Insane transmission theory

BenF · Post by **BenF** » 02 Feb 2021, 05:55

Quick background, I have a 2001 V70XC. It had the well documented transmission issue which was vastly improved by putting in a reconditioned valve body about a year ago ( actually can't really remember). I never got it it properly reset and re adapted with the VIDA as no one really wanted to help (local dealer can go suck it). Anyway it still has quite a harsh downshift especially slowing down at low speeds I guess 2nd to 1st maybe 3rd to 2nd.

So the last couple of days it seems to have been a lot better, I didn't find myself wincing so much on the school run. On Sunday I did some light maintenance, including checking the tires and adding about 15 psi to the passenger rear from about 20 to 36ish so now both rears are the same. Suddenly I seem to have smooth shifting.

Am I nuts ? Could this possibly be a thing ?

Post by **abscate** » 02 Feb 2021, 05:59

Bala ncing the wheel diameters?

Easy enough to run the experiment again!

Harvard would calculate the change in tire size from the theoretical tension of rubber and write a paper in the Journal of Engineering Report Knotes

MIT would let the air out of the tires and report the experiment.

Columbia would have stolen both of your tires during the experiment

Cornell would still be saying they really are an Ivy League Svhool, and Princeton is the fake

Post by **erikv11** » 02 Feb 2021, 09:41

Berkeley would have reasoned the rears are probably not affecting the transmission. Haldex engagement and function maybe, trans no. Then welcomed more data but suggested to let sleeping dogs lie.

Post by **abscate** » 02 Feb 2021, 18:12

But we would never get the paper written by Berkeley because they would be out at a “water bottles are racist “ rally *

* Google Tracy Ullman woke intervention for a laugh .....warning. NSFW

Post by **firstv70volvo** » 05 Feb 2021, 13:20

erikv11 wrote: ↑02 Feb 2021, 09:41 Berkeley would have reasoned the rears are probably not affecting the transmission. Haldex engagement and function maybe, trans no. Then welcomed more data but suggested to let sleeping dogs lie.

Stanford would point out any “reasoning” from Berkeley is flawed from the start and then ask why not buy a new car to solve this problem.

Back to the problem, I’m wondering if a rear wheel speed sensor is used for vehicle speed input and part of the transmission shifting routine. It’s a long shot because I think the transmission output shaft sensor could do the same thing but, if pumping up the low rear tire actually did fix the problem the rear wheel speed sensor is all that I can come up with that may affect trans shifting.

I'm for the experiment of lowering the psi again and see if the problems comes back.

Post by **BlackBart** » 05 Feb 2021, 13:42

Mayyybe it's not the transmission that's insane.....

U of Montana would have looked into that...Montana State would have asked, Why don't you just drive a pickup?

Post by **abscate** » 05 Feb 2021, 15:18

BrynMawr and Smith kiss passionately......

Post by **SuperHerman** » 05 Feb 2021, 20:11

Schools aside as they would never teach anything that will answer the OP's original question.

I don't think in a 2001 your theory holds. The AWD is fully mechanical.

Slamming is a pressure issue - what did you use for rebuilt solenoids and valve body? Original software "fuzzy logic" would have adjusted pressure which a reset/relearn would correctly reset. Ultimately the computer will learn what is required. So it should be getting better naturally.

My issue is a sticking solenoid, which exhibits a slam as it takes excess pressure to release - hence the slam when it does, would have the opposite effect once a properly functioning one was installed.

Oro · Post by **Oro** » 07 Feb 2021, 01:06

BenF wrote: ↑02 Feb 2021, 05:55 Anyway it still has quite a harsh downshift especially slowing down at low speeds I guess 2nd to 1st maybe 3rd to 2nd.

Have you checked your lower torque strut and the upper engine support? Once I replaced these, my harsh downshift went away. Also, is your TCV working and your turbo making good power/torque? Mine was not, and that was messing with my transmission shifts, too (TCM sending controls for different power expectations vs. what was being delivered).

Here is a cheap and more permanent fix for the lower mount:

https://www.volvoxc.com/forums/showthre ... ture/page3

Here is the same type of fix for the upper:

https://www.rmeuropean.com/Products/306 ... FG523.aspx

This is the TCV valve I use and what a difference a good working one made:

https://www.rmeuropean.com/Products/996 ... FG244.aspx

BenF · Post by **BenF** » 07 Feb 2021, 07:35

I used a rebuilt valve body from the internet. From memory, and I actually did talk with the guy because I initially got the wrong code number type and he swapped it quickly and efficiently he alluded to having a tech guy who does the rebuilds. I think it was somewhat a legit rebuild rather than a used one from a donor car. It was around $500-$600 not a $150 "rebuilt" one. Note I didn't do a reset/relearn, neither the dealer who basically won't touch anything as old as my car or the local independent Volvo guy want to help me with it.

Anyways, old lisbef was clunking around horribly the other day. So I have gone off the tire pressure theory. I think that was a coincidence caused by the fact that we have an unusually cold winter and I pumped the tire during a cold couple of days, by that I mean 30-50 degrees f outside, we don't have real cold here in GA. It also depends on the type of driving. If I have to do a lot of stop start low speed town driving in less than cold temps I think she's getting quite close to the junk yard. Then I have a cooler longer distance trip and she seems to do OK.

So time for the next silly question given that the shifting is better when cold. Has anyone ever diverged from recommendations and used a thicker transmission fluid ? Or is that a terrible idea.

I'm really close to looking for a new car, but if there is a cheap shot even if it's risky that gets me another few thousand miles I'm willing to give it a try.