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Old 05-22-2012, 11:53 AM   #1
dubmix
 
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Default 1991 D21 - 4spd Auto - RL40R1A no 4th?

Hey,

First post, and needing some help. I've been digging around the forums for quite a bit and wondering if there are any transmission savy people on the forum.

I've read the 1990 D21 FSM, and looking at the troubleshooting matrix, I don't really have a pinpoint way to diagnose the following problem I have with the transmission.

I bought this 1991 D21 recently and when I test drove it around town, I only took it around town and not highway speeds. SO, my own fault for not finding out that 4th wasn't engaging. Cash exchanged hands and now I got a bad trans. After digging around the truck, I found in the glove box that the transmission was leaking fluid from around the oil pump, and a local shop serviced and resealed (??) the transmission to stop it from leaking. Fluid level seems constant at the moment. It was a low at first and I added a bottle of ATF.

When driving the vehicle, it seems to shift a little early on vehicle speed/RPM, going into third at 35+/- mph, aprox 1.5k rpm, but then when trying to go above to 4th, it doesn't engage, it stays in 3rd. I've tried taking it up to 70+/- mph, according to the FSM shift points for 4th (at 4/8 throttle) and it never shifts. Stays in third, but continues acceleration. Checked the wiring from the OD switch, it works. The weird thing is when i drive it at low RPMs (sub 2k), the 4th engages (I hear it shift into 4th and RPMs match), but if I try to accelerate past 1.5k RPM, it drops back down into third as if the overdrive cancel is kicked in. It seems really strange.

I'm doing a fluid/filter change to start this weekend (according to FSM), and looking at the TV wire. Anyone have a way of pinpointing what this might be? I'm guessing I need to have the trans pressure tested after seeing the oil pump had been messed with.

Thanks for any advice / direction you can give me.
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Old 05-24-2012, 01:16 PM   #2
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Messed around under the trans yesterday and looked around the engine bay. Still no pressure test. Trying for it a shop this weekend. Found the TV wire next the the throttle body. It had a lot of slack in it. Tightend it up to full tension. Also, on the transmission side, the bolt securing the cable was left loose on the body of the transmission. I guess rushed work when they must with the oil pump. Took it for a test drive and now the shift points are much higher during acceleration for gears 1-3. Same problem though with 4th gear. Under 1500rpm, it shifts into overdrive. Over, it goes back into third. Can anyone tell me about how the Lock-up works for this transmission? Is it sole responsible for engaging the OD? I can't find much documentation on how exactly this transmission works during normal operations.

Can anyone please help?
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Old 05-24-2012, 02:31 PM   #3
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I wish I could offer some help but I know very little about the auto transmissions. Have you asked at nicoclub.com?
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Old 06-25-2012, 01:12 PM   #4
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Ok, so there is success in this story.... I would have taken pictures, but I was covered in trans fluid.

After hours of research on Google, Infamous, Haynes manuals, and etc, I ended up signing up for the alldatadiy.com website for the D21. There, I found a Factory Service Bulletin that had specific information on "if customer complains of downshifting/loss of 4th, no 4th, etc". In the bulletin, it details that on the lower valve body, there's a metal seal that retains pressure on the Lock-up control valve on the valve body. It states that the seal cracks, and when pressure builds up in drive up to 4th gear, it might have initial pressure, but will then soon loose it, causing the slip back down to 3rd gear. This explains the drop back to 3rd at 1500rpm. They recommend finding a replacement from a GM transmission that will fit the same diameter hole, but needs to be filed down a bit to secure seal. So, after another week of Google, trying to find this elusive seal, I found that most off the shelf transmission shift kits contain this seal, along with some springs, bands, gaskets, check balls etc. So I buy the kit for $60, just to replace a $5 part. This is all with the local transmission shop quoting me $950 for a "rebuild", without even looking at the transmission. If I wasn't a aircraft technician as my day job, I might have just paid it, but it bugged me that they were blind quoting me on a fix that they didn't even bother to diagnose according to the FSM.

