Mine was just the opposite. I got minimal heat when the diaphram failed. Maybe there is more than one way they can fail. Why not just hook up a voltmeter to the monovalve plug, spin the temp control from full cold to full hot and see if you are getting power. Make sure the keyswitch is on(I'd call it ignition but PEH would object - rightly so).

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold