For no apparent reason on day my parking lights decided to stay on. Both sides, front and rear. Only disconnecting battery gets them out. I am thinking switch but have heard these are not so prone to failure. Any tips? Thanks

Look for water ingress and dirty earth connections first.


Favourite places in a W123 =>


Windscreen leaks and bonnet / hood spring pocket areas - earthing / ground points near to steering wheel under the dash


Towards the back in the luggage compartment more earths to check


Check the bulb units themselves too


#######


Dry out anything that is wet


Look out for corrosion - if you find rust you are best off replacing parts rather than trying to clean them


Once clean and dry treat the outside of the earth connections to a bit of white grease

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

That's about the only plausible cause.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Unless there are rodents chewing on insulation causing a short somewhere.

__________________
Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic

That's possible. But the left/right sides are separate circuits. So that would require an improbable coincidence of wire chewing combinations.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

OK so light switch or crafty squirrels. I will order switch. Thanks.

I'd make a continuity check before blindly ordering parts


Check first and then if it is broken buy a replacement

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I think the OP can pretty confidently assume the switch is bad. There's no relays or anything else in the circuit to fail. If BOTH side parking lights are on, the only common source is the switch itself. Separate fuses, separate circuits, positive side switching, the only common point being that light switch.

__________________
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!)

What about a bad hazard switch? That could light up all four corners no?

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

That would light up the bright turn-signal filaments, not the parking lights.

__________________
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!)

Technically, that is correct. But how likely is it that the casual observer would make that distinction if the turn signals (hazard lights) were on but not flashing?

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Pretty easy to tell if the rears are red or amber. Red = parking/running lights, amber = hazard/turn signal.

__________________
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!)

Red is on, not amber in rear.
Thanks for all the input!

That was my thought

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

New light switch cured the situation. Thanks to everyone.