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I have a 2003 Silverado Duramax with Allison transmission. I…

I have a 2003...

I have a 2003 Silverado Duramax with Allison transmission. I have had some trouble with it going into limp mode when pulling a trailer but now its shifting very erratically at low speeds. It surges right before shifting. I also have some electrical issues and sometimes problems starting it. It idles fine

Mechanic's Assistant: How old is the transmission? Have you noticed any leaking?

Not that I have noticed.. 200,000 miles I know it needs a transmission rebuiild but I think this erratic shifting is something electrical or fuel related

Mechanic's Assistant: Are you fixing your Silverado yourself? What have you tried so far?

I havent tried anything yes myself

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Yes that my abs light is always on and it downshifts extremely hard sometimes

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Answered in 1 minute by:
3/27/2018
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,628
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Do you happen to know what the ABS trouble code is? It might be related, might not, but I'd be interested to know one way or the other.

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Customer reply replied 28 days ago
I dont know, I just called the place that ran the codes and the guy was going to call me back. it did give a 5th gear pressure code

OK. We have had a number of problems with the pressure switch manifold in the transmission, it may all that's wrong with yours. When the TCM sees something wrong with this switch input value, it will raise line pressure all the way up and give you harsh/weird shifts.

I don't know that you need an overhaul to fix this problem, it sounds more like you might need a pressure switch manifold, which is easy to replace by removing the transmission pan. I would do more diagnostic work before getting there, but that sounds like what you are describing.

If you want to run those trouble codes by me, I am happy to look into them and give you an analysis.

Please take a moment to rate my answers by clicking on the stars at the top of the page, that's how I get compensated. Let me know if you need anything else.

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Customer reply replied 28 days ago
what is a test I can do to verify that the pressure switch manifold is the problem?

The trouble codes will give it away, most likely. If you have a capable scan tool, you can shift through the gears and verify each switch closing and opening as it should, but the computer will definitely tell you if one of them is not working right, the TCM is watching these inputs very closely and seemingly sets a code for any hiccup out of this manifold.

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Customer reply replied 28 days ago
you are saying in need to get a scan tool and do what exactly? just to make sure we are on the same page, the main issue im having is that its shifting erratically especially at low speeds . Its impossibly to drive it smoothly.. The code it gave was a P07xx transmission code

I would need the rest of that P07XX code to point you in the right direction, but you mentioned that it was related to a pressure switch.

When looking at data on the scan tool, you can simply drive it while watching (or recording for later playback, to do this safely) the pressure switch activity. There is a data line for each switch, and the TCM will simply tell you what the switches are doing. If the TCM commands 5th gear and the pressure switches do not respond the way it expects, you get a code and you get default line pressure and possibly limp mode, and you can record this/watch this as it happens and pick apart the data to see exactly what is going on.

Let's start with a trouble code, though, to make sure we are going in the right direction.

Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 5,628
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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Customer reply replied 27 days ago
U1000 - class to data link malfunction
P0522 - Oil pressure sensor
C0040 - right front wheel speed sensor circuit/ABS

U1000 is a very generic code that indicates a communication problem. It's probably stored in history, not current, and diagnosis of it is impossible unless the symptom/code is current. It is not unusual to find U1000 codes in just about any GM car or truck, any hiccup will cause this code to set, and it usually isn't related to anything the vehicle is doing, and is not a symptom by itself. It won't turn on a warning light or anything, if you never checked codes with a scan tool, you'd likely never know that there's a U1000 banging around in one of the computers.

P0522 is pretty straight forward, the ECM sees low voltage (or has seen low voltage at some point) at the oil pressure sensor. I would just put a new sensor in to correct this.

C0040, as you describe, indicates a problem with the RF ABS wheel speed sensor. Not related to any transmission or shifting concern.

Where is the P07XX code you mentioned earlier?

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Customer reply replied 27 days ago
I ran the codes a second time and that particular area did not come back up. I don't remember the exact code but I know it said that it was the load line pressure in the fifth gear and that is why it triggered it. Yesterday I messed with the wiring harness is in the TCM to make sure nothing was broken, I unplug the batteries for a little while and plug them back in. I also found that the electrical connection for a map sensor that was on top of the turbo I had broken and I fastened it back together. When I got into drive it it worked really well for an hour or so and then started doing all the same symptoms again. The symptoms slightly worse and until they were back to the exact level as before. I don't know if this helps but it is definitely what happened

When you disconnected the batteries, it sounds like it reset the adapts, which is the learning portion of the TCM. It goes back to a default, then over time it learns the shifts again, watching the shift time in milliseconds and making small adjustments to the line pressure. It sounds like it's relearning, but it's also going back to its old habits.

If there are no transmission codes, I wouldn't replace any parts on it yet. Let the computer figure this out, if it's acting weird enough, it will eventually give you a trouble code that you can use to sort out what's causing the issue. It might be a worn out transmission, but it could also be a bad switch manifold, a bad PRNDL switch, a bad shift solenoid.... it can get expensive just throwing parts at it. Wait until you get the trouble code again, and address it specifically from that perspective.

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