Question About Diff Swap

[BMW_Noob]

Ok i am not very mechanically inclined but i have read about people changing differentials and have read about people's sucesses. My dad says the speedometer is driven by the output shaft of the transmission and that changing the differential will make my speedo slightly off, is this true? If so how would one go about solving it or can you? I have a base model (stripped) 1995 525 manual. I believe it has a non-LSD 3.23 rear diff. I found a nice 3.73LSD on Ebay and want to put it in my car. I hope i made sense and any help would be greatly appreciated.

[e34.535i.sport]

Ok i am not very mechanically inclined but i have read about people changing differentials and have read about people's sucesses. My dad says the speedometer is driven by the output shaft of the transmission and that changing the differential will make my speedo slightly off, is this true? If so how would one go about solving it or can you? I have a base model (stripped) 1995 525 manual. I believe it has a non-LSD 3.23 rear diff. I found a nice 3.73LSD on Ebay and want to put it in my car. I hope i made sense and any help would be greatly appreciated.

Hey I'm in the process of checking this out myself... This is what I've got so far: http://www.bimmernut.com/forum/showthread.php?t=39345 .

I can't remember which thread I was reading but apparently the speed sensors for the different diffs are universal so there shouldn't be a problem with the speedo... :D

[Martin in Bellevue]

There is a tone ring in the differential specific to its gear ratio. The sensor is in the differential, not the transmission on e34's.

[BMW_Noob]

Hey I'm in the process of checking this out myself... This is what I've got so far: http://www.bimmernut.com/forum/showthread.php?t=39345 .

I can't remember which thread I was reading but apparently the speed sensors for the different diffs are universal so there shouldn't be a problem with the speedo... :D

My dad seems to think that the speed sensor would have to be changed because it cant accomodate a different rear end, but then he doesnt know german cars at all so who knows.

[BMW_Noob]

There is a tone ring in the differential specific to its gear ratio. The sensor is in the differential, not the transmission on e34's.

well thats good news! i was hoping it wouldnt be an issue because i cant afford much else at this point. thanks!

[e34.535i.sport]

My dad seems to think that the speed sensor would have to be changed because it cant accomodate a different rear end, but then he doesnt know german cars at all so who knows.

Here it is:
http://www.bimmernut.com/forum/showthread.php?t=9651&highlight=diff+speed+sensor

[BMW_Noob]

Here it is:
http://www.bimmernut.com/forum/showthread.php?t=9651&highlight=diff+speed+sensor
Thank you that was a lot of help. Now i am excited and will probably bid on that diff later today.

[BMW_Noob]

There is a tone ring in the differential specific to its gear ratio. The sensor is in the differential, not the transmission on e34's.

Another question though, if i get the new 3.73LSD, should i have the dealership install it? I will be starting back at Rasmussen BMW in portland oregon on the 16th and will save myself a lot in labor by working there. My dad is a good mechanic and could probably do it but i was reading about the preload and having someone do that who was a pro.

[bubba966]

Another question though, if i get the new 3.73LSD, should i have the dealership install it? I will be starting back at Rasmussen BMW in portland oregon on the 16th and will save myself a lot in labor by working there. My dad is a good mechanic and could probably do it but i was reading about the preload and having someone do that who was a pro.

There's no worry about the preload unless you have to change the input shaft on the diff. If the diff has the same input flange as the diff that's currently in the car then it's a plug and play type of swap.

So, you need to decide if it's worth your $ to have it done at the dealer by someone that knows BMW's, or if you should have your dad do it. That's a decision that none of us here can really comment on.

[bubba966]

Oh, and if that diff is out of an M5 or 540 you're going to need to change a good bit of stuff to make it work in your 525. It can all be swapped so that the larger M5/540 diff fits the other E34's, but it's not usually worth the hassle.

[e34.535i.sport]

Another question though, if i get the new 3.73LSD, should i have the dealership install it? I will be starting back at Rasmussen BMW in portland oregon on the 16th and will save myself a lot in labor by working there. My dad is a good mechanic and could probably do it but i was reading about the preload and having someone do that who was a pro.

I'll be doing mine myself... Depending on what diff you're swapping to you may not need to worry about preloading - i.e if you get the right diff you do not need to change the flange on the front that connects to the driveshaft and therefore do not need to preload it. Other than that it's nuts and bolts from what I've read and the research I've done. Have you seen this? http://www.bmwe34.net/Wiki/tiki-index.php?page=Differential+swap

Are you sure you want to go all the way from a 3.23 to a 3.73? Thats a ~15% increase in revs... I'm looking at an ~8% increase if I jump from a 3.46 to a 3.73 and I'm concerned it may even be too much... Considering a 3.64 too.