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Hal
Hal, Chevy Master Tech
Category: Chevy
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Experience:  ASE and Chevy Master , 31yr Dealership exp,
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10 blazer: 98 chevy..The fuel pump doesnt come..fuses..relays

Customer Question

I have a 98 chevy S-10 blazer. The fuel pump doesn't come on when I turn the key on. I have checked all the fuses and relays. One thing I have noticed is that when I have the ignition on and pull the fuel pump relay, then plug it back in, it doesn't click. So I swap out the relay and still nothing. It has done this about 4 or 5 times in the past year. And everytime I would just let it sit a few weeks and then it would crank up. ECM maybe? Its got to be something besides the fuel pump.
Submitted: 4 years ago.
Category: Chevy
Expert:  Hal replied 4 years ago.

Hi my name isXXXXX to just Answer.

So your concern is the fuel pump will not run.

1st the fuel pump relay will only energize for 2 seconds on key up, then wont energize again until the key is in crank position.

Jump the terminals at the fuel pump relay( look on the relay for terminal position)Listen for the pump to run, If it doesn't, go back to the tank and tap on it with a hammer to jar the pump. If it takes off and runs, Replace the pump. This is a very common symptom of a fuel pump.

 

Make sure the service engine soon lamp( check engine , amber lamp) comes on with the key in run. If it doesn't then cycle the key on and off 5-6 times and see if it comes on.

These vehicles have a problem with the ignition switches not making contact sometimes. The ses lamp must turn on when key is on! This lets you know the computer is working and has power.

Customer: replied 4 years ago.

I have power at the relay but just on one pole with the relay in. Also the relay doesn't click when I put it in. Ive already tried another relay. What energizes the relay? Where does the power come from to make the circuit?

Expert:  Hal replied 4 years ago.

The ECM commands the relay on for two second when the key is turned to run. It has to see engine rpm before it energizes it to run. The oil pressure sensor also puts power back to the fuel pump once oil pressure comes up.

Did the check engine light come on when the key is in run???

Here is the wire diagram for the fuel pump. Make sure the ECM B fuse is good and has powergraphic

Customer: replied 4 years ago.
Yes the check engine light comes on. And the ECM B fuse is good. The engine will turn over when it is in the start position but won't crank. The fuel pump used to always hum before the truck quit running. I just don't wanna go through the trouble in replacing the fuel pump if that's not the problem. Normally a fuel pump just goes out and you will be lucky to knock on the tank to get it back running. I've never heard of one doing this. Run great a few weeks then quit and then do the same thing 3 or 4 times. Its weird how it just sat a few weeks and then crank up fine. Its got me puzzled. Plus I hardly have any time to work on it.
Expert:  Hal replied 4 years ago.

Jumping the fuel pump relay so there is power back to the fuel pump, and if it doesn't run/hum, then you can tap on the tank . this makes the brushes in the pump make contact. If the pump now runs, you know the pump is bad, and is the cause of the no start. This is how we have checked for a pump that will not work. It is a very good and very quick test to perform. It may sound out of the ordinary, and you might question the validity of it. But it does work. This same principle has been performed on starters that do not turn over but just click, for years.

It takes 2 minutes tops to determine if the pump is not making connection. It doesn't mean the pump is good. It is just a quick test to prove the pump is the failed component.

It is easier to do this and drive the vehicle in the stall then it is to push it in the stall.

 

You stated it is weird that it could set for a few weeks and then crank up fine.

This is due to temperature and humidity changes. Poor connections don't care what makes them move to get connection. Heat and cold temperature fluctuations or a tap on the tank.

You have to do what you can as fast as you can when your tying to diagnose an intermittent concern.

I have seen fuel pumps run good. drive the vehicle in my stall for something else. Go to start it to pull out, and it won't start, test fuel pressure 0psi, get the customer to come and look at it, and hit the key and it starts up. Customer doesn't replace pump, then gets stuck in a parking lot , and is mad at us for not repairing his vehicle. while it was in our shop.

I have personally experienced this condition every week, at least once.

The best time to check the vehicle is when it won't start.

The ECM B fuse is good,This powers the fuel pump. In front of the fuel pump relay you will see a open terminal slot. This is the fuel pump test terminal( it goes directly to the fuel pump.) Put 12 volts to this terminal and the pump should run. If not then remove the fuel pump connector . check for 12 volts at the gray wire. If you have voltage, Now test between the fuel pump connector terminals C and B ( gray and black) with a test lamp( 12 v) if the lamp lights, then replace the pump. If it doesn't light then check the ground splice pack 423 and ground connection G402. They are on the left rear frame rail

Check for corroded connections/ poor connections. This could be a very intermittent condition that could be affected by heat, and humidity that could change in 2 weeks. The diagram I sent you gives you all the connection points for the fuel pump.

To test the relay hook a test lamp in the fuel pump test terminal and to ground. Turn the key on , it should light up, for 2 seconds then go out. If it does, the ECM is working

 

If you are having difficulty testing this circuit or don't have the test equipment. Have someone crank the engine over and tap on the tank, It works about 98% of the time to quickly test for a bad pump. If the pump is really bad it usually wont work. But for ones that ran one time and just won't start now. Give it a try, It is quick and easy.

Thank you Hal

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