2003 2500hd Rear Power Locks

John288 · July 2, 2009

Hello

I have a 2003 Sierra SLT 2500HD, crewcab, shortbox. It has low milage and I have owned it since new. Over the last few years, the rear door locks would intermittenly stop working. The dealer was not able to figure out the exact problem other than to try switching the door lock relays in the fuse pannel beside the drivers door. I did that the the problem shifted from not unlocking to not locking so I went and bought a relay. It worked for about a month and then the problem was back. The parts department would not take the relay back because they figured there was a problem with my truck. The service department that charge me to diagnose the problem would not do anything other than to say they needed to book my truck in for an appointment. Before I buy another $35 relay (from a different parts department), is there a chance that there is a problem with something else in the wiring or ? that is causing the relay to malfunction(for example, body control module, short in some wiring)?

Thankyou

John288 · June 1, 2010

Dealer sold me the wrong relay. Problem fixed.

bill190 · July 2, 2009

What kind of diagnostic device did you build? It sounds very interesting. Any specifics would be cool. Thanks!

Well this was a long time ago before PC's and built out of relays, diodes, switches, resistors, and LED's. I can't discuss what it was for, but will translate it to the above problem...

Just get one of those aluminum metal boxes from Radio Shack for making your own electronic projects. Or use an old computer or electronic cabinet, then you will have plenty of room for wiring inside.

Then drill about 10 holes in it for LED's. And another 10 for switches.

Then with the relays, you can wire them so they "latch on" if voltage is applied. You get a double pole double throw relay, then wire it so once power is applied, one set of contacts keeps the relay tripped. And the other set of contacts can go to the LED.

You could wire this in the above example to say the wire going to right rear door latch at the floor wire. Then when it got power, it would latch on the relay and the LED would stay on.

And the polarity on the same set of wires (in the above example) might change depending on if you were locking or unlocking, so you could use diodes to turn on one relay/LED if locking and another relay/LED if unlocking.

And you could repeat this for monitoring multiple points in the wiring.

Or you could wire a relay to monitor for power loss. Maybe two relays. One relay latches on via a push button on the box at setup (if power is present on the test wire). And this would provide power to a relay. Then if power is lost, the 1st relay would drop out and disconnect power to the 2nd "latched on" relay. Then that relay would not come back on if power returned to the test wire (without pressing the button.) You could wire the 2nd set of contacts to turn on an LED.

And you could have several of these power loss relays which would trip if at any time power was lost to the particular test wire.

And you can power this with a small backup battery so that the readings on the monitoring LEDs would remain independent of the power available in the vehicle.

So basically you could configure one or two relays to monitor for if power was applied or if power was lost. Or to check for these only at certain times like when the ignition is on.

Or only check if the switch is pressed. "Logic circuits" were made from relays before IC's ever came along, so you can wire a bit of logic into these.

The problem with the above is it will not measure voltage. And this can be a problem in a vehicle. A better thing to do these days might be to use "data logger" like the following...

http://www.dataloggerinc.com/selection-voltage-current.html

Latching relay circuit...

http://www.eleinmec.com/article.asp?24

TexasAl · July 2, 2009

Well, my experience, the relay will either work, or it won't. In my opinion, your door actuators are going/gone bad. If the door locks are no longer working, pull the panel off and check to see if there is power going to the actuator. If there is, then you have a bad actuator. Let us know what you find out.

bill190 · July 2, 2009

Relays are pretty reliable. Usually millions of operations before they fail. And when they do fail, it is typically because the contacts are dirty or the contacts have gone bad due to arcing.

So I would suspect it is some other problem.

The way to troubleshoot an intermittent problem like this is to be able to "see" what is going on when it happens.

I would start with a GM factory wiring diagram for this vehicle, which would show me everything involved with the circuit in question, the connectors, wires colors, connector pin numbers, the associated fuses (sometime more than one), relays, devices, and where on the vehicle all this stuff is located.

Then I would start by monitoring power to the relay. With many relays in vehicles, power to the relay "coil" is on one circuit/fuse, and power to the contacts/power to the device is on another circuit/fuse!

So first I would connect test wires to the relay "coil" power wires and connect a multimeter to those wires and leave it in the vehicle. (Actually this time of year the heat would wreck a multimeter, so take it in/out of vehicle.)

Then just drive around and before using the door locks, turn on the multimeter. Then use the door locks. Then if the problem happens, see if power was getting to the relay coil or not. With the multimeter in place, you could "see" what was going on at that point in the circuit. You would need to be watching the multimeter as you pressed the button.

