Thread: What is the purpose of the ECM relay?

To me it reads white powers the coil, which is fed to a ground through the ecu (white/green), which in turn allows power from white to flow through to orange/blue which feeds the aforementioned sensors. I'm looking at a '93 diagram though, and what D-unit says makes much more logical sense.

The ECM relay DOES get power straight (fused) from the battery without going through any switched power source. The reason why it's not on all the time must be because the ECU must complete the ground for the ECM relay coil (white green) at some point, whether it be accessory power, on power, or start signal. This would agree with what I'm seeing in the FSM for the 93 models, as well as agree with the simple logic D unit is talking about. I guess a simple check would be to break out the multimeter and probe orange/blue for when it sees 12v+ - whether it be on accessory, on, or start. I was assuming white/green was a straight ground but it must be linked conditionally to ground, otherwise the ECM relay would be on all the time.

The '91 FSM pictures the ECM relay with 4 connections, two of which are to the ECM. Those two are pictured within the relay as the relay coil which is denoted by the "squiggly" lines. White is the power to the relay, but it does not flow through it until the ECM sends power through the coil and "pulls" the relay in.

This is only theory in my head at this point and I wish someone else would chime in to confirm or disprove.

Is it a safety feature for corrupting power? The relay will be destroyed instead of the ECM? Just a guess

White is power to both the relay and the ECM, but I don't think the white to the ECM powers everything, as orange/blue flows power as well to different pins when the relay is activated. If white/green was a power source then you'd be grounding through white, which is impossible due to the fact that white is a power source.

White is the power feed to the coil, which must be grounded through white/green (pin 4) otherwise it would not activate. orange/blue is the recipient of the power from white via the switch. The question I have now is how is the coil grounded through white/green... is it an internal relay in the ECU? Is it sinking the load from the coil through a chip's pin? The ECU must be doing something with another signal or power source and then using that as the signal to ground white/green, otherwise if white/green was a normal ground then the relay and ECU would be on all the time.

I wanna see a 91 FSM, there could be two totally different wiring designs going on here with regards to the ECM relay

WTF? That relay wiring makes no sense. Why would the coil be hot and pulling in the contact constantly? Must be a goof because that ain't how any relay works. I'll see if I can scan the '91 diagram here on the home scanner, if not it'll have to wait until Monday. I also have a '92 FSM I can reference...

Like I said the only way it could work is if the ECU pulls the white/green voltage to ground someway, either through sinking through one of the chip's pins or through an internal relay. White/green can't be a direct connection to ground or like you say the coil would be hot constantly. It's gotta be regulated by the ECU somehow. I made the same mistake thinking white/green is a direct ground, this can't be - it has to be controlled by the ECU someway so that it's dependent on some sort of logic. I.E. a starting signal, rpm signal etc

Okay, after looking at the '91 and '92 diagrams turns out they're the same wiring, albeit the diagrams are drawn a bit differently. Power is at one side of the coil with the white wire from the battery then the white/green side makes ground in the ECU completing the circuit and firing the coil in the relay. It's the only way that could work like a proper relay.

I also followed #3 of the ignition switch to ECU pin #36, which is the "ignition on signal". I'm guessing this is what tells pin #4 to make ground/fire he relay. This #3 of the ignition switch also powers up the ignition coil and the resistor/condenser.

My head hurts from looking at all these diagrams.

Back from the dead..
I have a 94 se-r the issue is with the ecm relay plugged in the car starts but seems to be sputtering and runs lean then when I shut the car off the relay starts Buzzing. ..When I unplug the relay the car starts right up with no issue...I tried changing the relay check all the wiring for feed back and switched ecu's..Stumped..