!!!!HELP!!!!2001 Ally 1000 issues!!!!!!!!!!! - Chevy and GMC Duramax Diesel Forum
01-04.5 LB7 Duramax Powertrain Discussion of components that are directly involved in the power production and all that is needed to get and keep the truck moving . Engine , Transmission, Etc.

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post #1 of 3 (permalink) Old 08-17-2019, 06:06 AM Thread Starter
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Join Date: Aug 2019
Location: Florida
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Unhappy !!!!HELP!!!!2001 Ally 1000 issues!!!!!!!!!!!

Here goes nothing, I have an 01 SIlverado 275k 2wd Duramax with the Ally 5 speed. The issue I am having is when you put it in reverse it works fine but when you put it in drive it goes in gear lurches and then kicks out of gear into what I assume is neutral! I checked the wiring harness, TCM and connectors. I bought new batteries and put them in. I cleaned all the grounds everywhere! I changed the fluid and filters just because I checked the internal harness but the pan was clean all except for minimal build up on the magnet. The fluid was clean and didn't smell burnt at all. I cleaned and inspected all the pins on the trans connector and the checked for push back on the TCM connector pins. I had issues with wheel speed sensors and repaired that issue. I also changed the NSBU and the hall-effect sensors in the trans. I had my brothers push the truck and it started to go under its own power and shifted through all the gears but I had to baby the throttle to get it to do so! When I got home I pulled in because I new I had reverse but to my surprise I didn't! The next morning I went out and reverse was back but no forward aside from the lurching forward and the kicking out of gear! The shift indicator flashes when this happens and it does it in D-3-2-1! I have read the codes with my Bosch 1300 and it does read TCM codes. I got the following... P0700 generic you got a problem code... P1720 trans solenoid controlled clutch engaged, also P1720 Solenoid A controlled clutch stuck OFF...P1875 4wd low switch circuit to electrical but my truck is 2wd...and 0500 vehicle speed sensor A even though I changed them all already! Man I need y'alls help bad!!!!! Thanks in advance guys!

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post #2 of 3 (permalink) Old 08-17-2019, 06:20 PM
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Join Date: Jan 2019
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Boy oh boy, you have a mess it sounds like, I really have no idea to help you besides telling you to take it to a shop (not dealer) that knows about the Allison an is good with them, hopefully the bump will get someone in here that knows

2002 LB7 5 inch exhaust, ppe boost valve, boost/egt/rail pressure gauges, fass 95
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post #3 of 3 (permalink) Old 08-18-2019, 10:36 AM
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Location: Berryton, KS
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Here is information gleaned from other sources:

"DTC P1720 sets when the TCM detects an incorrect oncoming ratio, range to range, for an accumulated number of occurances".
This DTC indicates the on-coming clutch being controlled by Solenoid "A" is not applied or applied too slowly. This could indicate a leak or obstruction in a specific clutch or apply circuit. Check scan tool failure record for data for previous or current range information when the DTC was set to determine the specific shift when the DTC was set.
Important: Clutch failure due to installation of an engine power upgrade in not covered by Manufacturer Warranty. When engine HP or torque is increased over factory rating, a shift flare condition may occur.

Reference Helms 2002, page 7-874.

There is a PI for a 1720 code, among others to inspect for burnt C1 clutches. Remove pan and check for clutch debris...


Ron Nielson
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator
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