Output Speed Sensor and Various Other Sidetracks


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As most of us know AM General lends factory support on all the major Hummer Club events. What they usually do is send a trailer full of parts, a couple of Hummers and experienced mechanics to the event. When it comes to technical help you will always find one of these two guys, John Ward or John Klotte at every event. It's their job to take care of all the dealers' technical and warranty problems. Ward covers the dealers East of the Mississippi and John Klatte is the technical rep in charge of the other half of the country. I know for a fact that they also get sent overseas to solve foreign customers problems. Both of these guys are walking encyclopedias and have witnessed about anything and everything there is to see about maintaining and troubleshooting a Hummer. I've had the pleasure of meeting both of these guys at a number of Hummer Club trips. Each time I run into them I'm able to pick their brain for a little insight into what makes our trucks tick. While I was out in Colorado at the Hummer Club Ouray 2000 event this year I had a chance to talk to John Ward about a number of subjects. We were out at the trailer talking maintenance when he started telling me about a little known problem that has an easy fix that's become a real nuisance. As we all know the Hummers have a bunch of sensors that are located all over the place. When a sensor goes bad it sends the computer incorrect information and the truck doesn't run right. The sensor that's been causing problems is the Output Speed Sensor.

Note In a NA diesel, the speed sensor in the t-case is only for the speedometer and cruise control. It does not connect in any way to the TCM, and is unnecessary if you don't care how fast you are going.

Speed SensorI filed away what John had told me figuring that I'd look into it when I got back home. Thursday was a free day in Ouray which meant that we didn't have any scheduled trails. Usually people went sight seeing and shopping or just relaxed and chilled out. Usually there is one group that just can't stand still and of course we had to go out exploring. Mark Sanders, Bill Kame, Gabe Vlad, Bob Davis and me as the trail leader. We went out to explore Carson, a ghost town and to find another route back to Silverton through Stony pass. We had a great time exploring the area. Around 4 or 5 o'clock we found ourselves on the back side of the mountains in the rain. At least we were on the highway. We started heading for Creede the place where Stony pass was supposed to be. By the time we got to Creede it was raining hard, getting dark and we hadn't found the road to the pass. We decided that it would be too dangerous to drive a road on any pass in the dark when it was raining. Our only alternative was to drive 250 miles around the mountain and back to the motel. Gabe was tail gunning on the highway and I was in front of him. We were always in touch with each other on the CB. Sometime around 11:00 pm I lost Gabe on the radio. I finally heard him saying his 96 turbo-diesel was in limp mode, it wouldn't up-shift past second gear, the yellow engine light was on and he was pulling over. By that time the group was at least a mile ahead on a two lane dark highway in the middle of the night pulled over, very tired and anticipating having to work on a truck. I radioed Gabe and asked him to attempt to drive up to where we were. He got back to me after a few minutes and said he was coming up and his truck was running fine now. What a relief. What he had done was shut it off for two minutes and restart it. Luckily it ran fine till 1:30am when we got back. The next morning John looked at the truck and found that the output speed sensor was the culprit. What a coincidence. When John pulled the sensor out of the transfercase I couldn't believe all the junk that came out with it.

SensorAccording to the AMG 98 shop manual the output speed sensor is a magnetic unit just like an electric guitar pickup. It consists of a permanent magnet surrounded by a wire coil. The sensor is mounted on the output of the transfer case opposite a gear-type rotor pressed onto the output carrier. The rotor and gear teeth interrupt the sensor magnetic field as they rotate. This induces an AC current in the sensor coil. The output sensor provides a voltage signal proportional to the vehicle speed. The sensor signals are used by the TCM (Transmission Control Module) / PCM (Power train Control Module) to control shift speed, pattern, torque converter clutch apply and the speedometer.

The TCM/PCM also monitors the Automatic Transmission Input (Shaft) Speed Sensor (A/T ISS) along with the Output Speed Sensor (OSS) during shifts in order to determine if a shift is occurring too fast or too slow. The TCM/PCM adjusts the signal to the transmission pressure control solenoid valve in order to maintain a set shift feel. The TCM/PCM also monitors the sensors after a shift to calculate the amount of slippage in that gear. It then adjusts the signal to the Transmission Pressure Control Solenoid in order to maintain slippage below a set amount. This is the way the 4L80-E transmission continuously adapts for normal wear in the transmission.

TransfercaseA failure of this sensor will affect shifting and converter clutch apply. Shifts will be erratic and the converter clutch applies at the wrong time. You can also experience and intermittent or loss of speedometer reading. It can cause the truck to go into backup / limp mode where it will only drive in 2nd gear. If you do get stuck out on the road with this problem sometimes shutting down the truck and then restarting it will allow the TCM/PCM to reacquire the correct signal from the sensor long enough to get home.

