GM 4L80E Transmission Shift Problem

Reply

  #1  
Old 07-16-11, 05:17 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Cool GM 4L80E Transmission Shift Problem

I'll keep this short. Transmission shifts perfect--THEN-it drops out of high gear to low gear without warning. On ocassion parking for a while or hitting a road bump it will shift OR after it has been shifting satisfactory, park it for a while and then it will not shift. Here is what I have done: Dropped transmission pan installed new internal wiring harness, installed new external wiring harness, installed new Throttle Position Sensor, and replaced both Variable Speed Sensors. NOW, my question: WHAT OTHER PART(S) INTERNAL OR EXTERNAL WOULD PREVENT TRANSMISSION FROM SHIFTING???? I had to splice the external wiring harness to the existing wiring going to thr ECM (computer) eleven splices. All suggestions appreciated.

REBEL
 
Sponsored Links
  #2  
Old 07-18-11, 07:51 PM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
Did all of this start before you swapped the harness and the rest of the work? What was the order of repair?

Could be a number of things but need to know in what sequence things started happening.

How many miles, what vehicle?

You may need a reflash of your ECM but make double sure all the splices are tight.
 
  #3  
Old 07-19-11, 07:14 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Red face i6pwr--more info

1991 Chevrolet/one ton dully/454cid/4L80E/125,000 miles.
Problem has been intermittent for two years. Sometimes sets code 24 "vehicle speed sensor circuit". Replaced both sensors three times and each time the shift improved for a few months. Now, however, vehicle is not trustworthy 'cause it may drop out of gear or not shift out of first gear. Taken to Aamco and Cottman transmission dealers. Aamco suggested new wiring harness for $600.00/Cottman suggested total rebuild for $2100.00+. I installed internal, external wiring harness, new pigtails to both speed sensors, and new throttle position sensor. I believe it is simple electrical fault based upon the code 24 fault. I don't have an electrical schematic showing speed sensor to ECU (computer), but I think that is the next step. On a few occasions I have "wiggled" the wires going to the ECU and it would shift for a while. When transmission does shift, it is very smooth. Any suggestion on tracing wires from the speed sensors to the ECU other than an ohm meter?? Thanx for the reply.
 
  #4  
Old 07-19-11, 08:34 PM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
Does your trans have a speedo cable or a electrical harness in place of the cable...or both? Just want to be sure, I believe being an "E" trans it's all electrical but not sure about 1991.

Does your speedometer ever stop working?

When you replaced the VSS was it at the trans or under the dash?

Was the 2nd sensor you replaced the OSS ( Output Speed Sensor)?
 

Last edited by i6pwr; 07-19-11 at 08:55 PM.
  #5  
Old 07-20-11, 04:01 AM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
I wanted to add that you could also be possibly chasing a ground problem. Make sure there's no fluid contaminating the wiring harness at the trans and double check all the grounding points. Look at the pins in the ECM harness to ensure all are clean also.
 
  #6  
Old 07-20-11, 04:16 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
Just looked up the troubleshooting for the 24 code in the factory manual for my wrecker ('94 Chevy W4/5.7L/4L80E). I have wiring diagram and troubleshooting tree if you need them. The wiring diagram may or may not match what you've got.
 
  #7  
Old 07-20-11, 05:01 AM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
I want to say the connectors changed in 93 but I the procedure should be similar.
 
  #8  
Old 07-20-11, 10:21 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
Could be; there are two different case designs, too, but I'm guessing the internals and electronics may be the same.
 
  #9  
Old 07-20-11, 01:21 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
ip6wr/the_tow_guy

ip6wr--Speedometer works fine-I believe it is electric, no cable. There is a single wire coming off the top of the transmission going into the bundle of wires going to the ECU, but I have no idea of its function. The speed sensors are located on the driver side of the transmission/ one toward the front and one toward the back just aft of the wiring harness connection. Both sensors are the same part number and interchangeable. Not sure of the function of each sensor. I have not tried to access the ECM yet to check the connections 'cause I am not sure which connector to check or even where the ECM is located. Probably under the passenger side dash?

the_tow_guy--The internal and external wiring harness(s) I installed are an upgrade to the 1994 configuration. The schematic and troubleshooting guide may help/how do I get a copy?

Appreciate the suggestions.
 
  #10  
Old 07-23-11, 05:16 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
PM me your email.
 
  #11  
Old 07-29-11, 01:14 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Update--No Cigar

Update-removed ECU--found no corrosion or loose connectors. Reinstalled ECU and took test drive. Went 10 miles transmission shifted perfectly. Stopped at post office for five minutes, started vehicle, transmission would not shift--returned home in low gear-did not set a trouble code. Next step check wires from VSS to ECU with ohm meter and also check VSS--should be 130 ohm. Troubleshoot using schematics received from the_tow_guy.

Rebel
 
  #12  
Old 07-29-11, 01:23 PM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
Whenever it acts up, does it work again after disconnecting the ecm?
 
