There is a components location page

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

yes! the fuse has 5 pins which are marked: 85, 30, 87, 87a, 86 - and it goes to ground g102 which is behind the cluster. so that's the answer. the wires are even nicely color coded on the diagram. thanks!

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1980 300D, 128K

it's a little blurry - it looks like #4 corresponds to 86 and #5 to 85. Am I reading that correctly?

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1980 300D, 128K

Blury here also. Relay coil is not polarity sensitive so mixing them up is okay. Relay gets unfused power (corrected my post above). That is probably what started the fire.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Some relays have a flyback diode across the coil making them polarity sensitive. The diode absorbs the inductive kick when power is removed and the magnetic field collapses. Think what happens with a gasoline engine ignition system.

If a relay that has a diode is hooked up backwards the diode will pop like a fuse but you will never know it happened since the diode is very small.

I didn't read this thread all the way through. it would be a real good idea to find the main power feed from the battery to the " car" ( not the starter cable ) and add some large fuses / fusable link. Finding out where the main power feed branches out would allow for a set of smaller fuses to be installed further protecting the circuits.

I added a bunch of fusable links / push breakers to my mid 70's backhoe to prevent a fire. The original system had zero in terms of fuses.

Agreed I have studied the diagrams for 123, 124, and 210. The amount of unprotected wiring is just mind boggling to me.

I design electrical equipment for a living. I could never get away with what they did on these cars in my designs.

No wonder the first thing the fire department does for anything worse than a minor dent in a wreck is pop the hood and snip the battery cable.

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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

That is true on most electronic devices but I do not think it is true on relays in W123 Mercedes.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

If it turns out to be worse than you thought, I have several parts cars with wire harnesses in them, should you need all or part.....Rich

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All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html

But it does have an AM radio that can pickup a pop when the relay cycles.

I'm not saying for sure the original relays have a diode but replacements might so it would be good practice to adhere to wiring standards.

DIN 72552

https://en.wikipedia.org/wiki/DIN_72552

Relay
85	relay coil -
86	relay coil +

To expand on this for others. A fuse should be on the non-grounded side of the battery ( older Brit and some American cars had positive ground so the fusing would be on the negative side ) and nearest to the power source ( usually battery ) as possible.

The fuse only needs to be as large as the largest load + some small margin. The elevated current associated with electric motor starting loads needs to be taken into account so the fuse will need to be upsized a bit. ( I've never seen a time delay fuse in an automotive application. )

Fusing gets complicated as as it is based on a current / time curve for how quickly the fuse / breaker will clear. For automotive use there are very limited options and luckily they seem to work out OK.

Ideally you would bring every bus to a common point nearest the battery and place fuses there. Pretty much start at the battery, ignore the starter wire, follow the wire that is left and see where it starts to fan out.

There would be a single wire / fuse leading from the battery to the ignition switch, another wire / fuse leading to the IG SW for accessories , another leading to the existing fuse block and so on.

You will still need smaller fuses for each branch circuit as a 50 amp fuse won't clear if you have a thin shorted wire that is drawing 20 amps.

If I were to put a fuse on this particular circuit, I should put it on the red wire right? How many amps?

okay, so i hooked up the new relay

30-red
86-black
87-red and blue
85-ground

car starts, no fires, but the windows still don't work!

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1980 300D, 128K

You have the wiring diagrams. All you need now is a test light to trouble shoot.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Less of it than you think. Several of the relays have internal fuses, which do not appear on the wiring diagrams. For example, the OVP, fan relays and I believe the seat relay are internally fused. For several of these, when the fuse blows, the only option is to replace the relay. I think Mercedes made a bad decision with this, as it causes endless confusion.

But that only protects if the fault is down stream of the relay.