[SOLVED] Continuing the Mystery of a 2003 Tahoe with electrical issues

[Firebirdracer82]

Hi All - so the Tahoe I got now runs... kinda. (2003 Z71 with 184K miles)

What is happening:

Runs great and shifts great until... you turn on the AC, turn on the headlights at all. It goes into reduced power mode when you do those things.

If you hit the high beams then the thing shuts down as if you turned off the truck with the key.

I know this is electrical so, to that point.

I have done the following:

Cleaned all grounds except for the two under the dash (going to do that soon). I have put ringlets on the grounds to the passenger head, the drivers side bumper, the ground on the drivers side engine block, new ground from the drivers side engine block to firewall, new negative battery cable, as well and cleaned the surfaces very well. I paid specific attention to the grounds under the driver's door.

I have replaced the ignition switch.

I have traced the ECU ground back to the passenger side engine head, and I checked continuity with the ECM and it reads .8 resistance. (Which I think is good.)

I have removed the temp control head unit, the light switch, the blinker switch wires in turn individually, and no change in that.

I know that it's an electrical issue that is surging the system when I hit any of those switches causing the throttle position sensor to loose voltage an dgo into limp mode.

CODES:

BCM - B1480, B2580, B2585 (code read when I had the light switch removed)

Powertrain - B1518 (done when the reduced power mode was engaged)

Ideas:

All fuses have been replaced and the fuse box inspected and I did not see issues. But, maybe, a fuse box (under hood or in cab) has become problematic.

Welcome to any other ideas!

 

[Doubeleive]

Hi All - so the Tahoe I got now runs... kinda. (2003 Z71 with 184K miles)

What is happening:

Runs great and shifts great until... you turn on the AC, turn on the headlights at all. It goes into reduced power mode when you do those things.

If you hit the high beams then the thing shuts down as if you turned off the truck with the key.

I know this is electrical so, to that point.

I have done the following:

Cleaned all grounds except for the two under the dash (going to do that soon). I have put ringlets on the grounds to the passenger head, the drivers side bumper, the ground on the drivers side engine block, new ground from the drivers side engine block to firewall, new negative battery cable, as well and cleaned the surfaces very well. I paid specific attention to the grounds under the driver's door.

I have replaced the ignition switch.

I have traced the ECU ground back to the passenger side engine head, and I checked continuity with the ECM and it reads .8 resistance. (Which I think is good.)

I have removed the temp control head unit, the light switch, the blinker switch wires in turn individually, and no change in that.

I know that it's an electrical issue that is surging the system when I hit any of those switches causing the throttle position sensor to loose voltage an dgo into limp mode.

CODES:

BCM - B1480, B2580, B2585 (code read when I had the light switch removed)

Powertrain - B1518 (done when the reduced power mode was engaged)

Ideas:

All fuses have been replaced and the fuse box inspected and I did not see issues. But, maybe, a fuse box (under hood or in cab) has become problematic.

Welcome to any other ideas!

have you done any voltage testing on the battery, alternator, ohm testing the alternator to battery cable
resting voltage
engine on voltage
voltage monitoring when high beams are kicked on

 

[Firebirdracer82]

have you done any voltage testing on the battery, alternator, ohm testing the alternator to battery cable
resting voltage
engine on voltage
voltage monitoring when high beams are kicked on

I’ve tested the battery off and tested alternator (can’t remember exact values but holds 12.5 off and 14.5 running). Have not looked at voltage drop yet since I can’t seem to isolate which system is the problematic one (headlights or AC unit head)

 

[Jimmyy]

Voltage drop is a better test than a ohm test. Ohm test is not going to give you good information on if the wire can handle amps. Think of a multi wire cable with only 1 small wire making a connection. The ohm test will give a low reading but as soon as you pull current through it, it will fail.

 

[Doubeleive]

I’ve tested the battery off and tested alternator (can’t remember exact values but holds 12.5 off and 14.5 running). Have not looked at voltage drop yet since I can’t seem to isolate which system is the problematic one (headlights or AC unit head)

I would be probing all over with the meter checking ohms, inspecting all wiring for chaffing, pinched wires.
your description makes it seem severe so it shouldn't be to hard to find
I know you say it happens when the headlights are on but part of that same circuit is the tail lights/running lights/blinkers so check there as well

 

[Doubeleive]

Voltage drop is a better test than a ohm test. Ohm test is not going to give you good information on if the wire can handle amps. Think of a multi wire cable with only 1 small wire making a connection. The ohm test will give a low reading but as soon as you pull current through it, it will fail.

possibly but this sounds pretty severe, if it is enough to shut the engine down a ohm test should reveal it
the basic starting point is to check the integrity of the lan buss thru the obd port, by ohm testing
that bad of a short should relatively easy to find

 

[Fless]

I would definitely load test the battery using a commercial load tester; most parts stores can do this for you. Batteries can have bad cells yet seem to work OK most of the time.

I would also test both the main ground circuits (battery to engine block, battery to body) as well as the positive connections using the voltage drop method. How's the connection inside the red jump access on the DS of the engine, in the main positive cable circuit?

In the weeds a little, but verify fuse pin tension in the fusebox, especially those fuses that feed the BCM.

By "light switch" do you mean the one on the dash, or the turn signal stalk?

 

[Firebirdracer82]

I would definitely load test the battery using a commercial load tester; most parts stores can do this for you. Batteries can have bad cells yet seem to work OK most of the time.

I would also test both the main ground circuits (battery to engine block, battery to body) as well as the positive connections using the voltage drop method. How's the connection inside the red jump access on the DS of the engine, in the main positive cable circuit?

In the weeds a little, but verify fuse pin tension in the fusebox, especially those fuses that feed the BCM.

