Bad relay? How do I know?

[tviano]

I think I have a bad relay for my electric windows in my 72 280 SE 4.5. However, it doesn't look burned or fried. ANy way to tell if it's still good w/o hunting for the part and replacing? Thanks!

[ctaylor738]

Get a circuit diagram or fool around with a test light to figure out how to bypass the relay and see if the windows work.

[stevebfl]

Most every simple relay used by MB has its wiring diagram printed on the case. It surely could be unreadable by now.

The relay will attach either terminal 87 or 87a to term 30. That is its job. Jumping these terminals will operate whatever the relay controls.

Terminals 86 and 85 control the relay. Convention says that 86 is powered and 85 is ground but they will work with polarity reversed except when a diode is in the circuit for spike protection.

The simplest relays only have 4 terminals. Most MB relays have 5 terminals. These relays are two types: "either/or" or "on/off". The either/or relay will switch the power from 87a to 87b (with one of them always hot). The on/off relay will turn both 87 and 87a on at the same time. Either the wiring diagram on the relay or the cars diagram must be consulted to see the intended activity. The 87 terminals can have a number of different labels: 87, 87a, 87b. The terminal "87" will always be some form of output but sometimes is attached to nothing.

So to test one should verify that term 30 is hot and that term 85 and 86 will light a test light (one is ground the other hot). With these conditions the output at 87 should be turned on.

[ctaylor738]

Steve -

As we say on this Forum - U da man! You are welcome here anytime!

Chuck

[tviano]

Is there no one else on the planet who can explain this? Steve, your advice on the relays functions is fantastic.

The wiring diagrams for the round relays are printed on the side and barely legible. THe square relay has the diagram imprinted on the top, so that was helpful.

The Mercedes dealer had no clue (no surprise) from looking at their info. My mechanic would have required $ome time with the vehicle to diagnose the problem. I took some time thinking I was jumping the relay, but I think I ended up jiggling the sockets enough to fix the problem. There is a loose connection in there that I fear and know will come back soon, and next time I bet it will be with the windows in the down postion. Thanks very much for your help.