Because the primary part of the speed sensor is a strong magnet it has been attracting all the loose iron particles circulating in the transfercase since it was new (see picture of my sensor). These iron filings which range from the size of dust to slivers of metal pile up on the sensor eventually causing the signal to degenerate. These false speed readings are fed to the computer which in turn causes all kinds of problems. John Ward tells me that this is not a problem on the input speed sensors because the transmission doesn't create the wear that the transfercase does. I also suspect that most of the junk floating around the transmission is picked up in the very fine transmission oil filter. John also said that "the transmission has a output speed sensor that is not used on the Hummer because we get the output speed from the T-case. This sensor is the same as the input speed sensor, which can be used as a spare in case the input speed sensor fails". John says "the failure of the input speed sensor is very unlikely".

Speed sensorFortunately maintaining the Output speed sensor is very easy. The speed sensor is screwed into the top of the transfercase (see picture). You might want to remove the undercarriage protection plate (2 screws) under the transfercase first. All you have to do is remove the sensor which is a plug from the top and unscrew it from the case. Carefully clean off all the iron particles. If the sensor is chewed up in any way you will have to replace it.

The connectors and wires for the speed sensors are known to be troublesome at times. The connector is a standard GM part. Most failures are caused by water intrusion, due to a connector that is not well sealed.

AMG has come out with a 10 thousands shim washer (see picture) that goes under the sensor to space it out a bit from the rotor. Screw it back in and plug back the wire and you're all done. The shim is important. Owners that had light wear on the end of the sensor and were having intermittent problems with the truck going into limp home mode reported the problem solved when they installed the shim.

Trouble Shoot the Speedometer

The output speed sensor produces the signal that the cruise control and the speedometer uses. A quick way to test everything up to the speedometer is to try the cruise control. If the cruise control works and your speedometer doesn't then your speedometer is bad. If the cruise doesn't work then check the sender, DRAC and associated wiring .

ECU and DRAWhile we're down there I would check the transfercase fluid which is Dexron III. Looking at the picture there are two rearward facing plugs in the case. These plugs use a hex wrench to open and may be on too tight. I usually have to take a piece of pipe over the hex wrench to get more leverage. Open the top plug and make sure the fluid level is 1/8" below the lower edge of the hole. Take a drop of fluid and put it on a white towel to check it's condition. According to the shop manual "normal fluid color ranges from dark red to light pink". In my experience fluid in good condition is a nice red color. Fluid that has been overheated will be dark red to brown in color. Burned fluid will be dark brown to black and smell burned. I can tell you from experience that if your truck has been driven 70 mph or faster, especially on a road trip you should check the transfercase oil ASAP.

You can drain the oil by removing the lower plug. Clean off the area around the drain and replace the plug. You can use a little liquid Teflon pipe sealer if you want. Stay away from Teflon tape because sometimes small pieces of it find there way into the case and can plug lubrication holes. Don't over-tighten the plug. If you find that your oil is burned or off color you might consider replacing it with one of the synthetic Dexron ATF's such as Amsol or Mobile. These products will take a higher temperature before they break down and it only takes 2 to 3 quarts to fill (buy 3 because you will spill some). The auto stores sell a "bottle pump" which screws into the oil bottle and allows you to pump the oil up into the fill hole. It's well worth the few bucks. I've filled my case with a funnel and gotten a Dexron bath at the same time.

TransmissionThe transmission has 2 speed sensors which are the same part that's used in the Hummer transfer case. I don't believe that the transmission output sensor is used on the Hummer. You may be able to swap the 2 sensors out using the one in the trans for the spare.


I wrote a couple of weeks ago regarding my 98 TD going into limp mode and my speedo not working. I took the advice, got the shim for the output speed sensor and still had the problem. I located the DRA (thanks to Chuck and the very helpful picture in his article), took it apart, cleaned it out and that seems to have solved the transmission shifting problem. Everything appears to be working well except the speedo still doesn't work. However, the check engine light came on and after hooking it up to a scan tool, the following codes came up. P0501 VSS Signal out of range and P0722 Output speed sensor CKT no signal. Does anyone have any idea what these codes mean and what I should do now?

The vss (vehicles speed sensor) code is a problem with the sensor on the t-case not the transmission. Check the wiring to it. You should remove the sensor and check it with ohm meter to see if it has an open circuit. the other code for output speed sensor could have set when you removed it to install the shim or the shim set it out to far and it is not sensing the tone ring. The best way to diagnose this is to monitor the vehicle speed signal and output speed sensor signal under engine data on the scanner. If these sensors are not working correctly they will not show a signal on the data screen or it will be very erratic. if you don't have access to a scanner you will just have to try and replace both sensors.