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03 Tahoe RWD 5.3l. Suddenly after sitting a couple months in…

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03 Tahoe RWD 5.3l...

03 Tahoe RWD 5.3l. Suddenly after sitting a couple months in the shop and attempting to start the engine, most accessories including windows, wipers, seats, lights, blower fan don't work with a no start condition. S

Mechanic's Assistant: Have you checked the fuses? Do you know if there is power to the wiper motor?

Sorry, sent before I wanted to. Code B1440 power mode master input circuit mismatch. About 4.6 volts to ground. The voltage is pulsing in 1 second intervals. The fuel pump is activated along with dimly lit 4 way lights in the dash which also pulse. This is all going on with the key out of the ignition. If I push the 4-way button, it triggers the left front turn signal to flash in similar 1 sec. Intervals.

Mechanic's Assistant: Are you fixing your Tahoe yourself? What have you tried so far?

Yes, myself. I noticed some light electrolysis on a 25a fuse slot and I'm beginning to wonder if it's not in the junction box. I've pulled all the fuses and checked resistance in all the slots. I've traced perage draw from the junction box back to the fuel pump with a generator voltage indicator, unhooked the harnesses at the fuel pump and have the pulsing voltage at the gray wire of about 4.3 volts. The other smaller harness at the fuel pump has lower voltage and just dimly lights up my voltage light. It's hard to know which way to go when everything has shut down. The voltage to ground can be measured at the ground straps on the firewall. I've pulled both the f.p. relay and the 4-way/ turn signal relay with a slight decrease in measured voltage but the pushing to ground continues.

Submitted: 8 months ago.Category: Chevy
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Customer reply replied 8 months ago
The are no blown fuses.
Customer reply replied 8 months ago
Also with B1440 there's U1000 as well.
Customer reply replied 8 months ago
Furthermore, disconnecting the PCM, BCM doesn't stop it.
Answered in 16 hours by:
9/29/2017
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,649
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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How is the battery voltage? All of these symptoms sound like a discharged battery.

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Customer reply replied 8 months ago
Battery voltage is good, had a newer battery, performed a load test and then eventually replaced that better with a new one. 12.8 volts. The odd thing is the pulsing voltage. It drives the fuel pump, the 4-way flasher dash lights (dimly lit), check engine light is illuminated with no key in the ignition, disconnected and the cylinder completley out. If I press the 4-way flasher button the left front turn signal flashes in a 1-2 second interval. Now ground voltage has increased from 4.6v to 8.4v in the last couple of days. Also the condition has degraded to no communication to scanner. No power to 10a Crank fuse. How common is it for the junction/fuse/relay panel to short out, seems suspect.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago

I rather suspect a battery cable or ground cable issue on this. No doubt you have low voltage, seemingly across the whole truck, which has me wanting to look at the cables. From battery to junction block and to starter, and also the ground side from battery to engine, and also the two ground cables that are attached to the back of the engine block. If you voltage drop the ground side from battery to engine, I suspect you'd find a huge voltage drop there, can you do that test?

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Customer reply replied 8 months ago
Actually low voltage is the measured voltage at the firewall ground strap(short to ground measured from battery ground with battery ground cable disconnected, then with test light connected to battery negative post and to firewall ground strap produced voltage 4.6-8.4 volts). Other voltage at fuses is good. I will perform the tests you mentioned and get back to you.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago

OK, let me know how it turns out.

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Customer reply replied 8 months ago
OK, I cleaned up the two frame grounds under the driver, secured the ground at the top of the tranny, the firewall ground strap is still off as I'm using it to measure voltage to ground and that strap has 8.4v. That voltage seems to fluctuate for some reason. Bear in mind the transmission is out and on the bench.
I ohm'ed out the junction box and get resistance at the btsi and crank slot and get 3.38k on the load side of the fuse slot with the fuse pulled. The crank slot has no voltage w/ key in crank/start position. The btsi has 8.4v with key on and 6.6v w/ key off. The w/s wash has 12.44v. I didn't pull the wiper motor connector to check voltage but I can if needed.
This thing is running away with me a bit. Now after pulling the 5* relay because I felt it getting hot w/ key on, the left front blinker started flashing and the fuel pump relay is tripping with fuel pump harness disconnected at the tank and the tempo of the pulsing has changed to a more rapid succession. Pretty crazy. Also, the 5* relay, upper right slot produces voltage that rapid cycles from 4.8-5.3v.
Also, no voltage at obd2 data port.
Customer reply replied 8 months ago
One more thing w/ key off, I removed the neg. battery cable and remeasured the 5* relay slot both upper and lower right have 12.54v, went up a bit I guess to loss of resistence. However the btsi, load side at the fuse now has 12.54 and the crank, load side has 12.54v
Customer reply replied 8 months ago
With key off, battery neg reconnection at the f.p. relay on #85, 4.3-5.8v. #87, 12.34v. #86, 0v. and #30, 0v.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago

The battery cable runs directly to that fuse box, and then to bus bars within that fuse block. Any fuse that has power should have the same voltage as what is measured at the cable connection to the fuse block, voltage shouldn't change within that fuse block. If it is, you have something seriously wrong with that fuse block.

Please take a moment to rate my answers, that's how I get compensated. If you have further questions, I am happy to follow up with you.

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Customer reply replied 8 months ago
I think I'll swap out the fuse box and I'll let you know how it turns out. Thank you very much.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago

You're welcome, good luck with it.

Please take a moment to rate my answers, that's how I get compensated. If you have further questions, I am happy to follow up with you.

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Customer reply replied 8 months ago
Jerry, I had an 05 Denali with the identical fuse box, I swapped it out and it still does it. Maybe I need to go through the diagnostic steps on the BCM? The wiring appears to be sound through the vehicle. It seems as though the back feed to ground may be component based rather than compromised wiring. What next? By the way this symptoms remain with the ignition switch disconnected & the cylinder out.
Chevy Mechanic: Jerry Newton, Chevy Technician replied 8 months ago

I don't want you to over complicate things, don't overthink it. You have fuses in the underhood fuse block that are reading a fraction of the voltage they should have....tell me some of the fuse numbers/names, and we'll pick one out and trace it just to see what its source is. BCM controls power, but only by energizing relays, it doesn't have the ability to cut the voltage in half.

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