Cadillac Owners Forum banner

1 - 20 of 32 Posts

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #1
Hello folks,

it's been a while, but at some point last year my 1978 Cadillac Seville (California model) suddenly stopped shifting from 2nd to 3rd gear. I can still drive and have no complains in shifting other than the 2-3 upshift that is not working. No noise or anything else. Need to mention that 2 or 3 times it does the shift after some highway driving constantly around 70 mph. Did not occured lately – maybe because I stopped driving at that speed within the 2nd gear.

So what the repair manual basically says is to do the preliminary checking procedure
1. Check trans. oil level
2.check outside manual linkage and correct
3. check engine timing and idle
4. check oil pressure in different manner.

And the diagnosis chart says the following about "1st and 2nd Speed Only, No 2-3 Upshift":
1. Control Valve Assembly: Stuck 2-3 valve, gasket mispositioned or leaking
2. Direct Clutch: Burned, check cause ...
3. Improper Vacuum: Check cause ...

And to complete the list of procedures and possible solutions, I'm gonna list other stuff I've found on the net:
- Definitely saying that to read the pressure FIRST and check if the PSI meets the specs
- Modulator problem (will rather cause slip shift or rough shift than no shift ...?)
- Cable(?) ...
- Stuck kickdown-switch above the gas pedal
- Stuck lockdown switch

So ... Do you have any experience on that? Any hints? Should I proceed as the manuals tell or do you know about some common cause of that problem?

A new modulator is pretty easy & cheap, so think it's worth swapping? Or how can I check if it's broken?

And I'm not sure if I would know what exactly to do beyond the filter (2-3 valve etc.). May end up letting a pro do that, as much as I prefer to do my own work. So if there are any recommendations on how to proceed – appreciate that!
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #3 (Edited)
Thank you!
Seems like the 2-3 shift valve could be the problem. Can you tell me, if I can fix a stuck valve without disassembling the housing; just by removing the oil pan and filter?

Before that I will check the oil pressure as described in the manual.

And even before that, a friend told me to check the oil pressure switch if there is one. I know that there is one, but is it for transmission oil or engine oil?
 

·
Registered
1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
Joined
·
6,738 Posts
Have you removed the transmission governor drive assembly? It's plastic and the gears can strip. Once you take off the speedometer cable you can unbolt and remove the governor. A small amount of fluid will drip out. Inspect the plastic gear teeth. If stripped replace the governor assembly. This is with the assumption that the trans is staying in low gear with your foot on the pedal and coasts with your foot removed as if you're in neutral. Also check that the down shift button under the pedal is not hindered by the carpet surrounding it. I read your post again and I think you mean it shifts from first to second but not to third, correct? If so, I suspect the downshift button or the wires to it first, governor second.
 

·
Registered
Eldorado
Joined
·
448 Posts
Check them out, but if it does 1-2 shift, I doubt it is some simple external
thing. My question is, has this 42 year old trans ever been apart and refreshed?
If not, the internal rubber seals containing full fluid pressure may be expected to
fail. Replacing these with the $30 kit might restore a lower mileage trans for a
couple decades, if all the hard parts are in good shape. Allowed to degrade
more, a clutch could burn up and fill the trans with poison requiring an expensive
overhaul. Bruce Roe
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #7
Have you removed the transmission governor drive assembly? It's plastic and the gears can strip. Once you take off the speedometer cable you can unbolt and remove the governor. A small amount of fluid will drip out. Inspect the plastic gear teeth. If stripped replace the governor assembly. This is with the assumption that the trans is staying in low gear with your foot on the pedal and coasts with your foot removed as if you're in neutral.
Thanks for your input!
Haven't inspected the governor, but as this looks to be done without removing the pan etc. I will do it as well.

Also check that the down shift button under the pedal is not hindered by the carpet surrounding it. I read your post again and I think you mean it shifts from first to second but not to third, correct? If so, I suspect the downshift button or the wires to it first, governor second.
Yes, you're totally right: 1-2 Shift works well. 2-3 not.
I will check the kickdown/downshift button first. What about the downshift switch somewhere above the accelerator? Think it's worth checking somehow as well?
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #8
Check them out, but if it does 1-2 shift, I doubt it is some simple external
thing. My question is, has this 42 year old trans ever been apart and refreshed?
If not, the internal rubber seals containing full fluid pressure may be expected to
fail. Replacing these with the $30 kit might restore a lower mileage trans for a
couple decades, if all the hard parts are in good shape. Allowed to degrade
more, a clutch could burn up and fill the trans with poison requiring an expensive
overhaul. Bruce Roe
Hi, thanks for your reply!
What do you mean by "them"?

No, the trans hasn't been refreshed. Just did a fluid change including pan gasket. But I think you mean some rubber gaskets inside the trans where I need to remove the whole assembly ...?
BUT the car sat most of the time until 5 years ago and was driven every now and then, so it got to roughly 20k miles (only). Since then I drive it more frequently but it is still at 25k or so.

