New GMC owner needs help.

pike_mazter · 12-01-2007, 12:22 PM

Great looking site!

Just bought a 98 GMC 1500 Ext. Cab 4X4 with an auto trans.

1st problem - Gas guage is always between half and full even if the tank is really empty. From what I already understand it may be the ground wire for the fuel pump or the fuel sending unit. Ran it out of gas (luckily in my driveway after a long day of driving) and it still showed 3/4 of a tank. Any thoughts?

2nd problem - Drove the truck yesterday all day with no tranny problems. This morning started it up to head out for work and the tranny would not downshift for more power when I would try and pass another vehicle. Temps. here in Michigan were below freezing so I just assumed the cold was the problem. Trans fluid is not burnt or dark and its full at normall levels. If its not downshifting because of the temp outside how or can it be fixed. I also think it may have something to do with an electrical sensor.

Any help would be great. Once I get some time I'll throw some pictures up of the new truck, its NICE...

75K30 · 12-01-2007, 12:43 PM

Welcome to the site.
Super cold here in So. Ca too. Might get to be around 71 deg today, burrrr.
Just jokin.
About the truck, you can try to verify the wiring of the sending unit, but since you have some range within the gauge, this tells me that the float in the sender is possible stuck and not working below half of a tank. You will undoubtedly be changing the fuel pump soon, and since the pump and sender are a comlete unit, you will kill two birds with one stone.
The trans is a 4L60E transmission and is electronically controlled. There are as a couple of solenoids that are responsible for the shifting operation. Many times these can be replaced and it generally takes care of the problem. They are very inexpensive, but require that the pan be removed. PLan on doing a full service while it is apart.
A quality trans shop should be able to diagnose the problem.
Does it shift this way after it warms up?
Does it shift through the gears normally, and just has a downshifting problem?

pike_mazter · 12-01-2007, 12:51 PM

Quote:

Originally Posted by 75K30

Does it shift this way after it warms up?
Does it shift through the gears normally, and just has a downshifting problem?

The truck warmed up and it still did not want to downshift, but yesterday it was not a problem, all day.

Have yet to shift it through the gears manually, was an hour and 1/2 late for work this morning. Shifting through the gears automatically works fine.

Also, the truck has had some (what I believe) computer chip upgrades. The reason I say that is becuase on the computer under the hood and on the fuel cap there are stickers that say "92 Octance or Better Only" . This leads me to believe that there is a slight possibility that the previouse owner may have adjusted the shift timing through the computer (if you can do so not really sure) and that I may be just driving it totally different than he did. Its a litter far fetched but I'm at a loss with this one.

75K30 · 12-01-2007, 01:15 PM

Man buddy without having the vehicle in fron of me, and being able to look at the transmission shift tables, I could not begin to tell you.
Did have a firechicken with a 406 Lingenfelter that had a similar downshift issue, I had to rewrite and knock down the P0121 diag.
Depending on what type of tune you have for the trans, it is very likely that the downshift for WOT was eliminated. I see this often for vehicles with bigger engines or Nitrous equiped vehicles where the extra torque is optimized by staying in a higher gear, and not downshifting. If the upshifts are correct, this might be the case. Will it downshift manually alright?

pike_mazter · 12-01-2007, 01:33 PM

Downshifting manually if fine. Just ran outside and drove around the block a few times shifting both up and down with no problem but when I floor the pedal it does not want to shift, feels like its having problems with shifting from 3 to D and then D to 3.

Also, there is a high pitched almost alarm like constant humming coming from under the truck towards the middle. This humming was not there yesterday and it hums no matter what the truck is doing as long as its running its humming.

pike_mazter · 12-01-2007, 01:40 PM

ON the fuel cap and under the hood on the computer it has an aftermarket sticker that says to use 92 Octane or better. Last night it ran out of gas and I had to put in regular gas from my lawnmowers can so I could make it to the station. Would that have anything to do with the way its shifting? It was only 1 1/2 gallons of regular at the most, but then I filled up with 93 octane assuming it would not hurt anything.

75K30 · 12-01-2007, 01:58 PM

The octane rating will have nothing to do with the transmission. The trans knows not what type of fuel you are running. This is there only so the engine can be run at with higher timing curves and prevent the dreaded "ping".
You humming noise is the fuel pump, it probably made this nose before and is not something that you notice a little bit more, although running a tank empty does tend to heat up the little pumps a bit, and they can make a little noise after a dry run, but it indicates that the pump is actually running. I have one truck that is silent as silent can be, and one that really lets you know that it is on.
The shifts indicates that the thing is working correctly, but the 3-4 shift you say is not quite right. These are directly related, and leads me back to my initial idea of a bad solenoid, however, it will require that a technician get into the system to look at the tune.
I would have to run scan at WOT and run a log to determine operation.
Some might guess TPS inop, but everything else seems to be shifting correctly, and does not seem to be the case.
Have seen little things like the harness at the trans become a problem, but usually this leads to more problems than just a 'no downshift'.

pike_mazter · 12-01-2007, 02:23 PM

Thanks for all your help.

Looks like I'll have to run it out to a tranny shop to have them look at it. I just dread the "your tranny is shot" line.

75K30 · 12-01-2007, 02:49 PM

Since itis shifting, I doubt that anyone could justify that, but I know what you mean, I hate that.
Hard to find a good shop.

pike_mazter · 12-01-2007, 03:51 PM

I'm thinking now (after taking it for another drive) that my problem may be that the pump is no delivering enough fuel. On top of its new high pitch whine after running it out of gas last night it just feels like its no delivering enough fuel when I gas it hard, normall acceleration is perfect but the minute I try and get up and go (in any gear) it chugs. Oh well, time for a new pump anyway.

