04 transmission in 03 Tahoe problem

[Hempwizard]

Hello all, I've gotten a lot of information from this site So I'm hoping you will be able to help me with the problem I'm having now. First let me say I'm new to the site even though I have been lurking around for a long time. Cars have been a hobby of mine for over 30 years. I do industrial maintenance professionally. I work on a lot of cars for people on the side, and one of the people I do work for has three tahoes and a Yukon Denali.

The problem I'm having is her daughter also has a 2003 Tahoe. She had someone(Her boyfriend) put a Transmission in for her and the one he got was from a 2004. Now they have brought me in on it because it will not shift properly. If you put it in drive, it shifts through first and second but when it gets to third it just free revs Like it goes to neutral. You can shift it manually through first and second but will not go into 3rd that way either. The transmission was apparently working fine from the donor vehicle and the fluid is proper fluid and brand new. Transmission came from a salvage yard with a warranty.

I have been told by multiple people that only the 03 is compatible with the 03 what's up making some modifications. Before I start trying to guess at this, I'm wondering if anyone on here would have an idea of what the problem is. I feel as though it is electrical, but I have no knowledge of the person who did the swaps skills. I'm wondering if this is a matter of swapping the O4 and O3 harness or changing the neutral safety switch or if there's some other problem I'm missing. I have searched all over this forum and the web and I can't find an exact reason why the O4 is not supposed to work in an 03. I drove it about a mile to get it to my house and I don't remember exactly what was going on, but I remember the PRNDL was not reading properly. I know it would not go into the third position even when shifted there manually.

I have changed the transmissions in these trucks before but always for the proper year transmission, I have no knowledge on the interchangeability of them, so I'm not sure what to check. She was telling me it Needed another transmission put in, but I'm not sure that's the case and I don't want to go through all of the hassle and charge her the money if I can fix what she has. Thank you all in advance for any help.

 

[justirv]

Hello all, I've gotten a lot of information from this site So I'm hoping you will be able to help me with the problem I'm having now. First let me say I'm new to the site even though I have been lurking around for a long time. Cars have been a hobby of mine for over 30 years. I do industrial maintenance professionally. I work on a lot of cars for people on the side, and one of the people I do work for has three tahoes and a Yukon Denali.

The problem I'm having is her daughter also has a 2003 Tahoe. She had someone(Her boyfriend) put a Transmission in for her and the one he got was from a 2004. Now they have brought me in on it because it will not shift properly. If you put it in drive, it shifts through first and second but when it gets to third it just free revs Like it goes to neutral. You can shift it manually through first and second but will not go into 3rd that way either. The transmission was apparently working fine from the donor vehicle and the fluid is proper fluid and brand new. Transmission came from a salvage yard with a warranty.

I have been told by multiple people that only the 03 is compatible with the 03 what's up making some modifications. Before I start trying to guess at this, I'm wondering if anyone on here would have an idea of what the problem is. I feel as though it is electrical, but I have no knowledge of the person who did the swaps skills. I'm wondering if this is a matter of swapping the O4 and O3 harness or changing the neutral safety switch or if there's some other problem I'm missing. I have searched all over this forum and the web and I can't find an exact reason why the O4 is not supposed to work in an 03. I drove it about a mile to get it to my house and I don't remember exactly what was going on, but I remember the PRNDL was not reading properly. I know it would not go into the third position even when shifted there manually.

I have changed the transmissions in these trucks before but always for the proper year transmission, I have no knowledge on the interchangeability of them, so I'm not sure what to check. She was telling me it Needed another transmission put in, but I'm not sure that's the case and I don't want to go through all of the hassle and charge her the money if I can fix what she has. Thank you all in advance for any help.

My first thought is that the 3/4 clutch pack is fried, as that is the typical symptom. Easiest way to confirm this is to disconnect the electrical connector at the trans. This should put it in "limp mode", which is 3rd gear. If it it moves in this mode, then it might be a solenoid or command signal issue. When this happened to me, I changed the fluid and filter, and was able to drive it around town for 2 years with (shift selector in "2") 1st and 2nd gear before I had the time for a full rebuild. If/when that time comes, I recommend a look at the Sonnax build sheets for some upgrades.

 

[Hempwizard]

I'm A little hesitant to think it is a mechanical problem at this point. I've done some checking into where the transmission came from and I know that it was working perfectly when the vehicle it was in was rear ended. It had 103,000 miles on it. Is there anything else electrical that would cause this issue? I just went out and started it, and while sitting still and not driving the shift indicator shows everything except park. When I'm shifting up to park it stops in reverse and then after a few seconds the indicator line goes OUT. I don't think it's ever lighting up over park, but it starts in park. I'm about to jack it up and get underneath it, just hoping for any suggestions on what to look for

 

[Hempwizard]

Plus there must be some reason why I keep being told 04 won't work in 03 without making some changes. But can't find what changes to make. When she told me she wanted me to put a new transmission in, I looked around for one and even the local salvage yards said it had to be in O3 to work right. Didn't know why it wouldn't work.

 

[justirv]

I'm A little hesitant to think it is a mechanical problem at this point. I've done some checking into where the transmission came from and I know that it was working perfectly when the vehicle it was in was rear ended. It had 103,000 miles on it. Is there anything else electrical that would cause this issue? I just went out and started it, and while sitting still and not driving the shift indicator shows everything except park. When I'm shifting up to park it stops in reverse and then after a few seconds the indicator line goes OUT. I don't think it's ever lighting up over park, but it starts in park. I'm about to jack it up and get underneath it, just hoping for any suggestions on what to look for

There are a few members here with expert transmission knowledge and advice; they should be chiming in shortly. By your response the control circuit does sound suspect. If you can get to that electrical connector, (or fuse possibly?), would confirm if your trans is mechanically working, taking control issues out of the equation.

 

[Hempwizard]

Okay that helps. From what you're saying and from what I have read, if I pull the fuse it should go into limp mode and operate in 3rd gear. This would show if 3rd gear is functional mechanically, am I correct in this assumption?

 

[Hempwizard]

I went out and pulled the Fuse that I believe goes to the transmission. From what I looked up it is the IGN 0 fuse when you open the door. With this Fuse pulled I was able to put it in drive and pull off but it never shifted at all. Not sure it was in 3rd, but it may have been. Just seemed to start off with more power than I would have expected from 3rd. Maybe Nick @NickTransmissions would have some idea why they say the O4 doesnt work right in the O3. Seems to really know his stuff about these things From what I have read around here it

 

[Hempwizard]

Just to make sure before I started taking anything apart, I just unplugged the main harness of the transmission then I'm still able to go forward in one gear. Seems a little strong to be third but I don't know for sure.