[T3] Fuel Pump relay not energizing

Jim Adney jadney at vwtype3.org
Thu Feb 23 13:50:15 PST 2017


On 23 Feb 2017 at 20:27, Dorough, Don CIV, NPPSC N3/PSD Pensacola 
wrote:

> My question is why is this signal not coming from the ECU and what
> does the ECU check for before it will energize the fuel pump relay? 
> I'm thinking there is a problem somewhere else and not in the ECU. 

Sounds like you have a good handle on the situation and have done 
good troubleshooting so far.

A few extra things to understand:

The circuit in the brain that connects to that wire to the relay is 
automatically grounded for about 1 second when you turn the key ON. 
That circuit gets refreshed by the trigger points each time a trigger 
point opens or closes. So yuou get another second every revolution of 
the crank. This keeps the pump on while the engine is running, but 
shuts the pump off after 1 second if the engine stops for any reason. 
This is an important safety feature.

So, if you just turn the key on and then try to check the relay, it 
will be off after 1 second.

The wire to the relay has an in-line connection in the engine 
compartment. It's located against the front "fire wall" just to the 
left of the intake air distributor. You can do another test by 
grounding that connector with the key ON and listening for the fuel 
pump relay.

None of this works if the brain isn't getting power. FI power is 
provided by the FI power relay under the back seat. You should be 
able to hear that relay click ON when you turn the key ON and click 
OFF when you turn the key OFF. That relay gets its ground via a 
ground wire to its mounting screw, so make sure that screw is tight.

I see you're in Florida. It's possible that your brain has suffered 
from corrosion that has damaged the transistor that controls this 
circuit. I've replaced that transistor in one other southern brain. 
If you trace this problem back to being inside your brain, then you 
can either look for a better 311 C brain, or I can replace that 
transistor for you. I can help you with either one, the prices would 
be about the same; one is all parts, the other is all labor.

If you get to the point of suspecting the brain, I can tell you what 
to look for.

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