12-01-2007, 12:22 PM	#1
pike_mazter Full Member Join Date: Dec 2007 Location: Oakland County MI Posts: 15	New GMC owner needs help. Great looking site! Just bought a 98 GMC 1500 Ext. Cab 4X4 with an auto trans. 1st problem - Gas guage is always between half and full even if the tank is really empty. From what I already understand it may be the ground wire for the fuel pump or the fuel sending unit. Ran it out of gas (luckily in my driveway after a long day of driving) and it still showed 3/4 of a tank. Any thoughts? 2nd problem - Drove the truck yesterday all day with no tranny problems. This morning started it up to head out for work and the tranny would not downshift for more power when I would try and pass another vehicle. Temps. here in Michigan were below freezing so I just assumed the cold was the problem. Trans fluid is not burnt or dark and its full at normall levels. If its not downshifting because of the temp outside how or can it be fixed. I also think it may have something to do with an electrical sensor. Any help would be great. Once I get some time I'll throw some pictures up of the new truck, its NICE...

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12-01-2007, 12:43 PM	#2
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	Welcome to the site. Super cold here in So. Ca too. Might get to be around 71 deg today, burrrr. Just jokin. About the truck, you can try to verify the wiring of the sending unit, but since you have some range within the gauge, this tells me that the float in the sender is possible stuck and not working below half of a tank. You will undoubtedly be changing the fuel pump soon, and since the pump and sender are a comlete unit, you will kill two birds with one stone. The trans is a 4L60E transmission and is electronically controlled. There are as a couple of solenoids that are responsible for the shifting operation. Many times these can be replaced and it generally takes care of the problem. They are very inexpensive, but require that the pan be removed. PLan on doing a full service while it is apart. A quality trans shop should be able to diagnose the problem. Does it shift this way after it warms up? Does it shift through the gears normally, and just has a downshifting problem?

12-01-2007, 01:15 PM	#4
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	Man buddy without having the vehicle in fron of me, and being able to look at the transmission shift tables, I could not begin to tell you. Did have a firechicken with a 406 Lingenfelter that had a similar downshift issue, I had to rewrite and knock down the P0121 diag. Depending on what type of tune you have for the trans, it is very likely that the downshift for WOT was eliminated. I see this often for vehicles with bigger engines or Nitrous equiped vehicles where the extra torque is optimized by staying in a higher gear, and not downshifting. If the upshifts are correct, this might be the case. Will it downshift manually alright?

12-01-2007, 01:33 PM	#5
pike_mazter Full Member Join Date: Dec 2007 Location: Oakland County MI Posts: 15	Downshifting manually if fine. Just ran outside and drove around the block a few times shifting both up and down with no problem but when I floor the pedal it does not want to shift, feels like its having problems with shifting from 3 to D and then D to 3. Also, there is a high pitched almost alarm like constant humming coming from under the truck towards the middle. This humming was not there yesterday and it hums no matter what the truck is doing as long as its running its humming.

12-01-2007, 01:40 PM	#6
pike_mazter Full Member Join Date: Dec 2007 Location: Oakland County MI Posts: 15	ON the fuel cap and under the hood on the computer it has an aftermarket sticker that says to use 92 Octane or better. Last night it ran out of gas and I had to put in regular gas from my lawnmowers can so I could make it to the station. Would that have anything to do with the way its shifting? It was only 1 1/2 gallons of regular at the most, but then I filled up with 93 octane assuming it would not hurt anything.

12-01-2007, 01:58 PM	#7
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	The octane rating will have nothing to do with the transmission. The trans knows not what type of fuel you are running. This is there only so the engine can be run at with higher timing curves and prevent the dreaded "ping". You humming noise is the fuel pump, it probably made this nose before and is not something that you notice a little bit more, although running a tank empty does tend to heat up the little pumps a bit, and they can make a little noise after a dry run, but it indicates that the pump is actually running. I have one truck that is silent as silent can be, and one that really lets you know that it is on. The shifts indicates that the thing is working correctly, but the 3-4 shift you say is not quite right. These are directly related, and leads me back to my initial idea of a bad solenoid, however, it will require that a technician get into the system to look at the tune. I would have to run scan at WOT and run a log to determine operation. Some might guess TPS inop, but everything else seems to be shifting correctly, and does not seem to be the case. Have seen little things like the harness at the trans become a problem, but usually this leads to more problems than just a 'no downshift'.

12-01-2007, 02:23 PM	#8
pike_mazter Full Member Join Date: Dec 2007 Location: Oakland County MI Posts: 15	Thanks for all your help. Looks like I'll have to run it out to a tranny shop to have them look at it. I just dread the "your tranny is shot" line.

12-01-2007, 02:49 PM	#9
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	Since itis shifting, I doubt that anyone could justify that, but I know what you mean, I hate that. Hard to find a good shop.

12-01-2007, 03:51 PM	#10
pike_mazter Full Member Join Date: Dec 2007 Location: Oakland County MI Posts: 15	I'm thinking now (after taking it for another drive) that my problem may be that the pump is no delivering enough fuel. On top of its new high pitch whine after running it out of gas last night it just feels like its no delivering enough fuel when I gas it hard, normall acceleration is perfect but the minute I try and get up and go (in any gear) it chugs. Oh well, time for a new pump anyway.