One of the drawbacks to modern transmissions, as well as engines, is that they are not built with too many components. It is hard to find them all when you are having a problem with your speedometer or other parts of your transmission.
The sensor’s location should be on the tail of the Allison transmission on the driver’s side of the vehicle. There may be other locations as well as some say it is on the front of the transmission and only on the side. But look on the outside where the drive shaft enters the transmission to find it.
To learn more about this topic, just continue reading our article. It provides the information you want to know about so you can make corrections if the speed sensor is off just a little. Sometimes it is the change in tire size that will throw off your speedometer and not the sensor.
If we are not talking just about Allison transmissions, then the speed sensor can be in a variety of locations. For the Allison model, it will depend on which transmission in the series you own. Some have one sensor and others have two.
For other models, their locations can be as follows:
- Toyota- on the bottom of their transmission or on the side
- Mini Cooper- it is on the right side of the transmission and it is about 3 inches long and looks like a plug
- Honda - this varies between models but it can be found in the air filter box of the transverse engine. With the box off, it is usually on top of the transmission
- Ford 150- the location is usually near the tail shaft on the driver’s side. The 2-wheel drive models are easy to find but you may have to remove the drive shaft of a 4-wheel drive model to get to it
- BMW- some models have their speed sensor in the fluid pan while others have external sensors near the back of the transmission.
Check your owner’s manual to make sure you find it on your specific vehicle or RV.
According to the diagram in the manual, this transmission option has 3-speed sensors. They are all placed on a wire harness and it should be on the side of the transmission near the front. That should be the driver’s side.
One of the more common problems with this model is that the speed sensor can fail on a regular basis. When it does fail, it will send your vehicle into limp mode. Limp mode in this case limits you to first and second gear if you are lucky.
The fault codes you should see if your speed sensor is failing will be P0500, P0501, P0502, & P0503 according to the Allison fault code list. A trained transmission mechanic should be the one to work on your Allison transmission to make sure you can repair and clear those codes.
Your code reader may not be sophisticated enough to read them all.
According to the manuals, the 1000 and 2000 series Allison transmissions have the same speed sensor set up. This model series also has 3 sensors and they are located in the same place on the wiring harness from the TCM.
These sensors were designed for 5 and 6-gear transmissions. You can see the harness at this link, just scroll down till you get to the right page. The sensor looks like a plug and one size does fit all three.
Interestingly enough, this part is not listed as a common problem for the Allison 2000 transmission. All three sensors, the input, turbine, and output, are located externally. Their exact locations are:
1. Input speed sensor- externally mounted in the torque converter housing and directed at the ribs protruding from the torque converter.
2. Turbine speed sensor- externally mounted in the main housing directed at the tone wheel or PTO drive gear attached to the rotating clutch module.
3. Output sensor- externally mounted in the main housing directed at the tone wheel or PTO drive gear attached to the rotating clutch module
These locations should be the same spots for the Allison 1000 transmission.
The service manual for the 3000 and 4000 series Allison transmissions are not as detailed about the location of the speed sensors as the manual is for the 1000 and 2000 series transmissions.
This is what their combined manual states:
“Three speed sensors—engine speed, turbine speed, and output speed—provide information to the TCM. The engine speed signal is generated by ribs on the shell of the torque converter pump. The turbine speed signal is generated by the rotating-clutch housing spline contours. The output speed signal is generated by a toothed member attached to the output shaft (except for the 3000 Product Family 7-speed models, where the toothed member is the transfer case idler gear).” (source- 1-5)
As you look at the diagram underneath those words, you will see that the 3000 and the 4000 transmission models share the same sensor designs. In general, the location of the sensor for this model series is at the front near the output shaft.
Sharing the same troubleshooting manual does not provide anyone with a lot more information. When you look at the diagrams of the different speed sensors you will see that different models had different designs. They may have slightly different locations as well.
The speed sensors on the 4000 model series may not be on the same side. According to one source, the input speed sensor is on one side of the transmission while the output is on the other side.
For all model series, the specific location will depend on the make and model of your vehicle. They may not be all in the same spot. As we said earlier, check your owner’s manual to find the exact location of your specific vehicle.
