# TH400 Shift Issues when Hot



## dvarghes92 (Aug 9, 2018)

Hey guys - I have a 68 GTO, TH400 transmission.
Car runs perfectly for the first 20 minutes when it is initially somewhat cold.

However, once the car gets to operating temperature, the transmission has issues.

I cannot quite explain it but it seems like the car is shifting from 1st to 2nd gear, but the car revs very very high but not enough power is going to the wheels. So I cannot get over 20-25 MPH without redlining. (Once again, this only happens when the car is hot). 

I tried manually shifting but the same issue continues.

This is what I have done:
Replaced Vaccum Modulator
Checked if appropriate vacuum is going to modulator
Checked fluid levels

Any help would be appreciated. Thanks,


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## dadspackard31 (Dec 2, 2019)

@dvarghes92 ,

I had the same issue with my 69 TH400 but mine happen cold or warmed.

Changed the vacuum module which others here advised me to and it fixed my shifting issue. It is a cheap fix.


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## dvarghes92 (Aug 9, 2018)

dadspackard31 said:


> @dvarghes92 ,
> 
> I had the same issue with my 69 TH400 but mine happen cold or warmed.
> 
> Changed the vacuum module which others here advised me to and it fixed my shifting issue. It is a cheap fix.


thanks - I changed the vacuum modulator as well but that didn’t seem to fix it unfortunately


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## Bopman (7 mo ago)

dvarghes92 said:


> thanks - I changed the vacuum modulator as well but that didn’t seem to fix it unfortunately


There will be help arriving soon ! 😊😉 I have some thoughts but really not a tranny guy. Worn clutches, band adjustments are my thoughts though. But like I said....Help is on the way 👍


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## O52 (Jan 27, 2019)

I had the same problem with the wife's Cadillac. Once the engine warmed up, the transmission started slipping. 

The Cause; The engine warm up was coincidental as the transmission clutches were found to be badly worn when the transmission was removed. 
Within a couple miles of starting, the transmission would begin to slip. The debris from the clutch slippage would start packing the oil pick up screen which cut off the oil supply which caused more slipping. 
When the engine shut down, the debris fell back into the pan. The cycle would repeat once the engine was started up again. 

Not saying this is your problem, but you may want to drop the transmission pan to check for metallic sludge plugging up the internal filter.


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## Bopman (7 mo ago)

O52 said:


> I had the same problem with the wife's Cadillac. Once the engine warmed up, the transmission started slipping.
> 
> The Cause; The engine warm up was coincidental as the transmission clutches were found to be badly worn when the transmission was removed.
> Within a couple miles of starting, the transmission would begin to slip. The debris from the clutch slippage would start packing the oil pick up screen which cut off the oil supply which caused more slipping.
> ...


I told him...Help is on the way.😊😉👍


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## lust4speed (Jul 5, 2019)

I've lost count of the times that my friends and I have started to encounter different shifting problems with our T400 transmissions. Most transmissions returned to normal operation with a fluid and filter change. Might be debris, might be moisture, but it's amazing how many problems go away with new fluid and filter.


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## armyadarkness (Dec 7, 2020)

Only time I ever had that happen on an auto is when the pump went bad or the filter fell off, causing the pump to suck air.

I suggest a fluid and filter change, and verify that your filter is pushed in all the way and not clogged.


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## dvarghes92 (Aug 9, 2018)

Thanks everyone, sounds like I will be changing the oil and filter tomorrow. If that’s not it, sounds like it’s the clutch.

much appreciated!


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## ponchonlefty (8 mo ago)

is the fluid burned. kind of smell burned?


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## dvarghes92 (Aug 9, 2018)

ponchonlefty said:


> is the fluid burned. kind of smell burned?


I don’t think so? Can’t quite tell.
I did change the oil and filter and dealing with the same thing.

could the governor be causing this issue?


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## ponchonlefty (8 mo ago)

dvarghes92 said:


> I don’t think so? Can’t quite tell.
> I did change the oil and filter and dealing with the same thing.
> 
> could the governor be causing this issue?


could be a broken spring in the governor.its a possibility.


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## dvarghes92 (Aug 9, 2018)

Thanks everyone.
Next step is checking out the governor for me as it is still not shifting.

2 questions:
1. I have heard of the kickdown solenoid being stuck in downshift. How do I unhook it to see if this will fix the issue?
2. How much vacuum should I be getting to the modulator?


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## ponchonlefty (8 mo ago)

the kickdown solenoid is the one wire on the trans. you will see what im talking about when you look. just pull the plug. maybe check the modulator line see if it is clogged. i dont know the exact vacuum. i can tell you its a full vacuum. if the solenoid is stuck im not sure unplugging will make a difference. the wire may be grounded somewhere.


