A/T - 4L60E Code 72 Diagnostics/Speed Sensor Loss
TECHNICAL BULLETIN # 302DATE 1995
TRANSMISSION: 4L60-E
SUBJECT: Code 72 diagnostic
APPLICATION: GM
4L60-E - Code 72 Diagnostics (1993-95 Applications with a PCM)
One of the most common and difficult to diagnose 4L60-E concerns is code 72, vehicle speed sensor loss. It has shown itself to be a very common concern for both the dealer and independent shop. This bulletin is designed to address the code, and proper diagnostic steps required to locate the root cause of the problem. This bulletin will be followed with another showing component locations, as well as the most common causes for the code on each vehicle application.
Some service manuals may not adequately diagnose a code 72 condition, therefore a "comeback" is likely. Use of the following diagnostic strategies to assist with code 72 diagnosis of the 4L60-E transmission.
Circuit Operation
The 4L60-E transmission uses a vehicle speed sensor mounted to the transmission (or transfer case on 4X4 units). Mounted to the output shaft is a 40 toothed wheel. As the output shaft rotates, an AC voltage signal is created. This signal is sent to a VSS buffer on all non-passenger-car applications. (Passenger-car applications do not use an external buffer, therefore the AC signal is fed directly to the PCM.) The buffer (also known as a DRAC or DRAB) performs the following functions:
^ It coverts the AC signal from the sensor to a digital DC signal for the various computers on the vehicle.
^ It provides different frequency rates for the various vehicle computers.
^ It allows for adjustment if the tire size or axle ratio is changed.
To perform these functions, the buffer is given a five volt signal from the vehicle's PCM on two circuits, 437 and 1697. Circuit 437 is used by the transmission shift points. Circuit 1697 is used by the PCM to determine road speed for uses such as vehicle overspeed fuel cutoff and idle air control valve position (figure one).
To signal the PCM, the buffer simply grounds and ungrounds the two circuits, based on the frequency of the signal received from the sensor. As the five volt signal is grounded, the signal voltage at the PCM is pulled down, close to zero volts. As the buffer ungrounds the circuit the signal voltage will return to five volts. The PCM monitors the frequency of these signals to determine the vehicles actual road speed. By using a scan tool you will notice two readings relating to vehicle speed, transmission output RPM and vehicle MPH. MPH readings represent circuit 1697 while output RPM readings refer to circuit 437.
Code 72 will set if the following conditions exist.
* The vehicle is in any range other than park or neutral.
* Engine RPM is greater than 200.
* Pressure switch assembly code 28 is not set.
* Output RPM indicates an RPM change of over 1000 RPM in less than 2 seconds.
OR
* The vehicle is in park or neutral.
* Engine RPM is greater then 200.
* Pressure switch assembly, code 28 is not set.
* Output RPM indicates an rpm change of over 2050 RPM in less than 2 seconds.
IF CODE 72 IS SET THE PCM WILL RESPOND BY
* Defaulting to second gear. Once the default has occurred the vehicle will not shift, and the unit will start and stay in second gear.
* High line pressure.
RECOVERY
Next ignition cycle.
(For Truck, Van and Sport Utility Applications Using a PCM)
Chart -A- Code 72 or 72 and 24
Chart -B- Scanner Indicates 0 MPH and 0 Output RPM
Gas Engine
Diesel to Gas "Pin" Conversion Chart
Above is a conversion chart for using chart "A" and "B" with a diesel application. If there is not reference made for a pin in question, it is the same as the gas version.
Diesel Engines