Hi all,

- W123 blower motor intermittent
- brushes replaced, and i have a spare motor -> does not follow motor itself
- fuses replaced

- brand new head unit and climate control amplifier - all confirmed good

whats the next thing to check?

cheers,
dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Is the silver box actually new/rebuilt, or is it a used one? I've repaired *several* now that had intermittent or erratic blower issues and temp regulation caused by the silver box. The electrolytic capacitors leak and dissolve the copper traces on the circuit card.

Another possibility is the relay box for the fan. It's a black plastic box that contains a bunch of relays in it and should be in the same area as the silver box.

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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!)

Make sure the fuse holder is not damaged. My 240D melted the fuse holder so badly I had to swap the wires over to a spare fuse holder. I think it was #5. Not sure if the 300D uses that fuse for something else.

Another trick, put an ohm meter on the motor terminals (disconnected from the car) and slowly spin the motor shaft. You should read a couple ohms of resistance as you turn the shaft. If the reading spikes up while turning then you have bad spots on the commutator. This happened on my W210, the brushes went so low they ate into the commutator.

Good call on the capacitors. If you have a car from the 80s/early 90s it's time to find every electronic device in the car and renew the capacitors in it. So many electronic gremlins caused by bad capacitors.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

thank you! the amplifier is rebuilt from Programma. ill dive into the relay box. was wondering what the other box was!

jay_bob - good points. ill check on those. and yes old caps are a total pain!

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Hook a small light bulb or meter on the power feed to the blower motor. Then when it is intermittent see if the light or voltage is on or off.

You do not want to go digging until you have an indication of where the problem is. Could be the blower motor or the blower control circuit. You have to start somewhere.

You want intermittent. We have a newer car that on average the blower fails to run about once a year. We have owned that one for about six years now. The warranty was over before the first time. So it has done this about four times. Of course when it occurs you do not have time to check it out. By the time you do it is back on again.