't see any fluid around the connection and it felt tight to me. Any tricks the old pros want to share with the rookie before he puts the pan back on for the first time?

02-03-2015, 09:31 PM	#2
greg64 Registered User Join Date: Dec 2007 Location: Kimberley, BC, Canada Posts: 799	Re: THM 350 issue with 85 C10 Okay, I can't claim to be an expert on auto trannies, but I had a similar thing happen with a th350 once. I'm pretty sure the problem was that the vacuum line wasn't connected. So the transmission figured I was floored all the time, and wouldn't shift until the governor kicked in. And idea how high you got the revs, while in first? __________________ Greg 64 GMC Suburban - 283, NV3500, 14 bolt 77 C10 swb - 292, SM465, 12 bolt

02-04-2015, 12:36 AM	#3
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 Not sure how high I got the revs. Not high enough to kick in the governor though. My son and I got everything put back together tonight and did the eyeball inspection for any leaks. There was some fresh fluid on the floor after we refilled, but I spotted where it was coming from on the fill/dipstick tube and am not too concerned about that spot. I ran it for a few minutes in the garage and shifted through the gears on the tree before we took it around the block a few times. The first pass around the block it didn't shift, but as we were getting back to the house and both feeling disgusted it shifted. We took it around the block two more times and it shifted perfectly every time it should have. Hopefully, we took care of the issue. Time will tell.

02-04-2015, 12:38 PM	#5
donut Registered User Join Date: Aug 2009 Location: S.C. Posts: 1,593	Re: THM 350 issue with 85 C10 I started typing a response early this am to tell you that when 2-3 went in my truck, it was only a week before r-1 went... Can you pull the transmission? A bench build is a lot cheaper than having your truck towed to a transmission shop. Downside is you have to do the RR. Up to you and your wallet. I was going to point you to clinebarger (sp), he's the resident trans expert. I find him hanging out in the engine/drivetrain section. __________________ '86 C-30 dually, 454/tbh400 '73 K-20 350/350/205 (sold ) I'm kinda like duct tape- no real purpose, but handy to have around.

02-04-2015, 03:30 PM	#6
Chaplain Registered User Join Date: Oct 2014 Location: TN Posts: 294	Re: THM 350 issue with 85 C10 Updates as you go please.

02-04-2015, 11:15 PM	#8
Drizzle Registered User Join Date: Dec 2014 Location: Knoxville,TN Posts: 54	Re: THM 350 issue with 85 C10 While replacing you vacuum modulator, replace the hose that goes from it to your intake. They sometimes collapse from vacuum and cause the issues you are experiencing with second and third. As for losing all gears, you may want to pull the tranny, flip it upside down and pull the trans adaptor out and either replace it or tune it up. This is the device that shifts all the gears and sends the fluid to the proper passages in the transmission. Make absolutely sure you read up on how to do it. It is easy to put a shift kit in but you need to know what your doing. There are videos in YouTube on how to do it. Also shift kits come with instructions But I wouldn't rely solely on them. YouTube helped me the most. Good luck.

02-17-2015, 01:06 PM	#14
GRADYS Performance Registered User Join Date: Dec 2013 Location: Columbus, Oh Posts: 367	Re: THM 350 issue with 85 C10 That is your shifting problem.

02-17-2015, 01:18 PM	#15
rs74 Senior Member Join Date: Jun 2010 Location: Illinois Posts: 2,957	Re: THM 350 issue with 85 C10 I was just getting ready to tell you to pull the governor and check the gear. Had the same problem on an old 350 turbo and just went through this last week on my 700r4. That is definitely the cause of a no shift. Here is what I pulled out of the 700r4 the other day. You may have to drill the hole out in the new gear in order to put the roll pin back through it. Not sure if your new dip stick tube takes the o-ring or boot but if it doesn't come with a new one then get one. Attached Images __________________ Roy 85 Short Bed Silverado- Work in progress. http://67-72chevytrucks.com/vboard/s...d.php?t=564818 1987 R10 shortie

02-04-2015, 03:33 PM	#16
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 I put some more fluid in it and got it rolling again. I am going to get back under it tonight and see if I can find any other potential leak culprits. The pan was bone dry, so it has to be coming from somewhere up top. I started a thread on the engine/drivetrain section too because I found a guy selling a turbo 400 for pretty cheap near me.

02-05-2015, 01:33 PM	#17
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 I didn't have a chance to get under it again and pull out the speedo gears yet. Should I expect it to be a 20 or 22 tooth driven gear?

02-18-2015, 12:52 PM	#19
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 Alright, this is the single most infuriating thing that I have had to ask for help with. The stupid governor cover clip won't go back on. Is there a trick to getting that idiotic thing to click back into place that I am just not seeing? The gear slid right in and seated fine. The cover itself is seated all the way down. I just can't get the clip on. This thing:

02-21-2015, 10:17 AM	#22
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 So I came home from work Wednesday and my wife told me that she got under there and got the clip on. Something about vice grips and her small hammer. She keeps her own set of tools hidden from everyone else just to be able to do things like this. I'm not complaining though. The transmission shifts like a dream. I am going to have to adjust the vacuum modulator I think because it is shifting very quickly through 1-2-3 on heavy acceleration, but that should be a minor thing. I have an appointment Monday to have a local shop replace my dipstick/fill tube because the bolt holding it on is in a very difficult to reach location and they are only going to charge me $45 to do it. Feels good to finally have the truck running again.

02-21-2015, 10:27 AM	#23
Slammed84 Registered User Join Date: Mar 2014 Location: Tulsa, OK Posts: 292	Re: THM 350 issue with 85 C10 Can we trade wives? Mine doesnt do any wrenching, hahaha. __________________ 84' C-10 Custom Deluxe 97' & 98' C-1500 Silverados

02-21-2015, 10:56 AM	#24
Chaplain Registered User Join Date: Oct 2014 Location: TN Posts: 294	Re: THM 350 issue with 85 C10 I'm saying...

02-26-2015, 04:55 PM	#25
peresich Registered User Join Date: Jan 2015 Location: Muscle Shoals, AL Posts: 20	Re: THM 350 issue with 85 C10 When we were dating our first argument was because I was changing the spark plugs on my Mustang 5.0 and couldn't reach the last one very well because it was darn near under the driver seat. She walked into the garage and took the wrench from my hand and finished the job for me. I was so mad that I wanted to scream, but I also knew that I was never going to let her get away. Her stepdad at the time drove a stock car in local races and she helped him wrench on it and she is as country as they come. It's nice to have the help. That said, I am going to have to drop the transmission and do a rebuild after all. It shifts like a dream now, but the clutches are going out. It is slipping pretty bad now.



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