Then if no power at that point, work backwards in the circuit. If power was present, work forward in the circuit. So move the test wires to the next location.

So you can "monitor" various spots in the wiring / circuit as you use your vehicle and find the EXACT problem.

You would want to monitor...

-Power to the relay coil. (Is the relay being activated?)

-Power to the relay contacts. (Is there power to the relay contacts?)

-Power after the relay contacts. (Is the relay switching on power?)

-Power at various points between the doors and the relay. (Wiring problem?)

And if there is no power being provided to the relay coil, what switches on the relay? Is it an electronic module? Monitor the output of that module right at the module. Then if bad there, monitor power to the module.

Also you would want to verify you are monitoring the *correct* relay. You can do this by looking at the GM factory wiring diagram and by disconnecting the relay and seeing that the door locks in question no longer work. (Don't want to waste a lot of time monitoring the wrong relay! )

Advanced intermittent troubleshooting for a shop: In the past in the non-automotive area, I have come across some very difficult to troubleshoot intermittent problems and the customer wants a quick resolution to the problem. In one case I built a device to monitor a circuit in 10 places at the same time. So I had wires strung all over the place connecting to the circuit at different points. Then I had the customer turn it on before they used their device. When the problem cropped up, the customer called me, then I checked my device and then saw where the problem was.

As vehicles become more complex electrically, something like this might be a way to more quickly find these intermittent problems.

5Alive · November 11, 2015

Thank you Bill, for your detailed post on step by step diagnostics.

And thank you John288, for returning to report what your resolution ended up being.

Here the dealer sold you the wrong relay that they wouldn't take back. The American automotive business seems to be built on the rape of consumers... from manufacturing to selling to service. I have a Japanese car with over 350,000 miles and 25 years of service, never garaged, and never had a problem with the door lock relays or actuators.

TexasAl · July 2, 2009

Relays are pretty reliable. Usually millions of operations before they fail. And when they do fail, it is typically because the contacts are dirty or the contacts have gone bad due to arcing.

So I would suspect it is some other problem.

The way to troubleshoot an intermittent problem like this is to be able to "see" what is going on when it happens.

I would start with a GM factory wiring diagram for this vehicle, which would show me everything involved with the circuit in question, the connectors, wires colors, connector pin numbers, the associated fuses (sometime more than one), relays, devices, and where on the vehicle all this stuff is located.

Then I would start by monitoring power to the relay. With many relays in vehicles, power to the relay "coil" is on one circuit/fuse, and power to the contacts/power to the device is on another circuit/fuse!

So first I would connect test wires to the relay "coil" power wires and connect a multimeter to those wires and leave it in the vehicle. (Actually this time of year the heat would wreck a multimeter, so take it in/out of vehicle.)

Then just drive around and before using the door locks, turn on the multimeter. Then use the door locks. Then if the problem happens, see if power was getting to the relay coil or not. With the multimeter in place, you could "see" what was going on at that point in the circuit. You would need to be watching the multimeter as you pressed the button.

Then if no power at that point, work backwards in the circuit. If power was present, work forward in the circuit. So move the test wires to the next location.

So you can "monitor" various spots in the wiring / circuit as you use your vehicle and find the EXACT problem.

You would want to monitor...

-Power to the relay coil. (Is the relay being activated?)

-Power to the relay contacts. (Is there power to the relay contacts?)

-Power after the relay contacts. (Is the relay switching on power?)

-Power at various points between the doors and the relay. (Wiring problem?)

And if there is no power being provided to the relay coil, what switches on the relay? Is it an electronic module? Monitor the output of that module right at the module. Then if bad there, monitor power to the module.

Also you would want to verify you are monitoring the *correct* relay. You can do this by looking at the GM factory wiring diagram and by disconnecting the relay and seeing that the door locks in question no longer work. (Don't want to waste a lot of time monitoring the wrong relay! )

Advanced intermittent troubleshooting for a shop: In the past in the non-automotive area, I have come across some very difficult to troubleshoot intermittent problems and the customer wants a quick resolution to the problem. In one case I built a device to monitor a circuit in 10 places at the same time. So I had wires strung all over the place connecting to the circuit at different points. Then I had the customer turn it on before they used their device. When the problem cropped up, the customer called me, then I checked my device and then saw where the problem was.

As vehicles become more complex electrically, something like this might be a way to more quickly find these intermittent problems.