PWM soleniods: failure due to heat
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1. Heat increases resistance of the coil. PWM solenoids typically have low ohm coils and generate more heat. This causes increased occurrences of coil failures.
2. Heat raises the coils resistance. PWM coils have lower ohms to begin with but generate more heat, thus more occurrences of heat failures. The solenoid in the first picture at room temperature runs at 114 C. That’s around 240 F. Imagine how hot it gets in the transmission! A bad thing about heat and magnetics is that the solenoid gets weaker when it gets hotter. In addition, the higher ohm coils at those temps provide for less current to convert to magnetic energy.
3. Plastics get brittle and harder due to thermal expansion. Everything expands at different rates. (Coefficient of thermal expansion). Thermal expansion stack ups change the stroke and can cause binding of VB spools.
Fluids breakdown. The chemical additives put into ATF to make it work its magic break down into radical molecules that attack the solenoid plastics and some metals. Local hot spots appear near PWM solenoids.
Click on an illustration to see an enlargement.
© Scott Kirkendall
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Replace and reuse rules
Failure review chart