Wednesday, August 15, 2007

Chrysler Dodge 42RE Governor Solenoid Problems.

The Chrysler, Dodge transmission series 42RE, 44RE, 46RE, 47RE (A500-ES) 1993 through 2003 are notorious for failing to start in low gear. The culprit is often the Governor Pressure Solenoid. The normal function of this solenoid is to generate a hydraulic pressure based on the road speed of the vehicle. The pressure should be low around 10 Psi at idle, and increase steadily with road speed, and throttle response to a high of 100 Psi.

As with earlier, hydraulic ( non electronic) transmissions, the governor pressure is used to move the shift spool valves in the transmission. When a valve is shuttled, the transmission fluid will be directed to a different circuit, causing the application and release of clutch elements. In simplifying the system, consider the 1-2 shift valve to have the weakest return spring. The weakest spring will compress first allowing the 1-2 shift to occur before the 2-3 shift, of which, the 2-3 shift valve has a relatively stronger spring. So the higher the Governor Solenoid pressure, the heavier spring it can compress. ( 1-2 valve moves before the 2-3 valve).

So what happens when you do not have first gear, and your Dodge Ram is trying to start off in second gear? Pretty simple huh? Yup, you guessed it, the Governor Solenoid pressure is to high, and has already moved the 1-2 valve into the 2nd gear position.

To understand why this happens, one must look into the design of the Governor solenoid. This solenoid is a proportional solenoid valve, which uses a tiny internal spool valve to allow the solenoid to give an output pressure that varies in proportion to the electrical current fed to the solenoid. The tiny spool is prone to getting plugged with metallic debris, which either causes it to stick, or eventually get worn out in its bore, allowing transmission fluid to leak by. In either case
the solenoid ends up putting out too much pressure, and hence the loss of low gear.

To make matters worse, the designers of this solenoid found a way to dramatically reduce the size and weight of this device as compared to other like products. Instead of using a large heavy copper coil, the designers used a small copper coil and an internal magnet within the Governor Solenoid. The combination of the small coil, and the magnet, produces the same amount of needed magnetic flux energy, as the larger copper coil used in like designed products. This reduced the overall product weight around 80 %, making the Chrysler Engineers very happy. (Lower weight =improved fuel economy). The downside is that magnets attract ferrous particles.
These little bits of steel that wear in the transmission get attracted right to the solenoid and end up plugging the spool valve, and or accelerating the wear of the tiny spool valve. Chrysler engineers even added a protective shroud around the solenoid to help minimize this occurrence.
But it's function is in my opinion, merely cosmetic.

So when you loose you 1-2 shift at somewhere between 60 & 100K miles, this is the most likely cause. Keeping your transmission fluid clean will help postpone the failure, but it will not prevent it. The solenoid is Mopar part number 4617210. It can be serviced fairly easily, but you will have to drop the transmission pan to get to it.

1 comment:

mikaljains said...

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