[T3] Fuel Pump Circuit

Jim Adney jadney at vwtype3.org
Sun Apr 21 11:33:50 PDT 2019

On 21 Apr 2019 at 17:56, bobsnotch at aol.com wrote:

> Jim, isn't there 2 wires that come into the engine bay in the left front
> corner that travel back to the under seat relay? I think 1 of them powers
> the ECU, while 1 is a ground. On the t-4, both are hot, which is why I
> remember them. 

Those 2 white wires, with 2 separate connectors, are both hot and both 
come from the FI main power relay under the back seat. They deliver 
separate 12 V power to the low power brain control circuits and the high 
power brain injector driver circuits.  

The 2 black wires, crimped into a single connector, are, as Dave pointed out, 
the conductors that lead to the backup lights on both sides. They should 
connect to the backup light harness which leads then to the backup switch 
on the front of the MT, then back to the engine, a fuse, and the hot terminal 
on the coil.

> Dennis, there should be a white wire (FI wire sized) that connects to
> the white wire sticking out of the harness where you've got your green
> wire plugged in. 

#19 coming out of the FI harness is white; the wire leading into the main rear 
harness is (wht/blu?) The latter wire leads to the pump relay.

> If you ground it (body side) with a jumper wire, and turn on the key,
> the pump should run. It's the ECU's pump ground wire, or #19 wire. The
> car harness side goes all the way to the front FI pump relay. You could
> pull the ECU out, and with an ohm meter check your wiring from terminal
> #19 to the other wires in the general area to find the other end of #19
> in the FI harness. 

If the green wire comes from something that's hot all the time, and is now 
connected to the white wire #19, the control transistor in the brain has now 
been blown. That's a big deal, but I can replace it. You'll know for sure once 
you get the #19 wire connected correctly: If the relay works correctly (2 
clicks) then everything's okay; if the pump never runs, the transistor is blown 
open; if the pump runs all the time the key is ON, the transistor is shorted.

Dennis, tell us what the other end of that green wire was connected to.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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