No power to fuel pump

Apologies in advance if this was already covered. Answer didn't appear in search results.

'04 base 5 lugger @ 81k miles

Anyway, got back from a week long vacation with the Trailblazer and went to start my parked truck. It cranked without a single attempt to fire. All other electronics in and out of cab work. 3/4 tank of gas. Never had a single hint of a problem before.

Checked all the colored plastic fuses in the dash and in the fuse block under the hood. All fuses check out ok.

No fuel pump noise when turning key.

Had a family member check the fuel line pressure with several attempts. No fuel pressure.

Dropped the gas tank and took the pump mechanism out. plugged the pump into the wiring outside of the gas tank. Pump is dead. Or so I thought. a 9v battery turns the pump smoothly and quietly... so the pump actually works. Using a voltmeter, I tested for voltage on the wire that powers the pump under the truck. No juice whatsoever. A power failure is preventing pump from operating.

Time to test for voltage upstream. traced fuel pump wires under the truck, through the cab and under the dash. Wires go to a relay (circled in green). This relay actually checks out ok. It passes a continuity test and is in tip top shape, as I have swapped it with a couple relays from the fuse block under the hood. Using a 9v battery, I was successful in sending voltage down the wire from the circuit opening relay (circled in green) to the fuel pump connector.

With the circuit opening relay, I noticed that the base (whatever it's called) that the relay plugs into, was only providing 2 volts. 2 volts apparently is not enough to switch the relay on to send power to the pump. Ok, time to find out why it's only receiving 2 volts. We'll look further upstream to see why

I check the next relay under the hood next to the battery labeled EFI relay. The relay passes all continuity and the block it plugs into is sending 12v downstream to the circuit opening relay that I just checked.

I've come to two assumptions:

some sort of connection issue between the EFI relay and the Circuit opening relay that drops voltage from 12 to 2. (not sure what)

Or

The ECM shown in the diagram is fouled up limiting the voltage. (No knowledge about ECM)

From here, does anyone have any suggestions?

 

Seems like you are on the right track and have the skills to figure it out. In your testing, keep in mind, the ECM will not allow the circuit opening relay to power the pump unless the crankshaft is rotating. Crankshaft rotation is qualified to the ECM by the crankshaft position sensor. Any testing for power to the pump must be done while the engine is being turned over by the starter. Turning the key to "on" will not power the pump.
Keep at it.

 

Sure enough, you're right. I'm used to GM cars the will activate the fuel pump before cranking. When I crank, I go from 2 volts to the full 12 volts back to the pump.
I reassembled and after a 10 seconds of cranking (probably pumping fuel through empty line) it sputtered back to life. Not sure why it wouldn't start during any of my bajillion attempts before I took it apart. I'm pretty relieved that I don't have to buy any parts, but baffled by this phantom problem that appears to have went away.

 

Well I had the same issue did the same things you did and it ran good for about a month then all of a sudden it quit again no fuel almost starts here and there maybe plugged filter

 

This happened once to me in the taco. I spent a couple hours checking connections, would just crank and crank and never fire up. I left it alone to check online for some ideas or other's solutions and went back and it started.... That muthacrunker. So I don't know what fixed it, and I hate that in back of my mind it can happen again

 

Did you guys ever figure this out? My Taco has the EXACT same issue.

Do/did your truck act any particular way before it died? I was driving along with no problems at all and it began to sputter, power drop, began to backfire, than completely died. I could restart it, drive a few hundred feet just fine then it would start acting funky again. ( I was able to do this cycling of the key, for for about 10 miles trying to get it home) Acting like its starving for fuel maybe? I had a half a tank. Truck then died, and wouldn't restart at all.

I tow it home, rip it all apart, traced all the wiring, tested for breaks in wiring, tested relays and fuses, everything fine. Pull pump out, bench test it, spins free and quiet, slap it all back together, fires up and drives fine. WTF is this? crank sensor? ECU? 3rd time this whole process has happened. I pull it apart put it back together again, and its fine.

 

That's definitely not the issue I had. Is the MIL on? What codes are thrown when this happens? Sounds like it could be fuel pump, fuel filter, something in ignition system as well. Those intermittent problems are tough to work on man

 

No lights at all. No codes at all. Pump, filter, plugs, wires, coils, MAF are all new. I am leaning towards a weakening crank sensor or ECU as they are the only things not new or tested. thanks for the reply.

 

So i diagnosed my power to the pump and pronged + and - . Had 5.56 volts at “ON” position and on the crank, it would jump down to 4.5. Any ideas?

 

Battery voltage ?? up at 12.5 VDC ??

Start at the EFI fuse and check for voltage to the EFI relay to the Circuit opening relay see what you have??

Any Codes at all??

 

Remember there is a fuel pump kill relay under the dash by the dash fuse box that is part of the ignition circuit, FYI

 

Was this at the pump? Its time to find a circuit diagram and start following that voltage back to the battery, looking for bad connections and failed relays along the way.

 

always had a p0450 emissions code other than that no. Is the circuit open relay in the truck left side of driver under sterring wheel?

 

yes at the pump, weird this is i was using a good battery with about 12.5 volts, left gas lines open, tried to prime pump but wouldnt squirt. Is it possible the low volts to the fuel pump did something to it? Pump wont pump with good battery. And getting low volts to pump

 

Wouldnt the kill switch show just 0 volts? Im having power but very little. Have to backtrack and trace it down. Thought it was my filter, but no. Pump doesnt want to pump even with 12.5 volt battery.

 

If its a complete circuit with 12.5 volts from the + on the battery to the pump and a clean ground back to the - terminal, I would suspect the pump.

 

But tested the power to the pump, low reading at 5.5 volts at “ON” and goes down to 4.5v on the crank to turn on.

 

Could be more than one problem. I wonder if the contacts in the fuel pump relay are fried.

 

relay under the hood? Switched the tail relay with efi, tail lights worked fine so im assuming the relay is still good.

 

Have you confirmed you dont have a voltage drop on the ground side?