By "light switch" do you mean the one on the dash, or the turn signal stalk?

Light switch - basically turning on the regular headlights will put the truck into limp mode. Turning on the high beams by the blinker switch immediately kills the vehicle.

I may go get it tested, but I pulled it out of my other 2005 Tahoe and it had been doing great.

I can check in the red box connection but I believe it is OK. When I thought it was a battery terminal issue I got a known good set form a junk yard, but ordered a brand new negative cable (that is now installed). Still using the junkyard positive, but it appears fine.

 

[Firebirdracer82]

possibly but this sounds pretty severe, if it is enough to shut the engine down a ohm test should reveal it
the basic starting point is to check the integrity of the lan buss thru the obd port, by ohm testing
that bad of a short should relatively easy to find

I've been trying to figure what is drawing power so badly, but I can't seem to find it.

When I hit the AC or the headlights, it'll go into power saving mode but I do not know how to find what is drawing power - I used my multimeter on the battery and the battery volt does not drop when it goes into limp. I've made sure each connection has a connection to ground too with my multimeter.

I think it's something spiking the voltage or back feeding through the ground effecting the TPS.

But, how to test out the circuits I am not sure of. Been trying to find the way to do this but I am not sure of how.

 

[Firebirdracer82]

Measuring the power at the terminals when it goes into limp mode I do not detect a draw on power. It stays stable at 14.32v as measured at the battery terminals even going into limp mode.

 

[Doubeleive]

Measuring the power at the terminals when it goes into limp mode I do not detect a draw on power. It stays stable at 14.32v as measured at the battery terminals even going into limp mode.

well, I would start here with testing the integrity of the network ohm's/voltage, don't worry his breakout box that just makes it easier, the diagram on the bottom of his chalk board is what you want to reference on the obd port. That might point you somewhere or at least tell you the network is intact or not.

 

[Firebirdracer82]

I forgot to add prior, but the driver's side headlight is very dim... much dimmer than the passenger side headlight.

 

[Fless]

In hopes that it will help, see the link below for wiring diagrams, but verify that I selected the correct vehicle and navigate as necessary.

Headlamp Diagrams — 2003 Chevrolet K Tahoe 4WD V8-5.3L VIN T Service Manual | Operation CHARM

Detailed repair manual for the 2003 Chevrolet K Tahoe 4WD V8-5.3L VIN T.

charm.li

 

[cermakf2017]

Hi All - so the Tahoe I got now runs... kinda. (2003 Z71 with 184K miles)

What is happening:

Runs great and shifts great until... you turn on the AC, turn on the headlights at all. It goes into reduced power mode when you do those things.

If you hit the high beams then the thing shuts down as if you turned off the truck with the key.

I know this is electrical so, to that point.

I have done the following:

Cleaned all grounds except for the two under the dash (going to do that soon). I have put ringlets on the grounds to the passenger head, the drivers side bumper, the ground on the drivers side engine block, new ground from the drivers side engine block to firewall, new negative battery cable, as well and cleaned the surfaces very well. I paid specific attention to the grounds under the driver's door.

I have replaced the ignition switch.

I have traced the ECU ground back to the passenger side engine head, and I checked continuity with the ECM and it reads .8 resistance. (Which I think is good.)

I have removed the temp control head unit, the light switch, the blinker switch wires in turn individually, and no change in that.

I know that it's an electrical issue that is surging the system when I hit any of those switches causing the throttle position sensor to loose voltage an dgo into limp mode.

CODES:

BCM - B1480, B2580, B2585 (code read when I had the light switch removed)

Powertrain - B1518 (done when the reduced power mode was engaged)

Ideas:

All fuses have been replaced and the fuse box inspected and I did not see issues. But, maybe, a fuse box (under hood or in cab) has become problematic.

Welcome to any other ideas!

Could be an issue in the wiring to the exterior or interior lighting. Also if there's any aftermarket head unit/ audio system that could be culprit also. Had that happen in a 04 trailblazer i put my system in.

 

[Fless]

SWAG I would bet if you track down that headlight issue you'll have found the main issue.

 

[Doubeleive]

I forgot to add prior, but the driver's side headlight is very dim... much dimmer than the passenger side headlight.

There is your clue, pull the headlight out, inspect wiring harness, see what the voltage is to the bulb socket, backtrack to the relay and see if you can find a full 12v
check to see if the wires melted together anywhere.

 

[Firebirdracer82]

I’ll likely have some time tomorrow and start tracing from that headlight back. It’s odd though that the AC also causes the reduced power. I was hoping they’d share a circuit.

As to the other question there was a prior head unit in it. I’ve already ripped out any remaining parts of it and it looked like they just used a converter. No spliced wires.

I’ve also been debating if it’s the BCM itself draining the power but I don’t want to just throw parts at it if I can avoid it. And since it starts the two BCM codes may be issues in the harness somewhere.

Gotta stop collecting this gen truck but I like em too much.

 

[Firebirdracer82]

Update on the headlights

3v showing on drivers side.

12v showing on passenger side

This is with lights on.

 

[JoeBlobs]

An obvious answer, but if you haven't already, I'd start chasing wires from that DS headlight to your fuse box, inspecting the wire for breaks or issues. Maybe take a voltage reading on both sides of the fuse as well.

 

[Firebirdracer82]

Here are the results across the headlight circuit measuring for resistance:

Drivers side

Low:

.4h

.4ground

High Beam:

.4h

.4ground

Pass side

High beam

.4h

1.3ground


Low

.4h

1.3ground


It seems I have higher ground resistance on the ground side of the passenger headlight. I’ve been inspecting the wiring and don’t see much. Not sure if that matters. Readings taken from the plug under the fuse box.