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Jared D
Jared D, ASE Certified Technician
Category: GMC
Satisfied Customers: 1660
Experience:  Shop Supervisor at County Fleet
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I have a 2003 GMC HD 2500 truck. I'm having some electrical

Customer Question

I have a 2003 GMC HD 2500 truck .
I'm having some electrical issues .
A few months ago my transmission stopped shifting into fourth gear and would not get in the first either .
From a stop I can tell I'm probably in second gear .
Maybe third gear .
The point is it won't shift at all and I end up at 4000+ RPM at 55 mph.
I found that one of the red wires at my ignition switch did not have voltage.
The other red wire did however .
I jumped one red wire to the other and the transmission started working correctly again .
As of this morning the transmission is doing what it used to do and I have power at both red wires .
I also have power at the orange and brown wires with ignition switch turned on .
The white wire at the switch seem to have an intermittent ground almost sounded like it was running through a flasher with my beeping for meter so I connected the ground to that white wire but it did not help.
I'm not sure what to check next .
Submitted: 11 days ago.
Category: GMC
Expert:  Jared D replied 11 days ago.

Good afternoon.

First, have you considered replacing the ignition switch to make sure everything is working normally there?

Second, do you have any trouble codes set in the system? Check engine light coming on?

Customer: replied 11 days ago.
I already replaced the ignition switch.
My code reader won't power up on the obd2 plug
Expert:  Jared D replied 11 days ago.

Ok. But the check engine light is coming on?

Customer: replied 11 days ago.
The gauges actually don't work either and all the lights on the dash stay on at all times including check engine
Expert:  Jared D replied 11 days ago.

I would probably start by disconnecting the SP205 splice under the dash and looking for corrosion or water damage.

Customer: replied 11 days ago.
Is that the big plug with the flip lock?
If so, it is clean
I also disconnected the plug from switch to fuse box and checked continuity through all wires.
Wherever the red wire comes from I was thinging I may find the problem, but I don't know where it goes
Expert:  Jared D replied 10 days ago.

You might check this ground strap as well. Then look underneath the fuse box for wiring damage- the rodents like to get in there and make a mess.

Expert:  Jared D replied 9 days ago.

Let me know if you need additional info.

Please don't forget to rate- that's how Just Answer compensates us for our time.

Customer: replied 9 days ago.
The grounds are fine.
I need to trace the power back from the shift solenoid to the ignition switch .
Is there a fuse?
Expert:  Jared D replied 8 days ago.

The instrument cluster on this model controls a number of the voltage streams and can even affect the shift solenoid and transmission control module.

Which engine do you have in your truck?

Customer: replied 8 days ago.
6.0 gas engine
I had the gauge cluster rebuilt awhile back, but it did not fix the gauges. the check engine, battery, abs lights all stay on. the rpm and speedo are the only ones that work. no temp, volts , etc
Expert:  Jared D replied 8 days ago.

That sounds like an internal short which could definitely cause this shifting issue. Perhaps the rebuild they did a while back was just replacing the stepper motors in the gauges. I'm willing to bet the cluster still has major problems given the way it's functioning.

Customer: replied 8 days ago.
I contacted the rebuilders after I got the nonworking gauges back and the guy told me their protocol was to test the gauges before they ship them
Customer: replied 8 days ago.
The thing is the transmission has been working fine up until a few days ago
Customer: replied 8 days ago.
Are there any fuses connected to the shift solenoid ?
Expert:  Jared D replied 8 days ago.

No it doesn't look like it. They are run straight to the PCM. Pin 17 and 18.

Expert:  Jared D replied 8 days ago.

Here is the PCM pin diagram:

Customer: replied 8 days ago.
Well I guess I can check these wires for damage
Customer: replied 8 days ago.
Which pin is power in to pcm on the pin 17 & 18 side? And ground
Customer: replied 8 days ago.
Is there a pcm fuse?
I would love to be able to interface with my scanner
Expert:  Jared D replied 8 days ago.

Ground is pin 1 and 24 power is pin 20.

Expert:  Jared D replied 8 days ago.

You might try unplugging the instrument panel connector and then see if the scan tool will communicate with the PCM. If it does, then the cluster is shorted internally.

Customer: replied 8 days ago.
I am at work now but I will check later
Expert:  Jared D replied 8 days ago.

Sounds good.

Customer: replied 7 days ago.
I unplugged the instrument cluster and still my obd2 scanner attempts to detect protocol but is unable to establish a link to the computer
Expert:  Jared D replied 4 days ago.

So it seems like it might be trying to communicate with the cluster unplugged but it still isn't working?

I'm still thinking the cluster is the cause of the issue, but I'm thinking the communication network will not complete with the cluster out. I was hoping unplugging it might make the DLC come back to life, which it seem like it does, it just won't communicate with the PCM without it.

Customer: replied 3 days ago.
I found a parts truck and am going to get the instrument panel out of it.
Also, I noticed another box under the dash with lots of plugs and wires.
Is this some kind of body control module?
Could it be my issue?
Customer: replied 3 days ago.
I replaced the gauges and it did not help
Customer: replied 3 days ago.
Which wire is hot going on to the tranny and which wire is the ground that the people sends when it is time to shift
Customer: replied 2 day ago.
Are you gonna try to help me?
Expert:  Jared D replied 1 day ago.

I'm sorry, for some reason your question dropped to my timed out list. Let me look up a wiring diagram for the TCM.

Expert:  Jared D replied 1 day ago.

