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moozieman

Fuel Pump Issues... AGAIN

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My fuel pump has never run correctly in my 1977 280z. Currently, I get no voltage coming off the wires leading up to the fuel pump when I crank. I swear I've read voltage off those wires before, but one day during the process of rebuilding, it just stopped. So what i've done is hard wired the pump to the battery with a switch in the middle and that's it. Is there any problem with this method? I just flip the switch and let the fuel pump run while running the engine. Ideally, I would like to fix this. I've read things about the fuel pump safety switch getting stuck and cutting off power. Is there anything else I could look into? How would I test this switch?

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Somebody should reply with an answer on the diagnosis but direct wiring a fuel pump is not a good idea. At the very least, I would add an inertial kill switch in case of accident. Sitting inside a Molotov cocktail never sounds like fun!

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Here is a quick guide to how that FI relay works.

First some notes:

N1- The FI relay (FIR) actually has two physical relays in it (call them X1,X2).

N2- X1 provides power to the fuel pump and the air regulator (both the pump and regulator +12V feed are connected together directly INSIDE the FI) When X1 fires, your pump should spin and your air regulator should have power. (remember this note for later when we debug the circuit)

N3 - X2 provides power to the ECU, the Dropping Resistors, and the AFM air flow switch(which incidentally is also one of the items that can fire X1 to make your pump run) This is also the safety switch that if your engine dies, your fuel pump turns off. Caveat: 78 and later they replaced the airflow switch with an oil pressure switch. Both offer the same exact function, just a different method of determining when the engine is NOT running.

N4 - X1 (which again will turn on your pump) gets it's signal and is fired by one of two methods. The AFM switch we just mentioned in Note3 & the start signal (CRANK) from the ignition switch. Either input will fire X1 and you should have a running fuel pump (as well as +12V feed to your air regulator)

N5 - Datsun wiring harness for the EFI is fairly easy to traverse as Datsun conveniently printed the wire number in light white or light yellow numbers. If I refer to W36 it means the wire with number 36 written on it. Many times, but not in all case, when W36 terminates at a connector, the connector will have "36" stamped in it as well. For example, the AFM connector & the ECU connector. Somewhat UN-convenient is the fact that most all of the wires are green. 🙂

-- Lets debug a little.

1 - If the pump isn't working, it can be one of the following: No power feeding X1, no signal telling X1 to turn on, or the power being routed through X1 is not reaching the pump.

2 - Remember Note2 where we said the air regulator and pump power are tied together. The air  regulator is right on top of the engine and super easy to reach and test with a meter. So the first step is to pull the air regulator connector, locate W48. Probe it with a multimeter and see if it has power when you CRANK the engine and/or push the AFM flap in. If you get 12V there with either of those tests, you know X1 is working, and the problem is downstream of the FIR with the wiring from W74 exiting  FIR to the pump, or the pump itself.

3 - If you don't get 12v with those tests, jumper 12v to W48 and you should hear the pump run. This will confirm your wiring to the pump is good and we are in fact chasing down a problem that is in the relay or power to the relay. IMPORTANT: The air regulator connector also has W34 which runs to the ECU. The ECU grounds this wire to turn on the air regulator. If you mistakenly jumper 12V to this wire you will be shorting 12v to gnd through the ECU!!! Don't guess, double check which wire you are working with. If you are worried, just unplug the ECU.

4 -   If it is the relay, move on to ensuring that power is reaching the relay properly. This is a little more difficult as the relay isn't the easiest to get to.

5 -  Before the next few tests, you need to confirm that the FIR is getting a good ground. It comes in on W72. Place the ground side of your meter on W72 at the relay and the pos side on the +12V and make sure you read +12V on the meter. If not, check the ground connection of W72.

6 -  With the key OFF, measure voltage at W73 at the relay. It should be 12V.  If not, check your fusible links.

7 -  With the key OFF, measure voltage at W70 at the relay. It should be 12V.  (The schematic shows direct from battery, but I believe it is coming from a fusible link as well) I'll confirm later..

8 - With the key ON (but not cranking), measure voltage at W71 at the relay. It should be 12V. If not, check the ignition switch.

9 - With the key ON (but not cranking), measure voltage at W39/W10 or W43. All should be 12V. If not, X2 inside FIR isn't working.An easy way to reach W39 is at the AFM plug.

10 - With the key in the START position, we are back up to Step 2.

 

Len

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