I will check the modulator hose as well.
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #9 (Edited)
1) Trans Oil fine

2) Fuse fine

3) TV Cable
- Check if streched or broken
- Some part missing?
- Check movement (free & return)
- Check if no resistance = broken or disconnected
- Adjust cable (tension); screw, locknut

I did a photo (see below) and a video of this. Can you tell me if you see anything wrong here and how to check if it stretched?
For me it seems to be all fine ...

Other steps 4), 5), etc. will follow ...

IMG_20190319_132931.jpg
 

·
Super Moderator
2014 ELR
Joined
·
10,108 Posts
Review my post, and this quote from your post, I think this needs checking out (and look at the link from my post):

"And the diagnosis chart says the following about "1st and 2nd Speed Only, No 2-3 Upshift":
1. Control Valve Assembly: Stuck 2-3 valve, gasket mispositioned or leaking
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #11
Review my post, and this quote from your post, I think this needs checking out (and look at the link from my post):

"And the diagnosis chart says the following about "1st and 2nd Speed Only, No 2-3 Upshift":
1. Control Valve Assembly: Stuck 2-3 valve, gasket mispositioned or leaking
My checklist doesn't end at 3. It has a lot more things like the control valve, but it makes no sense for me to start with some harder topics when there is a possibility to check the basics first.

I just wanna make sure that I don't miss anything about the TV cable and got this right.
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #12 (Edited)
...

4) Downshift Button - Check if hindered by carpet
5) Downshift Button Wires

Read this somewhere, but I don't think I have a 'downshift button under the accelerator pedal' worth checking. Any ideas on this?

View attachment 546302
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #13 (Edited)
6) Kickdown Switch
- Check if stuck closed:
- Unplug connector from switch and test drive.
- Reset switch

Can you confirm that it is not the kickdown switch(?),
when
I unplugged it and did a test drive, but the 2-3 shift problem remained?

If so, then I don't need to disassemble (and reset) it.

IMG_20190319_134523.jpg
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
If you can confirm there is no problem (see above), then I would continue moving below the car to check the hoses, modulator and governor first; before the messy stuff...
Thanks in advance!
 

·
Registered
70 Deville 78 Seville 92 Deville 03 Deville
Joined
·
821 Posts
Hmmm, I can't believe nobody mentioned to verify the vacuum at the trans. Usually bad vacuum affects the 1-2 shift too but I would check it anyway; it costs nothing. If it has good vacuum and the modulator works and the kick down switch has been disconnected and makes no difference, you may have an internal trans problem. I have never killed a th400 in my life. I have a couple that I have had for 40 years.
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #16
Thanks! Will check that definitely!

Do you think it's worth checking the oil pressure as well?
 

·
Registered
70 Deville 78 Seville 92 Deville 03 Deville
Joined
·
821 Posts
If you have the means to check the pressure it may be helpful. If the pressure is down everywhere then it most likely has internal problems If the trans has good vacuum, and the kick down wire is disconnected, it should be shifting to 3rd. If not I have one last thought, the kick down solenoid may be stuck in the kick down position so even though it is unplugged it is still holding 2nd. That solenoid is pretty easy to get at with the pan off. I don't remember what it looks like though but I'm sure there are pics and maybe other tips out there in the googlverse.
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #18
If you have the means to check the pressure it may be helpful. If the pressure is down everywhere then it most likely has internal problems If the trans has good vacuum, and the kick down wire is disconnected, it should be shifting to 3rd. If not I have one last thought, the kick down solenoid may be stuck in the kick down position so even though it is unplugged it is still holding 2nd. That solenoid is pretty easy to get at with the pan off. I don't remember what it looks like though but I'm sure there are pics and maybe other tips out there in the googlverse.

Thank you very much!

I'll be at the garage tomorrow and will do my best to figure out if it's the vacuum, the modulator, the governor and of course the kickdown solenoid as you hinted!

btw found a good article for that here: https://www.gmsquarebody.com/threads/th400-2-3-shift-issue.14305/
 

·
Registered
1978 Seville Elegante
Joined
·
48 Posts
Discussion Starter #20
So I was in the garage last weekend and verified that the following things do not cause the problem:

7) Metal Vacuum Tube

8) Vacuum Modulator Hose
I exchange the hose because it had some cracks and looked like it is going to break some day, but it seemd like it was still holding the vacuum and the test drives after the change didn't make the 2-3 shift.

9) Vacuum Modulator (itself)
Holds vacuum. No fluid leaking. Still all good.

10) Kickdown Downshift Switch / Detent
It clicks all the time when pushing down the accelerator.
That means it works right? Didn't look at it.


Unfortunately I can not access the Governor easily, as you can see on the image below. Would need to let the whole transmission a little bit down to be able to unplug it. Need more time for this OR I might let some professionals do it, as well as the 2-3 Shift Valve at the assembly, because I don't think that I wanna do that by myself.

Maybe I'm going to check the oil pressure before and/or can find a "transmission downshift switch" at the engine compartment that I can check if stuck.

Any thoughts from your side?


IMG_20190406_172052.jpg
 
1 - 20 of 32 Posts
Top