So there is a vehicle speed signal that runs through the instrument cluster to the PCM and the radio (for the speed compensated volume control) if something in the cluster is shorting it could be killing the speed signal to the PCM and causing the shift problems.

The pressure control solenoid that controls the shift from the PCM is pin 8 with a light blue wire on the green PCM connector.

Customer: replied 1 day ago.
pin 8 is the ground to the shift solenoid? I heard that the solenoid has constant power and the pcm sends a ground to it to make it shift.
If so, which wire should be hot going in to the tranny.
I want to make sure I have good power and a good ground wire going to the solenoid.
If my pcm is working, then my shift solenoid may be faulty.
I wish my scanner was interfacing with the pcm.
Expert:  Jared D replied 1 day ago.

I still think the cluster is killing the communication network and causing all of these issues.

Customer: replied 1 day ago.
I replaced the cluster
Expert:  Jared D replied 1 day ago.

Oh I thought you just swapped out the gauges (the stepper motors).

Well there goes that theory. Maybe we should focus on why the PCM won't communicate:

1. Verify the fuses are powered at key on. (PCMB, PCMI in the Underhood Electrical Center (UHEC) and IGN0 in the left Instrument Panel (IP) fuse block)

2. If OK check for 5 volts on the Mass Air Flow (MAF) Sensor Yellow wire at key on, engine off.

3. If greater than 5.1 volts are measured on the Yellow wire check and repair the grounds G103 and G104 on the rear of the engine.
Customer: replied 1 day ago.
Ok will do
Expert:  Jared D replied 1 day ago.

Also, check the 15 amp CIG fuse. It should be getting 12 volts with the key on. It shares a power source with the OBDII connector.

Customer: replied 1 day ago.
Ogn0 has no power .
Other 3 have power and magic has 5.06 v
Customer: replied 1 day ago.
Maf has 5.06
Customer: replied 1 day ago.
Please let me know what to check next, I am standing by.
Expert:  Jared D replied 1 day ago.

Are you seeing 12 volts on the TBC fuse under the dash?

Expert:  Jared D replied 1 day ago.

Sorry for the delay, I went to lunch.

Customer: replied 1 day ago.
Customer: replied 1 day ago.
Ign0, tbc ign0, veh chmsl all 0 volts
Expert:  Jared D replied 1 day ago.

So that means you need to check the white wire from the back of the ignition switch for 12 volts.

Expert:  Jared D replied 1 day ago.

The white wire supplies votlage to IGN0 and TBC.

Customer: replied 1 day ago.
The white wire from the ignition switch? The small connector on the back of the fuse box next to the flasher ?
Expert:  Jared D replied 1 day ago.

Your best bet is to remove the steering wheel lower cover and check for power there. Leave the connector plugged into the back of the ignition switch and probe the white wire with the key in the run position.

Expert:  Jared D replied 1 day ago.

You can also pull the connector at the back of the under dash fuse box- it will be a white wire there too. If you have power at the switch, you will need to look for power at the back of the fuse box, so you could skip checking at the switch and just check at the fuse box.

Customer: replied 24 hours ago.
I have ground at the white wire when it is plugged in to the fuse box and when I unplug it from the fuse box, the white wire shows power
Expert:  Jared D replied 24 hours ago.

Ground when plugged into the fuse box? That means there is a dead short in the fuse box, but it would be causing a ton of heat. As long as you have power on the white wire from the ignition switch to the fuse box then the ignition switch is ok.

Expert:  Jared D replied 24 hours ago.

If plugging it into the fuse box grounds it then the fuse box is bad, but again, 12 volts from the ignition switch straight to ground would cause a fire.

Customer: replied 24 hours ago.
Well, I have been using the white wire as my ground with my voltmeter to test all the other wires
Expert:  Jared D replied 24 hours ago.

White brown and pink are all power coming from the ignition switch.

Customer: replied 23 hours ago.
Should I cut the white wire going in to fuse box and power the 2 fuses ?
Customer: replied 23 hours ago.
Or do I split the fuse box and look inside?
Expert:  Jared D replied 23 hours ago.

I would figure out what is keeping the power from making it through the fuse box. You could try powering the white wire and see if the power shows up at the fuses, but then that wouldn't make sense if you are getting 12 volts from the white wire. Just plugging it into the fuse box would power those two fuses unless there is a problem in the fuse box.

Customer: replied 22 hours ago.
The white wire has power until I plug into the fuse box
Expert:  Jared D replied 22 hours ago.

It has to be a problem in the fuse box then.

Customer: replied 22 hours ago.
Ok my parts truck has pne
Expert:  Jared D replied 22 hours ago.

I'm leaving the shop for the day. I'll check in later to see how things are going.

Customer: replied 22 hours ago.
Customer: replied 14 hours ago.
there are 4 fuses next to each other, 3 in a row and one under the 3 that have no power. The one on the bottom is veh stop.
I pulled the 4 fuses one by one and tried to interface my obd2.
It tried to link but failed each time.
Is there another wire going into the fuse box that is connected to the pcm circuit that may be causing my white wire to lose power?
Which fuse allows the pcm to interface?
Expert:  Jared D replied 6 hours ago.

The thing that doesn't make sense is that the white wire loses power when plugged in. It's not a 5volt reference voltage or anything, it's a full 12 volt full battery amperage wire that supplies the PCM. Plugging it into something can't make the wire lose voltage since the voltage runs from the battery, through the ignition switch, to the wire.

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