So I go in the afternoon, drive the truck up on stands for the front, and drain the transmission for a few hours. Then get up underneath and drop the pan, and, while underneath the truck, with transmission fluid gently raining down on me, attempt to drop the valve body myself. That was no problem, and taking apart the valve body itself was a smooth deal, the only concern was separating all the different length bolts from each other and remembering where they went. Taking apart the valve body, I used a small magnet to remove the pin securing the valve seal, and low and behold, it was cracked in half (thus not allowing the proper building up of pressure in that hydro circuit, and making me lose 4th and TCC Lockup). Replaced it, sealed up the valve body changed a couple springs, cleaned up all the metal shavings I could, and then the hard part was trying to support the valve body with one hand, while trying to hold the springs in place with the other hand, and quickly grabbing the ratchet to be able to at least start a few of the bolts that would secure the valve body up to the transmission. After 2 gallons of ATF, truck was working in gear, just fine, with a nice and smooth 70mph in 4th gear @ 1500 rpm. All said and done, 6hrs for never working on a transmission or being up under the truck. Total cost, about $150 between the shift kit, 4 gallons of ATF, a filter, and the alldatadiy.com membership. Big saving from the near $1000 dollar quote to "rebuild", and definately a lot of experience earned from working it by myself. And I know what I did, by the book, and it came out right.
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Old 06-26-2012, 04:31 AM   #5
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Ha, nice job!!!
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Old 11-25-2019, 10:23 PM   #6
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Quote:
Originally Posted by dubmix View Post
Ok, so there is success in this story.... I would have taken pictures, but I was covered in trans fluid.

After hours of research on Google, Infamous, Haynes manuals, and etc, I ended up signing up for the alldatadiy.com website for the D21. There, I found a Factory Service Bulletin that had specific information on "if customer complains of downshifting/loss of 4th, no 4th, etc". In the bulletin, it details that on the lower valve body, there's a metal seal that retains pressure on the Lock-up control valve on the valve body. It states that the seal cracks, and when pressure builds up in drive up to 4th gear, it might have initial pressure, but will then soon loose it, causing the slip back down to 3rd gear. This explains the drop back to 3rd at 1500rpm. They recommend finding a replacement from a GM transmission that will fit the same diameter hole, but needs to be filed down a bit to secure seal. So, after another week of Google, trying to find this elusive seal, I found that most off the shelf transmission shift kits contain this seal, along with some springs, bands, gaskets, check balls etc. So I buy the kit for $60, just to replace a $5 part. This is all with the local transmission shop quoting me $950 for a "rebuild", without even looking at the transmission. If I wasn't a aircraft technician as my day job, I might have just paid it, but it bugged me that they were blind quoting me on a fix that they didn't even bother to diagnose according to the FSM.

So I go in the afternoon, drive the truck up on stands for the front, and drain the transmission for a few hours. Then get up underneath and drop the pan, and, while underneath the truck, with transmission fluid gently raining down on me, attempt to drop the valve body myself. That was no problem, and taking apart the valve body itself was a smooth deal, the only concern was separating all the different length bolts from each other and remembering where they went. Taking apart the valve body, I used a small magnet to remove the pin securing the valve seal, and low and behold, it was cracked in half (thus not allowing the proper building up of pressure in that hydro circuit, and making me lose 4th and TCC Lockup). Replaced it, sealed up the valve body changed a couple springs, cleaned up all the metal shavings I could, and then the hard part was trying to support the valve body with one hand, while trying to hold the springs in place with the other hand, and quickly grabbing the ratchet to be able to at least start a few of the bolts that would secure the valve body up to the transmission. After 2 gallons of ATF, truck was working in gear, just fine, with a nice and smooth 70mph in 4th gear @ 1500 rpm. All said and done, 6hrs for never working on a transmission or being up under the truck. Total cost, about $150 between the shift kit, 4 gallons of ATF, a filter, and the alldatadiy.com membership. Big saving from the near $1000 dollar quote to "rebuild", and definately a lot of experience earned from working it by myself. And I know what I did, by the book, and it came out right.

sorry for the threat resurrection....

I'll be attempting this fix myself soon. i have the exact same symptoms.

did you find the GM part numbers for it ?
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Old 11-26-2019, 01:24 AM   #7
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4 GALLONS of ATF?
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Old 09-05-2020, 09:08 PM   #8
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I have same issue. Can you tell me what brand and part number you used to fix your 4gear issue. Thanks
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Old 09-06-2020, 12:27 AM   #9
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As Dub only has 5 posts total and this was written in 2012 I don't think you guys will get an answer from him. Sorry.
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Old 09-14-2020, 10:17 AM   #10
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Can you please tell me what the brand and model # of kit you used.
I have same issue.
Thanks

Last edited by Old timer; 09-14-2020 at 10:20 AM. Reason: Incorrect quote.
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