Factory BMW Automotive Relay

I'm just a tinkerer and not a EE. Please help me make sense of this;

Below is a factory BMW automotive relay. It has four posts and the diagram shows that post 30 is both the input voltage and the switching voltage. In other words, when post 30 is provided with current, the relay seemingly activates and provides power through two separate post 87's.

Why would a relay be used in this manner? It seems that the power to post 30 must be switched power, otherwise the relay would be activated all the time. And if the wire feeding post 30 is robust enough to provide power for the circuit(s), then why have the relay?

In other words, wouldn't wiring the consumer(s) directly to the switched wire that is supplying post 30 have the same end result?

TIA.

85 could be connected to a low current switched ground with 30 being connected to a constant 12V+.

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the12volt • Support the12volt.com

Ah. Makes sense.

I never even considered that possibility.

Thank you.

I'd like to tack on to this post, since the title is relevant:

Below is another factory BMW relay. There are several of these installed from the factory on various high-draw circuits (eg; blower motor, rear defogger).

These relays have two 87 posts. I've verified that both 87 posts supply current when the relay is activated and neither supply current when the relay is at rest.

Combing through the factory wiring, as best as I can tell, in the factory application all of these are wired into sockets with the center 87 post unutilized.

I am (very slowly) swapping engines and have to create a new wiring harness. For the sake of packaging and tidiness, I would like to wire both 87 posts to provide power to two separate fused feeds. I have not been able to find any load ratings for these factory relays and can only guess they are rated for 30 amps.

I am wiring the single relay to two 15 amp fused circuits (one 15 amp circuit on each of the two 87 posts).

I find it odd that BMW would use relays with two 87 posts in multiple applications without ever utilizing both posts. But I would like to take advantage of this feature, unless there is something I'm not considering?

I wouldn't assume the current rating is 30 amps. Is there a part number on one of the other sides of the relay? If so, google it and/or contact a BMW service center.

The two 87 terminals always have continuity regardless of the coil being energized or not. The second 87 terminal on these types of SPST relays is for wiring convenience and potentially could be used as you described.

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the12volt • Support the12volt.com

You are correct. The 87 posts have continuity all the time. I hadn't really thought about that until you mentioned it. I don't think that should be an issue though?

Yes, I realize it's not smart to make assumptions. Unfortunately, I've researched the heck out of the part number and as far as I've been able to determine, BMW simply doesn't provide any kind of specifications for the relays.

The "same" relay has seemingly been manufactured for BMW by both Tyco and Siemens over the years. I have unsuccessfully tired to find a specifications using the Siemens/Tyco part number.

The Siemens part number on the relay is V23134-K59-X312. From what I've seen, that number is the same on the Tyco relay.   The BMW part number on the relay is "6136 8 373 700."

I have the BMW ETK (Electrical Troubleshooting Manual) but I have not been able to identify any specifics regarding the relay.

EDIT: I just realized I posted back in June, trying to find a way to determine the amp rating of these relays . I haven't worked on the project since last summer, so I'd forgotten I posted. The info in that thread also makes an educated assumption that these relays are 30A, but nothing definitive.