Main Power/Voltage Supply Relay 109 Function

I'm looking to further simplify the wiring harness for my Tacoma TDI swap and looking at the wiring diagram for the main power relay (#109) I cannot for the life of me figure it out. The relay appears to be always held closed/on because the red wire feeding it is hot at all times, and fused. Should the fuse blow, the relay would open and remove power from various circuits including the ECU. Because it's fused, would this not make the relay completely redundant? What am I missing? Can it be bypassed? Thanks all!

Did you look here?
http://forums.tdiclub.com/showthread.php?t=479730&highlight=109+relay

You could probably wire around it but you would need an ignition switch with pretty substantial contacts in it.

The diagrams I have seen show relay 109 as being connected to the battery fuse box, which is hot at a times and has nothing to do with the ignition switch. This is what has me confused.

A relay can have four or five terminals. They will be labelled 85, 86, 30 87 and 87A.

Terminals 85 and 86 are the coil, which when activated creates a magnetic field and pulls the contacts from the at rest position to the active position. What this means is the relay is an electrically operated switch.

Since the coil is a low power circuit, the relay can be used to switch high power with a low power control.

Terminal 30 is common to both terminals 87 and 87A. In the at rest position, 30 is connected to 87A. This is know as a normally closed contact. When the coil activates the relay, terminal 30 is no longer connected to 87A, but is now connected to 87. Terminal 87 is known as a normally open contact.

In the case of the relay 109, terminal 85 is internally connected to 30 which is connected to the battery, as you have noted, and 87A is not used. Terminal 86 connects to the ECU which grounds it when the ignition switch supplies power to part of the ECU. With battery power on 85 and ground on 86, the coil causes the contacts to move to the active position, and apply power to 87 to operate the engine controls.

Does your head hurt yet, or did this help?

Great post. Why would it give someone a headache? Clear concise and factual me thinks.

What gives me headaches are posts that have way oversized fuzzy pics and/or super long posts with no paragraph breaks, no capitalization and the only punctuation is a period at the end.

Alchemist said:

A relay can have four or five terminals. They will be labelled 85, 86, 30 87 and 87A.

Terminals 85 and 86 are the coil, which when activated creates a magnetic field and pulls the contacts from the at rest position to the active position. What this means is the relay is an electrically operated switch.

Since the coil is a low power circuit, the relay can be used to switch high power with a low power control.

Terminal 30 is common to both terminals 87 and 87A. In the at rest position, 30 is connected to 87A. This is know as a normally closed contact. When the coil activates the relay, terminal 30 is no longer connected to 87A, but is now connected to 87. Terminal 87 is known as a normally open contact.

In the case of the relay 109, terminal 85 is internally connected to 30 which is connected to the battery, as you have noted, and 87A is not used. Terminal 86 connects to the ECU which grounds it when the ignition switch supplies power to part of the ECU. With battery power on 85 and ground on 86, the coil causes the contacts to move to the active position, and apply power to 87 to operate the engine controls.

Does your head hurt yet, or did this help?

Click to expand...

This. This was the missing link. While I have a full understanding of how relays work (helps being the wiring guy amongst your car friend circle...), it was the specific function of this system that I did not understand. Thanks for clearing this up!

Ol'Rattler said:

Great post. Why would it give someone a headache? Clear concise and factual me thinks.

What gives me headaches are posts that have way oversized fuzzy pics and/or super long posts with no paragraph breaks, no capitalization and the only punctuation is a period at the end.

Click to expand...

+1

Did you look at my post in the Link Ol'Rattler provided?

Using that info, draw out those circuits on a piece of paper for a visual... it will really clear-up things.

When I did my conversion project, it become immediately clear to me that I would need the wiring schematics. It also become clear to me that I would always need them for a reference in the event of a problem (who's going to work on it for me was the big question........ answer, nobody). So, that in of itself is a very strong incentive to have reference schematics, notes and a working knowledge of the system (totally self-learned here). I spent hours and hours studying those schematics with very little help.

Yes, Relay 109 gets the Power from the battery that it distributes through three different fuses [32, 34 & 43] (all identified in my post in the link, even by wire size and color).

Simply put, power goes to Relay 109 for distribution. When the Ignition is turned ON, the ECU "activates" Relay 109.

AndyBees said:

Did you look at my post in the Link Ol'Rattler provided?

Using that info, draw out those circuits on a piece of paper for a visual... it will really clear-up things.

When I did my conversion project, it become immediately clear to me that I would need the wiring schematics. It also become clear to me that I would always need them for a reference in the event of a problem (who's going to work on it for me was the big question........ answer, nobody). So, that in of itself is a very strong incentive to have reference schematics, notes and a working knowledge of the system (totally self-learned here). I spent hours and hours studying those schematics with very little help.

Yes, Relay 109 gets the Power from the battery that it distributes through three different fuses [32, 34 & 43] (all identified in my post in the link, even by wire size and color).

Simply put, power goes to Relay 109 for distribution. When the Ignition is turned ON, the ECU "activates" Relay 109.

Click to expand...

Your post was actually the one that heavily persuaded me to cut and splice my own harness rather than buying one a simplified one, so thank you for the learning experience and headache . On a side note, I hope you enjoyed your stay in Nova Scotia when you visited!!!

I've taken meticulous notes in my build logbook (engineering habits) and have sourced out a couple of different wiring schematic sources. The Haynes one is good, but it has a couple of errors. It was the one wire from the MPR going to the ECU that had me stumped. When I read your post I had interpreted it as it being the ignition that somehow controls the MPR, not the wire going to the ECU. Now that I know it's a grounding wire it all makes sense.

Hey, we got rained out in Nova Scotia. The forecast was rain for the next 4 to 6 days. So, the trip being at the end of the Annual TDI Fest, we opted to turn back at the visitor center just inside Nova Scotia.

So, a return trip is on our bucket list!

Yeah, when you read the linked Thread to the end, it is revealed that the wire from the ECU is a ground ... I'd forgot about that!

It's still all confusing to me ...