Fuel pump testing with an Ohmmeter.

[sr acalito]

Hello everybody! I am Acalito from Saudi Arabia, a new member.. Please somebody help me where to post this thread, last time brooklynbay transfer my post to new thread to where it belongs.. I have a problem with my '92 xlt. Last week i fill the tank full, at the gas station i hear like there's a motor knocking somewhere in the fuel tank, after running around 2 km, the engine stop & never start again, i cannot hear the fuel pump when i switch the key ON. I've read a lot of threads here, very interesting, so i decided to change the fuel pump relay but still the engine won't start. There are 3 relays under the power distribution box, i removed them all just to compare one from the another. This is what i've found: the green (fuel relay) normally open & the coil (two small pins) has 55Ω; the black (don't know what is this relay for) normally closed and the coil has 55Ω, same as green: brown (don't know what this relay for) is normally open but like a diode, there is a risistance of 80Ω, but if you reverse the leads of the tester, it's open. Are these an indications of GOOD relay? Please share your help.. Thanks in advance!

 

[sr acalito]

I want to check the fuel pump motor also if it is still working by supplying direct 12v from the battery, but how can i be sure of the wire going to the pump motor? I am planning to use the line from the enertia switch and ground. Will this be safe and correct?

 

[BrooklynBay]

I've moved your posts into their own thread in the stock 91-94 section. Use a jumper wire in the fuel pump socket to supply 12 volts to the fuel pump. Your Ohmmeter could test the resistance of the fuel pump from the fuel pump relay's socket.

 

[sr acalito]

Thanks BrooklynBay! Where is this fuel pump socket located? One day i was under the fuel tank planning how to remove it but didn't see any socket that is possibly for the fuel pump. Ok i will check the resistance of the fuel pump motor tru the relay socket.. I am wondering also the voltage on the enertia switch is only 6.3v, even if i remove the fuel pump relay there is still 6.3v.

 

[BrooklynBay]

The fuel pump is located inside of the fuel tank. The relay is under the hood.

 

[sr acalito]

I mean the fuel pump socket sir.. Can I access it under the truck?

 

[BrooklynBay]

The connector for the fuel pump, and the sending unit is between the tank, and the body, so you will have to lower the tank a little in order to get to it.

 

[sr acalito]

Thank you very much sir! Now I'm going to check it and return the result to you.

 

[BrooklynBay]

Did you try to jump the fuel pump relay's socket with + 12 volts? Try this first before you drop the tank.

 

[sr acalito]

No, ok thanks! I'll do this first.

 

[sr acalito]

This maybe a big problem, I supposed to check resistance of the fuel pump motor thru the socket of fuel pump relay, I found 12v in the two terminal/pins (not the pins for the coil) even if key is off. So if I jump this two, a short circuit will result. Any help sir?

 

[sr acalito]

Ah you mean the negative terminal of the socket, I'll connect it directly to the battery? And so for the positive terminal to the positive of the battery also?

 

[rhauf]

Ah you mean the negative terminal of the socket, I'll connect it directly to the battery? And so for the positive terminal to the positive of the battery also?

I think what he means is to use a small piece of wire (preferably with blade connectors on the end) to fit into where the relays "switched" wires are. there are 4 pins of the relay used, two of them trip the relay, when the relay is tripped it's like a switch that connects the other two wires, closing the circuit to the pump and making the fuel pump run.

when you turn the key, if somebody has their finger on the relay, they should feel it click. a click is a good sign but does not mean for sure it is working. you should also hear the fuel pump run for a couple seconds if you listen carefully (when the key is turned to the run position, but engine is not started)

 

[BrooklynBay]

Let me explain this again. The relay socket has constant voltage. The coil only has voltage when the pump is supposed to turn on. The coil has thin wires, and the wiring on the contacts is much thicker. One of those wires has 12 volts all of the time. Take a jumper wire, and touch the other end to the thicker wire. That end is going to the fuel pump. You should hear the pump turn on when you do this. If the pump doesn't turn on, then connect an Ohmmeter to that wire, and ground the other end of the meter.

 

[BrooklynBay]

This link should help you: http://www.ehow.com/how_4615219_test-electrical-relay.html

 

[sr acalito]

Thank you guys! I've just finished jumping the fuel pump relay, so sad, I did not hear the pump motor running. Now I'll check the resistance of the motor. Maybe next is to remove the fuel tank.. Hard job!

 

[sr acalito]

Since the engine stop last week, I keep on trying to start it but it won't start. So, can we say now that there is no more pressure on the line? This is my first time on this job, I'm so thankful for this site!

 

[BrooklynBay]

One last thing to test is the inertia switch. Put the meter on continuity mode, and test the 2 connections on the inertia switch.

 

[sr acalito]

Ok I'll do this.

 

[BrooklynBay]

If the inertia switch is open electrically, see if you could reset it. You could use a jumper wire to see if the pump will turn on at this point to bypass the inertia switch.