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      Allison 1000/2000 series transmission technical tips
The Allison world series transmissions models 1000,2100,2200,2400,2500 models and variants are found in large variety of truck chassis and busses. Also found in 2001 and up Chevrolet/GMC pickup trucks 2500-3500 chassis. models come  wide or close ratio gear sets, with or without PTO provisions,  to properly id these units you need the serial number off of the Manf. tag on the right side of the unit, some earlier tags show what model the unit actually is, part numbers etc. give serial number to any Allison dealer and unit can be identified. beware of  wrong tags on cases as this can be a common problem. you may be able to identify unit thru OEM chassis Manf./VIN #. (Best bet is to ID unit with both Allison serial number and thru VIN etc. Match the information from both!)

Allison 1K/2K Transmission Tech Tips

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Complaint: banging, or erratic shift feel on one or all shifts, may correct temporarily if adapts reset but comes back, no codes present.
can be a damaged tone wheel on output (spacer on output can break causing this) damaged wheel or PTO gear on drum, damaged converter "bumps" and or erratic speed sensors/wiring from trans to TCM for one or more of speed sensors faulty. these are all very common issues with these units. can be seen using Allison DOC software, otherwise its trail and error! the slightest nick or dent in rear tone wheel, dented or out of round drum ring (non PTO) or damaged "bump" on converter where input sensor reads from can drive you crazy. wiring is common issue too, corroded wires, loose sensor connectors and so-on.
Codes P0871 or P0872 shift valve E stuck on or off.
Cause: Debris has stuck shift valve E in valve body. internal damage such as bearing etc can cause this. if happens on replacement unit right away may be debris from your cooler got in new unit. spring in VB is weak, company called Sonnax makes replacement spring that works wonders, part#37000-02 If you take valve body apart you may or may not actually find valve stuck, very sensitive, timing of stroke can create codes, this is extremely common issue. if pan is loaded with debris your wasting time trying to fix with spring/cleaning
Complaint: Truck will not back up, may or may not move forward, have upshift problems. May or may not have trouble codes.
Cause: can be NSBU switch on shift lever. if has code for this change out switch, no codes: its guess unless you can see data on DOC or equivalent. can also be other code issues and or mechanical problems inside. good test to try is with engine OFF, disconnect transmission harness at right rear, and start engine. should go into reverse and in forward would be in 3rd gear all the time. if you get back reverse you know internals of unit are ok. diagnose electrical/control problems.
Complaint: No reverse, no trouble codes.
Can be simple problem here: transmission is low on fluid. check and add as neccasary, and find source of your leak as well. Alot of 2006 and up trucks are having cooler line fitting leak problems, usually right at bell housing fittings.
Complaint: ATF venting out of trans vent, GM trucks, others.
problem found in 2001 thru 2003 GM pick up trucks with 1000 series transmission, When let sit overnight or extended idling before driving, worse in colder weather. Lube oil is not draining back into sump quickly enough causing pressure build up and venting. GM has updated dipstick Part #15115171 which has vent slot added in it to vent more pressure out of unit. This can happen in any Allison transmission, you can also drill small hole in top of fill tube by stick handle end, .062 hole will alleviate chronic venting.
Complaint: Broken P-2 planetary and or broken C-2 clutch hub in spline area.
This is due to driveline angle and or torsional vibration due to out of balance drive shafts and driveline problems. On rare occasions this can also be due to engine miss fires due to injector problems, may or may not be noticeable or have trouble codes.if this is found in a transmission being repaired you must find the cause or it will be back again. CLICK HERE for pictures.
Complaint: MLPS switch plugs wont come out, possible broken switch or connector upon removal (up to 2004 model years).
Cause: Heat has melted the sealant between the plug and switch, you can usually safely remove the plugs if stuck by warming them up with Bic lighter or similar low heat source. pry gently with small screwdriver etc. if you crack the switch or plug ends you will need to replace them, the switch number is: 24229422 or 12450016 Allison part # 29540479 (all are same switch) and fits 4L60E, 4l80E and Allison 1k/2k units up thru 2004, the repair ends for the harnesses are available: large connector part number:15305887 and small connector part number:15305925 from any GM dealer.
Complaint: Truck will not upshift out of first gear, may or may not have codes.
Cause: can be output speed sensor is no good, TCM doesn't know truck is moving. if Ckt is good may not have any codes. this can also happen with input sensor and TCM will simply not know engine is running. however should set code if moving eventually.
Complaint: Front seal/bell housing area leaks.
Cause: One big problem here is the bolt pads on the torque converter crack and cause leaks, this can happen with reman converter and we strongly reccomend you only install Reman torque converters with an upgraded billet front cover to avoid this comeback after rebuild or replacement of the torque converter. A small leak in GM pick up 1000 models can be you need to install bronze washers on the bottom bolts inside the housing to prevent weepage. and lastly on rare occasions you can have a cracked main pump housing right in the suction or pressure cavity. Obviously you can have spun pump bushing causing a leak etc. rarely is a front area leak just the front seal  alone.
Harsh downshifts, Gm 4x4 pick up only with NP 263HD transfer case.
Cause: The rear transfer case output shaft bushing is completely wiped out, this can cause the output speed sensor which is located in tail housing of transfer case instead of rear of transmission to grind into the speed reluctor wheel and causing harsh downshifts. May also have trouble codes P0776, P0746 in memory. Alot of times were seeing that the bushing is completly gone and the drive shaft yoke is grinding into the tail housing itself.
Codes P0746 sol.A stuck off, P0747 sol.A stuck on, P0776 sol.B stuck off,P0777 sol.B stuck on. alone or together and may be with other codes.
(if other codes diagnose those first.) Cause: You may automatically think this means solenoid A or B are bad? not usually the case at all. these codes mean a shift that was using these solenoids to happen did not occur when commanded, or did not occur on time etc. this usually means internal damage such as clutches, broken hub, gearset, stuck valves in valve body and so-on. usually replacing one or both of the solenoids is a waste of time and money. on rare cases it can be the solenoid! these two solenoids are very busy guys if you look at flow charts in manuals on all shifts.
Second issue can be erratic speed sensor signals which you will likley only find by graphing the sensor signals. This can cause TCM to set solenoid codes as sees what it thinks is a slip condition or similar.
Allison transmission ID issues
Allison transmissions are best identified with serial number and part number on tag which is located on right rear passengers side of transmission. There's a TON of wrong units in trucks, wrong tags and so on out there. (You may also encounter an Allison tag that's stamped wrong as well! It can be a real joyous occasion here!) The last guys got the laugh! Beware if changing one unit from truck to truck and so on. you can create all kinds of issues that can be very difficult to diagnose. See sample Allison tag below: 
allison transmission tag
Recommended fluid for Allison 1000 and 2000 series transmissions
Allison Transmissions currently only recommends the use of TES 295/468 fluids in all transmissions. this is synthetic based fluid and available under the following brand names/suppliers: Castrol: Transysnd, BP Lubricants: Autran Syn 295, Cognis Corp: Em Gard 2805, International Truck: Fleetrite Synth.ATF. John Deere: HD Syn tran, Mobil: Mobil Delvac Synth. ATF, and Volvo N.A.: Bulldog Synth. ATF. They no longer recommend the use of Dexron fluids in any transmissions. However if you have pre 2006 Transmissions the recommended fluid before then was dexron 3 and equivalents
allison transmission diagnostics codes
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