Differential swap
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
I ended up getting a used (45K miles) 3.15 diff and having the mechanic inspect it and install new seals before putting it on the car. Speedometer works correctly, and the RPM's increased by about 300 at 70mph. Noticeable difference in off the line grunt!
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
No I haven't. It was idling really well. Yesterday I drove about 20 miles on the freeway with no issues, then got off and stopped at a light. When the light turned green, I gave it gas and it shifted really hard from 1st to 2nd. It had not done that before. Did the same thing at the next stop light. 2/3 shift and 3/4 shift are fine, and the hard 1/2 shift only happens after the car has been driven at freeway speed. i.e. cruise set at 70. Today the check engine light came on, and gave me the P1870 H C code. From what I have read that has to do with the transmission slipping. I'm wondering - some of you have said the car will need to be tuned after the 3.15 diff was installed. I'm thinking it should go back to the shop that just did all the work and see if there is some quick programming adjustment that needs to be done. Remember - went from the stock 2.73 diff to a 3.15. Thoughts?
Last edited by Gunnmetalblue; Dec 16, 2018 at 10:27 PM.
Safety Car
Joined: Jul 2010
Posts: 4,018
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From: Chandler AZ
I am concerned you might cause transmission damage driving it like that!
Hope I'm wrong, but that would really suck if your transmission got messed up.
Also from what I understand the car will even drive better after the tune, seems I read you have some other mods done to the engine, what exactly?
Hope I'm wrong, but that would really suck if your transmission got messed up.
Also from what I understand the car will even drive better after the tune, seems I read you have some other mods done to the engine, what exactly?
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
I am concerned you might cause transmission damage driving it like that!
Hope I'm wrong, but that would really suck if your transmission got messed up.
Also from what I understand the car will even drive better after the tune, seems I read you have some other mods done to the engine, what exactly?
Hope I'm wrong, but that would really suck if your transmission got messed up.
Also from what I understand the car will even drive better after the tune, seems I read you have some other mods done to the engine, what exactly?
Last edited by Gunnmetalblue; Dec 17, 2018 at 02:49 PM. Reason: more info
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
If the PCM is not reprogrammed for the 3.15 gear instead of the 2.73, would it cause the trans to have a hard 1/2 shift after the fluid warms up? The shop is telling me it is likely the valve body needs to be replaced, but it seems awfully coincidental that this issue would come up right after the diff swap. The P1870 code is for 'transmission component slipping', and researching that code gives credence to what the mechanic is telling me, but it seems that the first thing to do would be to ensure the PCM knows about the gear change, take it for a good drive to warm up the fluid, and see if the hard 1/2 shift still happens. Thoughts / advice needed!
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Joined: Apr 2013
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From: Oregon
St. Jude Donor '14-'15-'16-'17-'18-'19-'20-'21-'22-'23
1) You need to re-tune for the gear swap
2) Make sure the trans is full after the diff. change.. There is a specific procedure to refill an A4 trans
2) Make sure the trans is full after the diff. change.. There is a specific procedure to refill an A4 trans
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
get it tuned, you will damage your trans if you keep driving it, TCC slip table and the WOT shift MPH value need adjustment, your shift points will be off
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
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From: Poway CA
http://www.tim-yvonne.com/mikemercury/342.htm
Advanced
Joined: Jul 2018
Posts: 85
Likes: 22
From: Michigan
if the tune doesn't fix it...the TCC bore/valve are very prone to wear in the 4L60E, Sonnex has kits to repair it, I'm currently doing this has a prevent maintenance measure, along with replacing the plastic accumulator pistons
Thread Starter
Instructor
Joined: Dec 2014
Posts: 241
Likes: 17
From: Poway CA
Car is home, finally. Thanks to all of you for emphasizing the re-tune after the gear change. It absolutely solved the issue, and saved me some money since the valve body did not need replacing (Also thanks to the forum for making me aware of that potential issue, and I will have that addressed at the next trans service). The car has so much more 'jump' off the line with the taller gears. Thanks again forum; you guys rock! 