  #13  
Old 07-29-11, 02:07 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Will try

i6pwr I thought about trying that, but I was too busy limping home and the connectors are hard to reach since I reinstalled the ECU. I did "tap" on the ECU and wiggle the connectors, but no luck. I will give that a try on my next road test. If it shifts after disconnecting and reconnecting the ECU what would that indicate? On one occasion when I was stranded because it would not shift, I unplugged the external to internal wiring harness and it shifted after I reconnected. Any thoughts? Also, the _tow_guy sent a troubleshooting guide for code 24 that references a "digital ratio adapter", but I don't find one in this vehicle.

Thanx,

Rebel
 
  #14  
Old 07-29-11, 02:17 PM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
Probably like a regular dtc code being stored....once you remove power to the ecm or clear it....the limp mode goes away until it rests the code.
I've had this prob with my throttle body since someone cut the harness.

I get a limp mode until I restart it. In my case its prob a bad crimp. Very small voltage differences will wreak havoc. Our jetta before we sold it kept throwing throttle body codes and dealer swore it needed replacing. A buddy and I went over it over and over and after cleaning all the grounds, added another small ground to chassis from battery and prob went away.

Like I said earlier in your case...it could very well be a bad ground. Find all the grounding points and remove the wires, cables and clean with sandpaper.

If I were trying to figure your prob out I would at least ensure ALL grounds are clean. All cables, both ends....trans prob has one bolted somewhere.
 
  #15  
Old 07-29-11, 02:24 PM
Member
Join Date: Feb 2009
Location: USA
Posts: 323
That just gave me a thought.......have you EVER removed the harness from your throttle body...for any reason?

Has it ever been cleaned around the time these probs started happening?

The holding pressure for the bands in the trans is related to the throttle position. Sometimes it may need a reflash by the dealer or anyone with a Tech II tool.

When I get a reduced power light or "limp" mode, I only get 1 gear and no shifts.
 
  #16  
Old 07-30-11, 06:27 PM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
I think my limp mode is 2 gears, but I wouldn't swear to that, might only be 2nd.

Several years ago I had a problem that in the same type category, but not quite the same. I would periodcially lose hi gear. The fix would be to pull over, kill the engine, and then restart. My recollection is that it was a harness problem and was fixed by some rewiring/resplicing work. Can't say for certain, but Monday bill-passing-out day so I'll be seeing my favorite tranny guy (also the one who fixed my glitch way back) so I'll print this thread out and try to run this whole episode by him while he's writing my check.

Besides unplugging the ecm, you can also reset things by pulling the appropriate fuse(s) and putting them back in.
 
  #17  
Old 01-05-12, 04:35 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Update--Still Broke

Time for an update-spent so far $350.00. Truck has been parked for months. Talked to i6pwr who suggested that I remove the two speed sensors and clean surfaces to ensure that no metal particles/contaminates were attached to face of sensor. I pulled both sensors and used alcohol to clean. Reinstalled sensors/started truck and the transmission SHIFTED. I drove about five miles stopped and bought a case of Millers. Started the truck for the return trip and it did not shift, limped home--there were no codes set. My theory is that problem is probably a loose wire to the computer or ground OR the computer itself is defective. Might just purchase a new computer since I know of no way to test. I used ohm meter to test sensor connectors to computer connectors and discovered continuity to several pins.

I'll keep atryin'.

Its all fun
Rebel
 
  #18  
Old 03-08-12, 04:20 PM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
The final answer!!!

After 'bout a year and five hundred bucks the transmission is fixed!! This answer is to all who, as I, look for the complicated "fix". I won't rehash all that I tried just to keep this short(I even tried a new computer). I stopped in a tranny shop to explain my problem(s) and ask for a look-see price. The guy said "I bet it is the "Shift Selonoids". He said it is an easy fix--drop the pan, located the two selonoids, remove the two torx screws, and install new selonoids. I did and the transmission is shifting like new. Cost: $25.00.

Thanks to all who made suggestions.

Its all fun!
Rebel
 
  #19  
Old 03-09-12, 04:29 AM
the_tow_guy's Avatar
Group Moderator
Join Date: Feb 2001
Location: SW Fla USA
Posts: 11,541
Cool. I don't recall, were the shift solenoids on the troubleshooting tree? I don't think I've ever had a shift solenoid fail on mine through 3 trannies and 609,000 miles.
 
  #20  
Old 03-10-12, 11:16 AM
Member
Thread Starter
Join Date: Feb 2002
Location: USA
Posts: 86
Shift selonoid location.

the_tow_guy

The shift selonoid(s) on a 4L80E are located rearward toward the driveshaft.
The selonoids are in the valve body held in by either a clip or a torx screw. Each has a connector from the wiring harness encompassing two color coded wires. They cannot be installed incorrectly due to the physical configuration of the part and the wiring harness connectors. It is claimed that they can be checked with an ohm meter and should display "bout 22 ohms resistance. However that is not assured due to the fact the reading may be accurate but the selonoid may be restricted as to fluid flow. In my case, the ohm reading was correct, but the selonoids were obstructed. At a Cost of $12.50 each, best bet is to replace.

Good to hear from you again.

Its all fun

Rebel
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
'