W123 PW Relay issue
 

Hello, I have a 1985 300CD with a power window issue. I've traced it back to the relay behind the instrument cluster so far.
As I understand this circuit from previous posts, and the electrical diagram in a Haynes manual, the electricity should flow to the relay from 2 sources. 30 amps enters the #30 prong and when the relay closes this 30 amps exits out the #87 prong heading thru fuses A and B in the fuse panel then to the PW switches on the center console. But in order for this to happen the relay must be activated by recieving electricity at the #86 prong. Thats my problem. I'm not getting juice at the #86 prong, and from the diagram I'm using it should be coming from fuse 4 in the main fuse panel. Well I have 12v on both sides of fuse 4, but only .3v at prong #86 at the PW relay. I did loosen up the fuse panel and verified the wiring behind fuse 4 is tight. My diagram shows no other gadgets between these two points...

ANY advise???

Thanks,
Ken

Check the Fuse chart in the Lid of your Fuse Box.

On my year and model I believe it is Fuese #12 that the Relay and some other items share.

I have a diagram for an 83 240D, YMMV.

Fuse 12 is for the coil circuit pin 4 (86) of the window relay, black and violet wire. Looks to be a straight shot between fuse block and relay. This fuse is live only in run or start.

Try jumping pins 1 to 3 (30 to 87) on the relay socket, see if you get window operation.

But if fuse 12 is no good you have lost quite a bit else in the car. Brake lights, turn signals, cluster gauges and brake warning lights share this fuse.

Double check you have a good ground on pin 5 (85) of the relay socket.

It's got to be something stupid simple. No switches in that coil circuit and it only spans about two feet in total of wire behind the dash.

Try with the Ign 'Off' and front driver's door Open.

Do the windows work then?

There's two relays behind inst cluster, (At least there's two on mine) one supplies the power for the coil on the 'window' relay--When the front door is open, and not when closed. Ign supplies when door is closed, so wont work with door closed Ign 'Off'.
One of those little 'oddities' that M.B. did to enable window operation without Ign on......

--This very fault had me going round in circles for quite a while. In my case, one of those relays was faulty.....

I replaced them both in the end, Tried switching them round but fault remained.

Problem solved! And I have all my taillights, and instruments back...The Haynes diagram shows fuse 4, not 12, supply electricity to the relay, but if I had paid closer attention to the print out attached to the fuse panel door I would have known different. As described before the fuse "looked" fine, but one end was disconnected. Looks like I've earned the stupid simple oversight of the day!

Thanks guys, really appreciate it,
Ken

Be sure to order the high quality copper/ceramic bodied fuses from our good friend Phil. Just go ahead and re-fuse the whole box, it's one of the cheapest things you can do to prevent electrical problems.

Also check your contact springs, but do not use anything abrasive on them, that destroys the plating. Use brass polish or tarn-x on a q-tip.

Twicky Fuses.

Yup--Seen a few like that over the years!

Best replace ALL the fuses with the Copper type or the Brass ones.

The white 8A ones tend to be Brass, and the Red 16A ones Copper.
--These will be fine. Best preventative repairs you can do for the elect. system!

Avoid like the plague the silver (Aluminium) fuses--They set up some kind of electrolytic reaction--especially when damp-- between the ally strip and the copper spring contact of the fuse-board.

They then fail exactly like that one pictured!