This car is flaring 2-3 and 3-4. Some of the time. Not always. Does this damage the transmission? What is the most common cause of this? I know there are alot of possibilities. But, what is the most likely?

Just like slipping a clutch too long, flaring during gear changes will wear out the friction materials in the transmission faster than if it were shifting properly.

Start with fluid and filter change if it hasn't been done in a long time. Consider changing the K1 and K2 accumulator springs or installing a Superior shift kit while you have the pan off. Write-ups are all over this forum and the Internet in general. Parts are cheap and you'll already have the pan off.

Modulator adjustment is the next most common issue. If it's adjusted too soft you'll get flaring. When adjusted properly a 722.3 transmission should be shifting FIRM. Not jolting, but firm.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Thanks. I'll do that/ Though, I might just take it to a tranny shop to do that. My time is limited these days. BTW, I don't mind a firm shift as long as it doesn't damage the transmission.

I used to have a BIG flare on 2-3 I changed the K1. It fixed it

A firm shift does far less damage to the friction linings than a soft/slow shift does. The transmission is designed for a firm shift. Too firm and you can shock the flex disk, U-joint, and differential, so there is a balance between "too soft" and "too hard". You're looking for a reassuring "bump" when the transmission shifts, not "someone just rear-ended me".

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Beautiful 300CD!

__________________
1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

I stopped in to the local transmission shop and he opened the hood and tightened the cable on top of the valve cover and it shifts very firmly now from 1-2 and 2-3.

3-4 is also more a little firm but maybe a little soft/flaring. I'll likely leave it is it s and take it to him to flush and change filter etc. since I'm unfamiliar with automatic transmissions.

Wow that is a gorgeous W123 coupe!!!

__________________
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'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)