I need some help troubleshooting a heater problem.
The heater hasn't worked since I bought the '79 300SD a year ago.
Even on Defrost I get no heat.
Last Spring I replaced the AC compressor, drier, high/low switch and the AC blows really cold.
The Auxiliary Pump was having a hard time spinning so I replaced it recently. The new pump moves hot water.
I replaced the interior AC on/off switch yesterday (which was bad) but I'm still not getting any heat.
The previous owner included some extra parts with the purchase and one was a regulator valve. The PO said he had been having heater problems but hadn't switched the regulator valve yet. It's used and has a crack in the plastic body. I'm planning to try to swap the regulator valve this weekend and see what happens.
By the way, I did a citric acid flush of the cooling system this summer. The Aux pump wasn't working so I bypassed it and flushed the heater core using the garden hose.
Does anybody have a troubleshooting pattern to follow? I don't have a vacuum pump (buy can buy one) or the Mercedes Climate Control Tester. I'm usually pretty good at troubleshooting but I'm out of my league with this Climate Control System. Anybody have any direction?