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## ponchonlefty (8 mo ago)

i think you may be looking at a rebuild.


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## dvarghes92 (Aug 9, 2018)

Alright guys, here is an update.

pulled the governor off and everything looked alright.

went for a quick spin today to see any difference, turns out the car will shift when it’s hot, but I have to be close to redlining it. Guess I wasn’t pushing it earlier.

so what could cause the car to shift fine when cold but different shift points when it’s hot? Wonder if it’s getting varied vacuum?

should I be looking at governor springs or start out with adjustment of the modulator?

what about kickdown linkage or solenoid?


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## ponchonlefty (8 mo ago)

dvarghes92 said:


> Alright guys, here is an update.
> 
> pulled the governor off and everything looked alright.
> 
> ...


probably the pump.


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## BLK69JUDGE (Jun 10, 2010)

so
did this just start out of the blue ???
or ?
have you had the valve body off ??
having the gaskets on the plate reversed caused my issue years ago
on a 69 w-30 sedan I had .... when I put a shift kit in ,,,,
and didnt pay attention ,,,, 

Scott


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## dvarghes92 (Aug 9, 2018)

What pump? 

And this is def out of the blue, it was riding fine a couple months ago…


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## ponchonlefty (8 mo ago)

dvarghes92 said:


> What pump?
> 
> And this is def out of the blue, it was riding fine a couple months ago…


the front pump in the trans. the fact that it works when its cold then screws up when warm it is a possible problem. im not a trans guy but have some experience with bad transmissions. even if it has been rebuilt it doesn't mean it was done properly. find a reputable trans shop. it may be best option. its better than guessing.


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## dvarghes92 (Aug 9, 2018)

So…to everyone who is wondering. I am still messing with it it haha.

I dropped the pan again so I can replace the kick down solenoid. This was going to be my last shot at troubleshooting before I take it to a shop.

Once I dropped the pan and the filter I realized, there is no connector the kickdown solenoid. I am assuming this could have been affecting my upshifts?

couple of questions -
1. The new kickdown solenoid comes with a 1 prong plug instead of a 2 prong. What was the second prong used for?
2. When I inspected further, there is a blue wire on the opposite side of the tunnel which could potentially hook up right the 1 prong. I am assuming that is correct?
3. See picture attached. What does this part do?

thanks guys, I am having fun with it so not giving up quite yet.


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## lust4speed (Jul 5, 2019)

Blue wire on passenger side of tunnel = oil pressure wire going to sender on the oil filter adapter.
The transmissions that had two prongs on the driver's side had one for kickdown and one for switch-pitch converter which wasn't available in a '68 GTO. The transmission could have been substituted at some point in the past.

The trans kickdown wire is something like a black with a pink tracer or maybe a yellow tracer.

Very late shifting could be a failed modulator or even simply the vacuum line to the modulator disconnected.


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## ponchonlefty (8 mo ago)

dvarghes92 said:


> So…to everyone who is wondering. I am still messing with it it haha.
> 
> I dropped the pan again so I can replace the kick down solenoid. This was going to be my last shot at troubleshooting before I take it to a shop.
> 
> ...


hope you find the problem and its something simple. i respect your attitude.that will take you far.


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## dvarghes92 (Aug 9, 2018)

Thanks guys, I’m guessing I’ll need to figure out where the wire for the kickdown solenoid is and see if that fixes the issue by connecting it. 

it already has a new modulator but might as well buy a new one just to troubleshoot since it’s so inexpensive.

Appreciate it


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## BearGFR (Aug 25, 2008)

dvarghes92 said:


> Thanks guys, I’m guessing I’ll need to figure out where the wire for the kickdown solenoid is and see if that fixes the issue by connecting it.
> 
> it already has a new modulator but might as well buy a new one just to troubleshoot since it’s so inexpensive.
> 
> Appreciate it


There should be a sliding switch up under the dash that moves with the accelerator pedal linkage. A wire coming off of that goes to the kick-down solenoid connector on the transmission. 
The switch itself looks similar to this:


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## dvarghes92 (Aug 9, 2018)

Quick update to everyone, got a new kickdown solenoid and hooked it up (previously not hooked up). That seems to have fixed the issue.
Thanks to everyone!


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## ponchonlefty (8 mo ago)

dvarghes92 said:


> Quick update to everyone, got a new kickdown solenoid and hooked it up (previously not hooked up). That seems to have fixed the issue.
> Thanks to everyone!


glad to hear it. now go enjoy a drive,its a very nice day. thanks for the update.


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