First of all, I've applied 12V directly to the prongs on top of the monovalve and I could feel the plunger moving inside each time I energized the coil. I have then taken apart and serviced the valve. Look at the photo attached and notice that the diaphragm is torn on it. So some questions:

1) Can I just buy the diaphragm replacement or do I have to purchase an entire new monvalve (around $50)?!

2) I don't understand how a torn diaphragm might jeopardize the operation of that circuit. Seems that the rubber grommet that's at the end of the plunger if in good shape would inhibit the flow of coolant regardless of the condition of the diaphragm... what am I missing?!

3) How far should the pluger normally extend out - looks line mine only extends maybe 3-5mm - not far at all - definitely not enough to stretch it and expose the rip in the diaphragm when voltage is applied. I had to physically pull on it to see the hole. Just wondering if the travel on mine is sufficient.... Maybe my next round of tests can tell me if it's sufficient.

So should I spring for a new mono valve !??? This is perhaps part of the problem but I don't think it's the entire problem. There's more...

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K