2002 Chevy 2500 HD 4WD 8.1 Allison Auto Trans

348tripower · December 16, 2012, 07:34:43 AM

I have one of these coming to the shop that keeps going into limp mode. I see a lot of chatter on the net about the neutral safety switch. I was told it has been changed before. I am kinda wondering if it has become a problem again. Anybody have any experience with these?
Thanks,
Don

wayne petty · December 16, 2012, 10:20:49 AM

check the TRANSMISSION CODEs with your scan tool...

sometimes they come up in normal mode.. they will be in the P0700 to P0799 and more above P1000...

post your codes..
there may also be a snap shot of data that was recorded when the codes set.. if you have not cleared them to allow you to examine what the truck was doing exactly when the codes set..

warning.. this can be caused also by the ignition switch contacts... i am hoping that you have a set of external torx sockets.. in the smallest sizes.. E-4, E-5-E-6 to get the column covers and the switches loose from the steering column. HF had a six piece deep socket set in their stores for under 10 bucks.. that included theses sizes.. no body else i can find has them other than snap on or mac, ect...
they are not on the web site and they are not ordering more. they are in a few stores still... check your local store.. invest if you can find a set..
item 67017 torx External deep socket set

people can take the ignition switch apart and burnish the contacts.. not impossible unless you don't have the tiny E sockets above.. oh... the music box ignition switches.. the contacts are closed with spring pressure.. not mechanical force by the pins on the roller. they are held open by the pins.. corvette C5 models have this same problem. the contact faces are silver.. so if not burned away are easily cleaned..

i am not saying the ignition switch is bad.. but voltage drop outs from the ignition switch may kick the transmission into default... as the computer thinks the solenoid has failed.. this is a KNOWN issue and i am just trying to EASE the hair pulling when you find no codes...

post some codes.. lets see where it goes..

lastly.. this chart if for OBD1.. but you will see some of the transmission sensor info scattered in there.. so you can take a hoist test drive or a road test with somebody else driving and see what the transmission and other sensors should be displaying.. notice the right side column in the chart below. that sends you to the proper trouble shooting section in the diagnostics manual...

this chart covers the 4L60E versions and the 4L80E versions the allison should have similar data but i am not sure about it..

phat rat · December 16, 2012, 01:06:48 PM

Don, read this about the neutral safety switch.

http://www.dieselplace.com/forum/showthread.php?t=338137

It's not hard at all to change.

348tripower · December 16, 2012, 02:42:21 PM

Thanks guys. The truck showed up about two hours ago. I am going to check codes and then drive it till it goes into reduced power and check codes again. It is ok right now. I will report in tomorrow with my findings.

348tripower · December 17, 2012, 03:58:06 PM

Drove this ting for two hours today and I didn't go into limp. Try again tomorrow. :?

wayne petty · December 17, 2012, 05:21:09 PM

its looking more and more like the IGNition switch contacts...

were there NO CODES STORED?

then run the voltage drop test i have posted...

then... examine the wiring diagrams for the ignition switch..

stick a test light or grab a piezo beeper and use it like a test light... so you can hear the connection problems by changes in tone... slowly work the ignition switch through the various positions... there are 5 or 6 wires.. out... that get individual power.. you need to deal with the ONE to the PCM.. unless the allison uses a PCM...

you can also ask the owner if he has a big ring of keys...

that could swing when he depresses the brake or the gas pedal... causing the ignition lock to rock slightly..

say.. how does that TPI motor run now?

wayne petty · December 17, 2012, 06:14:49 PM

module power diagram

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0996b43f/80/24/55/9d/large/0996b43f8024559d.gif

start with this one.. you can monitor the various fuses while you operate the Ignition key ..

i would think the three right fuses on the top of this diagram i would want to monitor...

underhood fuse block..
ECM 1 fuse...

on the left end of the instrument panel.
IGN 0
and
IGN 1

348tripower · December 18, 2012, 02:07:41 PM

No problem with the overload. No codes, not even a trans code. And the previous cod that he cleared was a P 1221 fuel pump circuit low. I will ask about the key ring. May just put in a switch. He is out of town for a few days. The TPI runs great now.

wayne petty · December 18, 2012, 05:10:54 PM

P1221 on a GM is not a LOW secondary fuel pump circuit..

