Frequently on the forum a question is ask and a single answer is expected. However, anwhere along the electricle circuit can case the problem. Other time our members can give components that often cause problems.
In your case you fixed that as it is usually the Motor itself that causes the probem.


That leaves 2 ways to aproch the problem from. You trace the circuit back from the Motor Conector to the next connection and see if you get voltage there if not continue tracing back.

Or you could start at the source of the Electricity and trace the circuit down towards the motor and see where you loose the voltage.

For either it is helpful to have a pic of the circuit. Perhaps someone can post a site with the Manual for you to look at to where the circuit goes.

Another way to go but less reliable is to pull the connector at the resistor and see if you get voltage there and then pull the connector at the switch and see if the Switch is getting voltage.

Or if you don't mind the Blower going at one speed you could bypass the whole mess and add a switch that turns the Blower on. One that only gets voltage when the Ignition is on and the engine is running.

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84 300D, 82 Volvo 244Gl Diesel

You make a fair point. The electrical system is an entirely new realm for me -even bought my first multimeter today. A year ago, I bought my first socket set and with generous help from the tremendous community here, have become a semi-almost-competent back yard mechanic

I've done a little goose chasing and it turns out the number 8 fuse is actually NOT getting power. Mistook the 6 for the 8 earlier. This seems more manageable, though is still something I've never before encountered...

That is actually a good clue. The key switch has a dedicated contact just for the blower motor. It is terminal 15X.

Here is the link to the factory service manual for your car (provided by MB). You will need to use a Windows PC and go to startekinfo.com (won't work from an iOS device for sure and may not work on a Mac).
On the left is a link to MB Workshop Resources, select that, follow down the table and you will find Model 123 Service Library. This has the complete manual including the wiring diagram for your car.

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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

Thanks for that. I have this manual bookmarked, but to be honest I have zero experience reading these diagrams. They seem to begin with the fuse at the top and work their way down across all the components. What I need seems to be the path of terminal 15x to the fuse box...

If you don't have experience you need to take your time and believe me we all have been there. What you learn correctly will help you in the future so it is worth the investment of time.

Yes electrical trouble shooting is a pain. Just think of the wires as if they were plumbing pipes with the current flow coming from the Battery.

One of the things that the wiring diagram shows is the color of the wires. The first stated color is the main color of the wire and if there is one the 2nd color stated is the color of the stipe on the wire.

Unlike on US vehicles the Brown Wires are ground wires.

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84 300D, 82 Volvo 244Gl Diesel

Sage advice.
When the key is turned, does anyone know if the #8 fuse is always hot?
I'm assuming the power flows from the battery to the fuse and downward from there. Unless of course the current runs from the battery to a "switch" that is toggled by the ignition, at which point the 8 fuse becomes hot.

General rule of thumb on MB
Brown = ground (circuit 31)
Brown base color with another color stripe is a switched grounded circuit (like the dome light switches in the door frames)
Red = unswitched battery (circuit 30)
Black = switched battery (circuit 15)
Red or black base color with another color stripe used for identification of various circuits

The following colors are commonly found on these circuits
Purple = engine start (circuit 50)
Blue = alternator D+ (circuit 61)
Yellow = low beam headlight (circuit 56b)
White = high beam (circuit 56a)
Gray = dash/running lights (circuit 58)

__________________
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

Yes the power flow is battery + terminal -> terminal 30 on key -> key contact only made in position II (running) -> terminal 15x on key -> fuse 8 -> blower switch -> blower resistor -> motor -> ground -> battery - terminal

Notice there is no fuse between the battery and the back of the key switch. Be very careful you don't cause a short probing around on the back of the key switch with your meter probes.

If you are not seeing power on fuse 8 try wiggling the key switch. Sometimes the key switch contacts can go bad especially on that 15x contact, since the blower motor is one of the largest electrical loads going through the switch on a 240D.

Another good tip especially on these older cars is to disable the glow plug relay if you are doing electrical troubleshooting that involves the key in II position but the engine not running. To do this lift off the top of the glow plug relay (on the inner left side fender) and unplug either connector. This disables your glow plugs and keeps from burning them out. Of course you will need to plug that back in when you want to start the engine.

The electrical part of the switch is separately replaceable from the key lock part.

__________________
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 so much. I'm done for today, mainly because my forehead hurts from hitting against the wall, but I'll have a look at that key switch tomorrow morning. I have a hunch that's where I'm running awry. Hopefully it's easy enough to access/clean and or swap out. Will report back tomorrow.

It's looking more and more likely the ignition switch -potentially the tumbler as well- are culprits in need of replacing. I've had intermittent issues with the key refusing to turn, the key sticking in the ignition/III position which I've had to turn back myself, and now this.

I'd prefer not to change the tumbler for obvious reasons, but does this sound like a comprehensive issue? One in which the entire assembly is due for replacement. I would hate to be stranded when it eventually went.

The Loch Tumbler, Steering Colum Loch and the Ingintion Switch at the Bottom are all related.
One thing for sure is remove the Lock Tumbler be fore it gets stuck.

Also the lock tumbler issue can be deceptive. I had the key stuck in position issue but after severdays of fooling with it managed to get it turned to the position where I could remove the Tumbler.
What I found was with the Tubler out it worked fine. So I investigated further and found that my Steering Colum Lock was busted inside.

when you replace the Steering Colum lock you can buy it as an assembly with a new Ignition Switch and the Vacuum Switch that is on the Steering Colum lock.

When I took off the old Ignition Switch I found that one of the contacts on the switch was almost burnded away.

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84 300D, 82 Volvo 244Gl Diesel

If the steering wheel lock is starting to fail and you can't get to it right away,
turn the steering wheel 90 degrees before removing the key to prevent the steering wheel lock from engaging.

You can also remove the steering wheel lock and drill out the locking button and reinstall it, if you don't want to replace it with a new or used unit.

It's frustrating when they fail in the locked position on a public street. BTDT.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Are these things much trouble to replace? Thinking I'll hop on eBay and replace everything at once.

Removal and replacement of the steering lock FSM job 46-640 is pretty straightforward (as long as the key still turns).

The releasing wire is easily made from an old bicycle spoke.
I borrowed my independent mechanic's releasing wire when I did this job

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Before I buy the new parts. I'd like to confirm this is a reasonable course of action...
The fan works as tested on a 12v battery, there is no power where it plugs in, there is no power at the number 8 fuse.

The next place to check is at the back of the ignition switch, the cable actually coming from the engine bay. In order to do this (from what I understand) I need to disconnect it from the ignition, place my positive probe in the 15x socket and the negative on the ground. I should be getting 12v. Provided I am, the issue would be in the switch itself.

Am I right?