# Relay 428 vs 100 Confusion



## ejg3855 (Sep 23, 2004)

So Its my understanding that the 100 Relay is for the ECU power and the 428 relay is for the SAI pump power.
But here is my conundrum:
IMAGE 1:
This supports my pre-existing knowledge, J271 (#2) Relay 100 is for ”Motronic Engine Control Module”

IMAGE #2/3:
Also supporting my claim is the wiring pages of the benltey, J271 being the ECM Power Relay
 

Image #4:
Relay 428 in the harness has wire colors matching the J271 relay in the diagram above, Specifically the lilac wire to pin #21 in the ECU.


IMAGE #5
The 100 Relay in my harness (originally thought to be ECU power), wire colors match that of the SAI relay per the diagram seen below in IMAGE #6


IMAGE #6:
Which is contradictory to my popular belief that 100 is the ECU relay. In the diagram below you can see that the color wires also matches.




In short my Bentley contradicts itself.

What is the proper ECU relay?

My 100 relay has a long power and ground with the SAI connector on the end.


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## cri-cri (Sep 27, 2009)

Wrong picture in Bentley... or right picture, wrong description (it all depends on how they're installed in your car: like this or like this)
Anyway,

*428* is definitely the *ECM* (J271) relay.

*100* is definitely the *SAI* (J299) relay

irrespective of their position.


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## elRey (May 15, 2001)

Purpose if relay is to allow ecu to remain powered on after ignition is turned off. The ecu it self has control of the relay after ignition is turned off. That's why you can cycle key with vagcom connected and not lose communication on later cars equipped with the relay, but not on earlier cars with out it.


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## ejg3855 (Sep 23, 2004)

cri-cri said:


> Wrong picture in Bentley... or right picture, wrong description (it all depends on how they're installed in your car: like this or like this)
> Anyway,
> 
> *428* is definitely the *ECM* (J271) relay.
> ...


This is good to know, thanks

They aren't installed in a car at the moment so its hard to use the visual comparison. Building A MK2 1.8T SWAP harness.


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## oathofoats (Aug 14, 2016)

just wondering then,

if SAI is deleted from tune and no longer in place is the fuse 100 then still needed?

my 428 relay just started acting up and found this, one ordered and hopefully this will get it back in order.

thanks


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