P1221 is a Throttle position sensor code and that can be enough to kick the transmission into Default..

i only have the info for a 97 not the newer ..

it shows that there are 2 throttle position sensors...

#1 starts at 2.1 volts and goes to about 3.5 volts at wot..

#2 starts high at 3.5 volts and drops to the 1.2 volts at wot.

so one increases and the other decreases...

OBD2 codes above P1000 are MANUFACTURER specific...

each manufacturer can designate them for which ever circuit they want... i think nissan uses it for the low secondary fuel pump circuit...

find your proper TDC section in print or online at eautorepair dot net or alldatadiy dot com.. if you don't have it already...

TPS issues can and will cause the transmission to act up..

here is the wiring diagram that covers the TPS

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0996b43f/80/24/51/eb/large/0996b43f802451eb.gif

if you look at the diagram above pins 1 and 11 you will see the resistor icon.. one starts low and the other starts high..

i sure hope this helps...

UGLY OLDS · December 18, 2012, 06:06:57 PM

We have seen multiple issues with the throttle body's on these trucks ....

They will cause NO END of driveability issues .... :evil:
Not cheap either ...

Bob....

wayne petty · December 18, 2012, 08:06:12 PM

Duralast Wells Throttle Position Sensor
For your 2002 Chevrolet Truck Silverado 2500HD 4WD 8.1L SFI 8cyl
around 60 bucks..
Part Number: TPS166

http://contentinfo.autozone.com/znetcs/additional-prod-images/en/US/wl2/TPS166/3/image/4/

http://contentinfo.autozone.com/znetcs/additional-prod-images/en/US/wl2/TPS166/4/image/4/

http://www.wellsve.com/custom_searches/showpinoutdiagram.php?partno=TPS166#zoom=100

348tripower · December 19, 2012, 02:23:32 PM

Wayne,
I thanked the owner of this for the wrong explanation of the code so it made me look like an idiot. :oops: I got some more info. He said that they changed the multi function switch in the column and shortly after he began having the problem. So I vote to change out the TPS first because that is the code. Then the switch next.
Don

wayne petty · December 19, 2012, 03:06:16 PM

Quote from: "348tripower"Wayne,
I thanked the owner of this for the wrong explanation of the code so it made me look like an idiot. :oops: I got some more info. He said that they changed the multi function switch in the column and shortly after he began having the problem. So I vote to change out the TPS first because that is the code. Then the switch next.
Don

that DTC caught me also... there is not a LOT written about it online.. i had to drag out my printed book from 97 and that one only listed the corvette

look at the live data from TPS 1 and TPS 2...

slowly open and close the throttle...

do the voltages smoothly ramp up and down??? and down and up..

the ignition switch is a PIA to remove.. good luck.. there are 2 or 3 pairs of wires that connect to the various items up around the column.. even harder to disconnect the plugs from.. take your time.. the shift interlock solenoid on the right side of the column is the hardest..

the steering wheel as i recall has to come off.. if air bag equipped.. i know you will have a tie wrap ready to catch the air bag clock spring so it does not unwind..

348tripower · December 21, 2012, 03:05:08 PM

Put a new neutral safety back up light switch in. It's on the road. Waiting for a report back. He gave me some more info. TPS and the connector had just been changed. The morning he brought it to me it was having shifting problems. Seems all the indicators point to the NSBU switch so that is where I went. The link that phatrat gave me also showed it to be the old style switch. We will wait for reports. I did add dielectric grease to both connectors on the NSBU and the TPS because there was none. Some one must have an allergy

wayne petty · December 22, 2012, 01:02:14 AM

here is a GREAT forum thread on cleaning the contacts in a C5 and C6 corvette ignition switch...

this is a great and well written thread...

should probably be read by many members as it will show them how to clean various switches ..

http://forums.corvetteforum.com/c5-tech/1951626-c5-ignition-switch-repair.html