Some will be harder to find than others and that is due to the extra equipment underneath your vehicle or lack of that extra equipment. Your transmission expert should know exactly where to look if you need those sensors serviced or replaced.
“There are three speed sensors available for use with 1000 and 2000 Product Families transmissions: the input (engine) speed sensor, the turbine speed sensor, and the output speed sensor (Figure 1–7). The speed sensors provide rpm information to the TCM. The speed ratios between the various sensors allow the TCM to determine the transmission operating range. Speed sensor information is also used to control the timing of clutch apply pressures, resulting in the best possible shift quality.”
That information comes from the first link above. It is kind of technical in nature but these sensors make sure your transmission shifts at the right time. Then from the troubleshooting manual for the 3000 & 4000 model series:
“The speed ratios between the various speed sensors allow the TCM to determine if the transmission is in the selected range. Speed sensor information is also used to control the timing of clutch apply pressures, resulting in the smoothest shifts possible. Hydraulic problems are detected by comparing the speed sensor information for the current range to that range’s speed sensor information stored in the TCM memory.” (source is the second link above)
That is the best way to describe what these sensors do when you drive your vehicle.
To know that your speed sensor is failing, you need to recognize the symptoms or signs that there is a problem. These signs include harsh or improper shifting, loss of cruise control, rough idling, unusual noises and vibrations as well as seeing the check engine light come on.
The problems that are associated with those symptoms are just one- speed sensor failure. However, there are many sources for that one problem. Those sources include the following:
- worn or loose connections between components
- electrical shorts or interference caused by faulty wiring
- oil contamination due to leaking seals or gaskets
To properly diagnose these problems you also have to check the following components- wiring harness for damage, corrosion, and so on and seals for any leaks. Any component related to the speed sensors must be checked if you do not want a repeat of the same problem.
To help prevent this problem you should do regular checks to see that the wiring harness is free from damage, etc., and that the seals and gaskets are still in top shape. Repair any that look to be wearing out or becoming damaged.
You will want to do this because replacing those speed sensors can range from $100 to several hundred dollars each.
When your car stops operating like normal and you start to experience a loss in performance, check the speed sensors for a sign of failure. You may have rough idling, hard-to-shift moments, spot leaking fluid on your driveway, or vibrations that are greater than normal.
While those can be symptoms for other problems, do not ignore checking your transmission to make sure the speed sensors are in good condition. Then the most obvious symptom will be the check engine light.
When that is illuminated, it is telling you that it is possible the problem lies with your speed sensor. A combination of the other symptoms with this light will narrow down your search.
If you have a code reader, that would be the time to get it out and check to see if any codes have been set. Don’t be surprised when you take it to your transmission technician and he finds more codes on his more sophisticated code reader.
Once the speed sensor is identified as the problem, make sure to get it fixed then and there. It is possible that the sensor will send your vehicle into limp mode if not replaced right away.
To put it generically, your car is not going to perform as it once did when the speed sensor was operating perfectly. One of the reasons why you need to handle this problem as quickly as possible is that you can damage other engine and transmission parts if you let the problem continue.
As we mentioned earlier one of those components would be your cruise control. This is one of many engine parts that rely on the data provided by the VSS. That data is used to make the proper adjustments to your vehicle as you drive.
Besides limp mode and the above problem, you may see erratic speedometer readings. Or you may not see anything on your speedometer yet you will be traveling at the same speed as you were before this took place.
Then, you may have shift problems because the TCM or the PCM is getting the wrong data. You can experience delayed shifts, abnormal shifts, or restricted shift options.
All of this boils down to the fact that you will not be able to drive your vehicle safely. You can still drive with a bad speed sensor but it is highly recommended that you do not.
The output speed sensor is the component that provides you with your speedometer reading. When that goes, so does the knowledge of your speed. When that happens do not panic but make an appointment with your transmission guy to get it fixed as soon as possible.
Aside from a safety issue, you do not want to be issued a speeding ticket or a ticket making it mandatory to have your vehicle fixed or pay a huge fine. There are different factors that make this repair a priority over anything else.