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URGENT: 4T65-E/4T65E-HD Transmission swap/interchange question


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Shill-O-Ette needs a transmission. I want to upgrade if possible...can I use a 4T65E-HD transmission in there?


Will the Silhouette's engine computer still make everything work fine? It's got a 3400 and the non-HD 3.29:1 trans in there now, the transmission he recommended I use is a 70K mile one from a rolled 1998 Riviera 3800SC and would be a 4T65E-HD with a 2.93 ratio. That would mean instead of running 2000rpm at 70mph it would be 2000rpm at 78mph, lowering my highway cruise rpm's by about 220-225rpm. The only thing other than the computer not recognizing it that I'm worried about is that every other cruise or shift point will lower and that either means 3rd gear converter lockup at 23mph instead of 26mph or the 3rd gear converter lockup would happen at a lower rpm and make the tailpipe buzz for twice as long as it does now. Either way I don't care, might make for better mileage and this van needs help in that area.


So basically, will the 4T65E-HD from a '98 3800SC be plug and play with a '98 3400? Can the Silhouette's PCM or ECU or whatever handle it? The old trans is out and sitting next to the other one and I can't tell a damn difference between them. If not, can I use other years of 4T65-E? They have a 2004 base Park Avenue in another local junkyard with only 42K miles on it and right side damage, this would be a 3.05:1 ratio 4T65-E, non-HD and would cost me about $100 more.


Or let me know if I have any other options because they just don't have a 3.29 ratio anything available with under 100K miles. They have a 2002 Rendevous which could be 3.29 or 3.69 but I don't know how to be sure, can't find a SPID label to get an axle code from. This is a truck and van yard where my mechanic works, honestly, they haven't got a lot of cars, that Riviera is about it for a transmission (all 4 of their 97-05 GM vans either trans is gone or have bad trans) and he thinks it'll work but I wanted to ask here before I let him go for it.

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Your PCM will need reprogrammed for the different shift points/ VSS


You will need a different Passenger? side axle as the HD has a bigger diff. I want to say one for a 98' GTP would be the right length


you may have to use a 3400 TQ if the 3800 one has a different bolt pattern.

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It will work, but you may throw an engine light for the computer noticing the lower gearing, that other bad thing is that the lower gearing will make your van even more dogshit slow... And revving that low on the highway means no passing power and constant shifting into 3rd on any incline... Since you have the HD trans, I would pull the differential out and put it in whatever trans your going to put in... Thats the only difference in the HD/nonHD transmissions...


Im looking up an interchange for you

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Ouch. So if it's installed with the GTP axle and either TC and I attempt to drive it home, will it eat itself? Or will the speedometer read oddly and it shift too early or late or will it not shift at all?


I can mess with it here at home but mechanic is strictly mechanical, he doesn't fool around with computers and programming.


I imagine a dealer won't touch it, either.


Check engine light is already on for a TPS/IAC issue of some kind. So I wouldn't notice if the trans throws a code. I imagine it would.


Can't change the diff over. The diff is what ate itself and took out the trans output shaft and chain drive.

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Does converter size matter? I mean, if the vehicle it's being pulled from has the larger diameter, it'll still have the same bellhousing and bolt to the 3400, right? Just might have a lower or higher stall speed?


Based on that chart I shouldn't have a 4T65-E at all. The 1998 Silhouette is not shown. I wonder if the transmission was changed before, and I need to get the codes off it and find out what it came out of.

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According to Wiki, you have the 65E, I dont know why its not on that list



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I would love to do that but I already spent $600 doing the head gaskets on the 3400. Also, it would require some custom fabrication, custom tuning, and all sorts of things I'm not prepared to do at this time.


Also I'd rather use a 4.9L Cadillac V8 with a 4-5psi turbo. The 3800SC build would be for a base model lightweight Celebrity coupe with a 5-speed. Van needs torques.

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  • 3 weeks later...

Transmission is in.


I have reverse, first, second, and above 42mph, fourth. No third gear. No check engine light, no stored codes.



7YA code

4/30/97 build date


This means it's a 4T60-E and has 3.29 final drive. Pulled from a 1997 Trans Sport with 120K. The transmission pulled from my van was a 4T60-E as well, 8AYW, turns out original to the van.


So...if the transmission is assumed good, does a single year change mean it needs flashed?


edit: 1-2 shift now happens about 20mph (was 15mph) and around 32-33mph it feels like the lockup is engaging in 2, then on light throttle it'll shift into 4 and lockup at 42mph. If you drop below 42mph and accelerate, it drops to 2 until you get above 42mph and you have to let off the gas to get 4. Otherwise it holds second until you either let off the throttle or drop below 12mph when it'll catch first.


Evidently my transmission was making so much noise before that I couldn't hear brakes squealing.


It's looking like I'm going to have to wait for the junkyard to get in another good 7YAW or 8AYW 4T60E and then spend $380 in labor and fluid to have him swap it again. My van IS a '98...couldn't I just use a 99-00 4T65E? I know the 4T65E has no vacuum thing on the front, but what other changes would I have to make other than flashing the ECU? Can a dealer do the flash? Can it be done with a laptop? I don't have a laptop old enough to have a serial port though, so I'd have to use a USB adapter...wait...hold on...nope not even the Dell has one.

Edited by occupant
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