Fuel pump

[conedodger]

I have the service manual so I suppose I should just go research this myself...

Ok, I have a very stock 'looking' 73 240Z with triple Webers and some other very tasty modifications. But mostly, I have stayed with the stock wiring and so forth so that I don't become talked about as one of those 'Dumb-arse previous owners' you hear about one day. So here is my dilemma; I had a problem with my fuel pump and I was away from home, I had it towed into a shop and the guy said the fuel pump wire was disconnected. He 'rewired' it. So there goes a $100 but it works. Next, I am having a buddy of mine balance the carbs and he calls and says the fuel pump is toast. I buy another from Summit and send it to him. I'm out $150 but it works again... Now, I have it back home and the thing keeps stalling out and leaving me stranded. I check each time and sure enough, fuses are blown. Top left and the second to the top occasionally (probably unrelated). I take it to a shop here in Reno and they install a relay... Things are fine for a couple months but truthfully only about 50 miles since I tow it to events so it has had one track day and one autocross since this relay was installed. So, this weekend, I take it to Reno-Fernley Raceway for a track weekend and after the first session, it blows a the top left fuse and I am towed off the track.

I took it back to the shop that installed the relay and they just called to say that they think it should be wired to a different source of switched power. If you read my intro, you know that I would like to have it wired they way Mother Datsun intended. Isn't there a harness hook-up back by the tank for the fuel pump? Why do they have wire running from the new fuel pump to the front of the car? What fuse should it be on?

In my mind, there should be just a short length of two wires running to the harness for the new fuel pump...

Edited September 10, 2012 by conedodger

[conedodger]

Apparently, I would only have the electric pump and associated harness if my car had been brought in for some recall?

[Walter Moore]

My 71 has wiring for an electric fuel pump, but it isn't hooked up under the dash. I ran my own wires because I put an oil pressure switch in the circuit to shut the pump off if the engine stalls. The stock wires require the addition of a inertia cutoff switch I am told, otherwise the pump runs whenever the ignition is on which could be a problem in a crash.

There should be wiring at the pump location on a 73, but who knows what the wiring is like 39 years down the road.

[Carl Beck]

All the US Spec. Datsun 240Z's 70-73 had wires in the harness for an electric pump. They were wrapped in the harness, and not used until later production 73's. Yes, you have to put a jumper in the harness under the dash to supply power to the rear.

When the 73's first arrived they did not have an electric fuel pump installed. It was only after the 73's were found to have significant problems with vapor lock and percolation that the cars were retrofitted with electric pumps - and after a certain date they started coming from the factory with electric pumps as standard equipment.

When they were retrofitted with the kits supplied by Nissan - the Dealer's installed a relay. See attached image.

Before the kits were available - most people just put a jumper in the harness under the dash - and installed the electric pumps in the rear. {no relays or safety shut-off's}.

If you have 12volts at the HOT wire in the rear - and a good body ground - the pump should work fine. If a shop installed a relay - and installed a new wire - that should work just fine - as long as you have 12Volts on the hot wire - and a GOOD BODY GROUND.

As I recall - there is a body ground on that rear harness - on the right side, inside the car {not certain of that - it might be outside the cabin?} ... but it sounds like you have a floating ground ...I'd make sure that you have a clean body ground from the pump - and clean grounds in the engine compartment {from the battery to the body}.

[Carl Beck]

Apparently, I would only have the electric pump and associated harness if my car had been brought in for some recall?

Easy enough to tell - look behind the Right Side Kickpanel and see if you have the relays show in the image I posted. If you do - a failed relay "could" have caused your initial problem.

FWIW,

Carl B.

[conedodger]

Carl, Thanks for the archived data. I have always had an electric pump installed since the 1980's when I bought the car. But, it was always wired to the front with a long power wire and a short ground. Owner mod I am sure. I will check for the relays. I doubt that a failed relay could have caused the problem as it has never been hooked up that way for me...

[conedodger]

Ok, turns out the electric fan and the fuel pump are both wired to the top left circuit on the fuse box. The fan did not have a relay. My fuse box is now slightly melted but usable. As a temporary solution, the fuel pump now has a 30amp fuse inline as well as the relay. The fan has a relay and a 30amp fuse inline.

I intend to wire the fuel pump in the same way as Carl's archived data from 1973 in the near future as well as replace the fuse box with one of the bladed fuse circuit board units sold by MSA.

Any thoughts?

[madkaw]

http://www.classiczcars.com/forums/thread45836-2.html

post 45

[SteveJ]

FYI, if you still have a voltage regulator in the car, you can wire the (a) fuel pump relay like the factory did. Nissan used two sources for the coil on the fuel pump relay. The first one was from the solenoid for starting the car. The second was from between the alternator and voltage regulator. This way the fuel pump would no longer get power if the engine wasn't turning. If you have an internally regulated alternator, you should look into an oil pressure switch for the fuel pump relay.

[madkaw]

Sorry I didn't elaborate on my post, but you can modify what the shop did with your car, and still be somewhat stock. The switch I installed used the 2 wires that are behind your radio. With the switch you have the option to have key on wothout your pump running.

You should have the green and black wires by the tank right nect to the fuel sending unit wires. That wire is 14 guage and can handle about any pump. Whatever wire the shop used to run to the backof the car- off of the relay- could be wired into the existing green wire under the glove box. This way you have a relay hooked to the stock wire running to the pump.

Not sure if I would be using a 30 amp relay-how big is your pump? I think I am running a 15 amp fuse for my pump.

[SteveJ]

Even with an inertia switch, the fuel pump should have some kind of cut-off when the engine is stalled.

[madkaw]

No doubt-just another way to wire it.