TRANSMISSION TROUBLE SHOOTING
Heating Up with Diagnostic Trouble Code P0218
By Pete Huscher / ATRA Technical Advisor 
(Reprinted with permission from GEARS Magazine)
Today’s vehicles are very sophisticated and incorporate numerous computer systems to control vehicle operation. Diagnosing today’s computer systems can be quite intimidating at times.
To aid with diagnosis, vehicle manufacturers incorporate self-diagnostic capabilities into each of the on-board computer systems.
Most on-board computer systems provide diagnostic trouble codes (DTCs) to identify possible problems in the computer system.
Some diagnostic trouble codes are very specific and have very specific diagnostic routines — such as solenoid circuit codes — while other diagnostic trouble codes can be vague and can only be diagnosed by their symptoms — such as fault after shift, mechanical fault, or transmission over temperature.
In this issue of Keep Those Trannies Rolling, we’re going to take a brief look at diagnostic trouble code P0218 (transmission over temperature) and the possible causes for this code being set.
Diagnostic Trouble Code P0218
Depending upon the vehicle’s manufacturer, diagnostic trouble code P0218 indicates the PCM or TCM detects a high transmission fluid temperature — exceeding about 240º–260ºF (116º–126ºC), depending on manufacturer — for a long period of time.
When diagnostic trouble code P0218 sets, the PCM or TCM:
- Doesn’t light the malfunction indicator lamp (MIL).
- Freezes shift adapts.
- Records conditions when code sets.
- Stores the code in history.
Code P0218 can be caused by several conditions, such as:
- Faulty transmission fluid temperature sensor, wiring, connections, or a faulty PCM/TCM.
- Heat transfer from external sources (exhaust too close to transmission or cooler lines, engine overheating).
- Lack of cooler flow.
- Inadequate heat dissipation through cooling system
- Transmission components slipping
Pre-Diagnostic System Check
Before beginning any diagnostic routines, always perform a Pre-Diagnostic System Check and Diagnostic System Check. The Pre-Diagnostic System Check includes verifying the:
- Battery is fully charged.
- Battery connections are clean and tight.
- Fuses are intact.
- Wiring harnesses and connections are secure.
Use a laser temperature gun to verify TFT sensor accuracy. - No aftermarket devices are installed on the vehicle that may affect computer system operation.
Diagnostic System Check
The Diagnostic System Check includes:
- Verifying the customer’s complaint.
- Performing a recall and bulletin search for related problems.
- Determining whether the condition is mechanical or electrical.
- Checking for codes in all on-board control modules.
- Identifying and documenting codes.
- Determining an appropriate course of action.
Once you’ve completed the Diagnostic System Check, you’re ready to begin the diagnostic routine.
Diagnosing DTC P0218
Diagnosing diagnostic trouble code P0218 is usually pretty straightforward. The first steps to diagnosing DTC P0218 are to:
- Road test the vehicle and verify DTC P0218 sets.
- Check transmission fluid temperature (TFT) sensor operation.
- Verify TFT sensor accuracy with a laser or infrared temperature gun.
If you can verify transmission fluid temperature sensor accuracy with a temperature gun, you know the transmission fluid temperature sensor circuit is working properly.
If you can’t, then the first step would be to repair the transmission fluid temperature sensor circuit and recheck transmission operation.
After verifying that the transmission fluid temperature sensor circuit is working properly, it’s time to diagnose the transmission over temperature condition.
Heat Transfer from External Sources 
Missing heat shield or routing of exhaust to close to the transmission can cause overheat issues.
Heat transfer from external sources to the transmission fluid can be caused by a couple of conditions.
One possible cause for heat transfer on rear wheel drive vehicles is if the exhaust system is too close to the transmission case or cooler lines (aftermarket turbo systems; figure 2), allowing heat to transfer from the exhaust system to the transmission case or cooler lines.
Another possible cause for heat transfer to the transmission is if the engine is overheating. If the engine cooling system overheats, it’ll transfer excess heat to the transmission fluid through the combination radiator/cooler.
Lack of Cooler Flow
Lack of cooler flow continues to be a problem. Transmissions equipped with thermal bypass valves or systems are becoming a big consideration.
Lack of cooler flow is usually caused by TCC control problems, thermal valve problems, or a restricted cooler. Cooler flow should be at least one quart in 20 seconds.
Lack of Heat Dissipation through Cooling System
Lack of heat dissipation through the cooling system is usually caused by a restricted cooler or lack of air flow through the cooler. Comparing the temperature of the transmission fluid going into the cooler to the fluid coming out of the cooler is usually the easiest way to determine if you have a restricted cooler.
Check temp of the transmission fluid flow in and out of the cooler for proper heat drop.
A drop of 20º–30ºF is usually what’s recommended (figures 3 and 4). Restricted airflow through the cooler can also cause an over-temp condition.
Transmission Components Slipping
A slipping transmission component may cause the transmission to overheat and cause an over-temp condition.
Transmission slip may be caused by a slipping torque converter clutch, a slipping clutch, or a torque multiplication problem inside the torque converter itself.
Verify proper transmission operation before attempting to diagnose code P0218.
Well, there you have it. We’ve taken a brief look at the possible causes for code P0218 (transmission fluid over temp). With a better understanding of code P0218, you should have no problem keeping those trannies rolling.—Pro
