GMC Troubleshooting Questions? Ask a Mechanic for Answers
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?
Ok. But the check engine light is coming on?
I would probably start by disconnecting the SP205 splice under the dash and looking for corrosion or water damage.
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.
Let me know if you need additional info.
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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?
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.
No it doesn't look like it. They are run straight to the PCM. Pin 17 and 18.
Here is the PCM pin diagram:
Ground is pin 1 and 24 power is pin 20.
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.
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.
I'm sorry, for some reason your question dropped to my timed out list. Let me look up a wiring diagram for the TCM.
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.
I still think the cluster is killing the communication network and causing all of these issues.
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:
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.
Are you seeing 12 volts on the TBC fuse under the dash?
Sorry for the delay, I went to lunch.
So that means you need to check the white wire from the back of the ignition switch for 12 volts.
The white wire supplies votlage to IGN0 and TBC.
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.
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.
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.
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.
White brown and pink are all power coming from the ignition switch.
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.
It has to be a problem in the fuse box then.
I'm leaving the shop for the day. I'll check in later to see how things are going.
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.