TH350 No First Gear Problem

[DixieScottsdale]

Hello All!

I am currently experiencing a problem with my TH350 in my 1986 C10. When I am starting from a dead stop, my transmission is not starting in 1st gear. Additionally, regardless of what speed I am at, the transmission will not shift into first gear, manually or otherwise. I am not sure if this would be a problem, but I recently converted from the factory rochester quadrajet (I believe an E4ME) to a brand new Edelbrock 1405. When I was disassembling and setting up the throttle linkage, I saw no kickdown cable for the trans anywhere. I am using the factory throttle cable and bracket. My only modification to the throttle cable was bending the bracket so that the throttle had full travel. When it comes to automatic transmissions, I have very little knowledge, so I have no idea what this problem could be, so if someone could point me in the right direction, it would be greatly appreciated.

Thanks!

 

[animal]

@HotRodPC

 

[HotRodPC]

How about when you put the shifter in L1 or 1st gear what happens?

 

[HotRodPC]

I have a feeling you don't have a Th350 anyway. You've likley got a long tailshaft slip yoke Th400, so no detent cable is required and doesn't exist. Yep, weird I know, but 4.3 V6's came with a Th400 not a Th350 in the mid to late 80's. Find that out for sure too.

 

[DixieScottsdale]

How about when you put the shifter in L1 or 1st gear what happens?

When I shift it in to 1st it doesn't make any difference. It bogs if I try to get on the gas at low speed, and burns up the clutches almost like I'm starting in 2nd in a car with a M/T. Also, I looked up the RPO codes that were on the info sheet in the glovebox, and the code MX1 indicates that it has a TH350. However, it also has a code V02 for H.D. RAD and Trans? I'll attach some pictures of the option sheet to this post in case anyone is able to give any further information.

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[HotRodPC]

Code MX1 Indicates it has a 3 speed automatic transmission, doesn't mean it's a Th350. My 86 C20 with a 454 also has VO2 and MX1.
IF the transmission has never been swapped, I'd be 99% certain it's a long shaft Th400 with a slip yoke. Unlike My Turbo 400 with a short shaft and a bolt on fixed yoke. I wouldn't make it up, these that came with 3 speed autos and a V6 in the Mid 80's got a Th400. I think it had to do with Th350's being phased out and 700r4's being phased in, but since the C20 and C30's were still getting Th400's all the way up until 90, they were put into the half tons too.

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[DixieScottsdale]

Code MX1 Indicates it has a 3 speed automatic transmission, doesn't mean it's a Th350. My 86 C20 with a 454 also has VO2 and MX1.
IF the transmission has never been swapped, I'd be 99% certain it's a long shaft Th400 with a slip yoke. Unlike My Turbo 400 with a short shaft and a bolt on fixed yoke. I wouldn't make it up, these that came with 3 speed autos and a V6 in the Mid 80's got a Th400. I think it had to do with Th350's being phased out and 700r4's being phased in, but since the C20 and C30's were still getting Th400's all the way up until 90, they were put into the half tons too.

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To my knowledge, the transmission is original, which means that my mood has definitely been boosted a bit! So thanks!



Now that it's established that it is most likely a TH400, then do you have any clue as to what my problem could be as far as not having first gear?

 

[HotRodPC]

Whether it's a Th350 or Th400 does matter since they have different characteristics. But it doesn't really sound good at all when you say it's bogging down and burning up clutches. Is the trans fluid black yet? It's going to be if it's not already.

 

[DixieScottsdale]

I haven't checked the trans fluid within the past two weeks. However, I checked it about a month ago, as I initially thought that low fluid may have been the problem. But it's probably been a week or two since I've checked it. The truck is my daily driver, so I've been driving it in its current state for close to a month now. I'll have to check it tomorrow morning. If the trans is bad then it gives me a good reason to take the NV3500 out of my '96 parts truck and swap it in. However, if possible, I'd like to know if there's any other possible source to the problem that I could fix without rebuilding the trans.

Thank you for your help btw!

 

[HotRodPC]

To my knowledge, the transmission is original, which means that my mood has definitely been boosted a bit! So thanks!



Now that it's established that it is most likely a TH400, then do you have any clue as to what my problem could be as far as not having first gear?

Usually with this kind of issue, where it's obviously trying to be in 2 gears at the same time, binding and bogging, burning up clutches it's some type of bleed in the valve body. I'm not all that great at diagnosing this type of problem since it could be several different ones. The bad news is, when I do hear other people tell me what there's was doing and this has happened on a Th400 once, in fact it's why I bought my truck, the guy threw the towel in on it and wanted it hauled off with a burnt up transmission cuz it was bogging and he said it didn't have 1st gear but he continued to drive it. Next time he came to a stop, it wouldn't even take off. This was the result. Roller sprag had broke. Chain reaction things happened and transmission was burnt to a crisp melting the plastic parts and melting rubber seals. It wasn't pretty at all.

How does yours act when you finally get up to speed, and then does it go into 3rd OK???

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[DixieScottsdale]

It shifts into third fine. Once I get above 15-20 mph then its fine. The only time I have an issue with bogging is if I go past 2/3 throttle below the aforementioned speeds or from like a dead stop on a semi-steep hill. Other than that, I'd say that the trans operates normally (not including the lack of 1st gear obviously).

 

[HotRodPC]

I'll have to find my Th400 diagnostics flow chart and take a look what's needed for first gear. You've mentioned you tried L1 on the shifter too and still no 1st gear also. That's important to know because 1st gear is made differently in L1 than it is in D. Different clutch packs, roller clutches, bands etc are applied and non active in different scenarios so it does help a little to know those things and I don't know by memory it's been so long.

I see no reason for the simple things like Vac Modulator or Governor to be causing the problem since it is shifting. So whatever it is, at the bare minimum the pan is going to have to come off and get into the valve body. Looking for a hung up valve or shuttle valve. I'll see if I can find my Th400 flow chart. It's in one on these books I have around somewhere.

 

[DixieScottsdale]

I'll have to find my Th400 diagnostics flow chart and take a look what's needed for first gear. You've mentioned you tried L1 on the shifter too and still no 1st gear also. That's important to know because 1st gear is made differently in L1 than it is in D. Different clutch packs, roller clutches, bands etc are applied and non active in different scenarios so it does help a little to know those things and I don't know by memory it's been so long.

I see no reason for the simple things like Vac Modulator or Governor to be causing the problem since it is shifting. So whatever it is, at the bare minimum the pan is going to have to come off and get into the valve body. Looking for a hung up valve or shuttle valve. I'll see if I can find my Th400 flow chart. It's in one on these books I have around somewhere.

While that's not exactly what I had hoped for, it's at least somewhere to start. I would greatly appreciate it if you could upload that flow chart. I'll have to get my beater on the road and try to get to taking the pan off within the next couple weeks.

Thanks again for all of your help!

 

[HotRodPC]

I just talked to one of my trans buddies, and he does think it's possible to be a stuck governor valve. 1 thing is right about, whether it's true or not, it's external and easy to get to, so at least try it first before dropping the pan.

In a Th400 it'll be a small square plate with 4 bolts holding it. It's not under pressure at all, but it might leak out a few table spoons of fluid when you remove the cover. Governor will slide right out by hand. Point it up toward the sky, then move the weights. Make sure the valve is moving freely in both directions and make sure the teeth on the end of it aren't chewed up. If all good, put it back in and drop the pan. He's thinking more along the lines of a stuck 1-2 shift valve but won't say for sure either without driving it and doing a line pressure test. As he says, if it's original to 1986 it's overdue for a build. Yank it and build it then it's good for another 30 years.