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03 GMC fuel problems, work in progress.

M

Mike in Mn.

Member
Posted a while back. 03 GMC K3500 cab and chassis. 8.1 gas engine not using fuel from rear tank. finally getting some time to check this out, from your suggestions and reading the book here's what I know so far, I have no power to the rear pump, engine running and front tank half full. Finally Found the secondary fuel pump relay under the fuse box under the hood, the fuse that works with the relay is the power train control module fuse and it's good. By the wiring diagram looks like both the primary and secondary fuel senders send a signal to the power train control module and it sends signals on to both the relays for both fuel pumps. Does any one know where the power train control module is located?? I think that is where the problem is, any suggestions welcome, thanks. By the way this truck only has 6000 miles and has never been in the snow, so no corrosion any where.
 
The one I worked on had a module locate @ inner frame near front tank that was connected to switching circuit--maybe a year-2 earlier than yours--
 
The PCM is located above the steering box, under the battery. I doubt that is your problem. The rear pump only runs to transfer fuel to the front tank, it does not run all the time. The PCM reads both fuel level sensors, then transfers fuel to the front tank as needed. The PCM also uses both level sensor readings to provide an average for the fuel gauge. You need to monitor the data list and see what the front and rear sensors are telling the PCM. My bet is one or both are giving a false reading, such as saying the front tank is full or the rear tank is empty. Fuel level sensors are failure prone. BTW, there is no frame mounted module on an '03, that was on '90s models.
 
Thanks for your post, I have been running with just the front tank all summer and the fuel gauge seems to be accurate. When the front tank is almost empty the gauge reads one half, when I fill it the gauge reads full, of course the rear tank is full all the time, seems like it's averaging the two, I will keep looking, when I get done delivering this fall maybe I will just pull the box so I can pull the senders and check them out, thanks again.
 
It would be a good step to check for codes, more than likely the PCM would store a code related to a malfunction with fuel transfer. A code reader or better yet a scanner that can read the fuel level sensor data could be valuable here.

The fuel level sensors are known to cause problems on these trucks. Im not saying you need to replace the part(s) but keep this in mind. A malfunction in either fuel level sensor will disable the system. So the front tank level sensor could be faulty too. Most malfunctions detected by the PCM should cause the fuel gauge to drop to "E"

It also seems like its a bad idea to fill the rear tank first, or fill fuel with the key on and/or engine running. This can "freak out" the PCM when it see's strange fuel level readings and if the PCM see's that and stores a code then the transfer system will be shut down. Remedy for this might be to reset/erase codes and check system from proper operation. If you don't have a way to erase codes, try unplugging the battery for a few mins.
 
Thanks for your post, twice when I have used it the fuel gauge has stayed on empty and the low fuel light is on, turned it off and re-started and then it was ok, I guess my next step will be to get it hooked up to a scan tool and see whats going on. I have the system figured out, I have just been looking for something obvious, like a bad pump or fuse or relay, all good.
 
Interesting that they did away with the external module on the light trucks. The medium duty trucks used the frame mounted module until the trucks were discontinued in July 2009.
 
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