443 955 532C

Time Control Relay 443 955 532C is used in the VW T3 Microbus to turn the auxiliary water pump on if the engine is hot, and to keep it running for a while after the engine is stopped.

Firstly, the power supply. Power is derived from both "30" (always on, via fuse S20 in this application) and "15" (ignition) via a complicated network of diodes. I don't really know what the purpose of D2 is -- it will supply power to the relay if S20 blows, but since S20 also supplies the load there won't be any power to the pump in any case. On the other hand, this specific unit is used in other applications (Golf and Audi) so maybe it's just part of being multi-purpose.

From the schematic it's clear that the relay will turn on if the "SL1" terminal is grounded. "SL1" is however not connected to anything in this application -- but as mentioned above, multi-purpose is the word.

The other way for the relay to be on is if the BC517 Darlington transistor is baised properly. This means that the ignition ("15") must be off (otherwise the BC517 emitter will be high via D5), "T" must be grounded (otherwise the emitter will be high via the 2k2 resistor and D4 to "30") and there must be a positive voltage on the base (if "30" = 12V and T is grounded, the emitter would be at (12-0.7)*(560/(2200+560)) and the base needs to be ~1.1V higher -- I calculate about 3.4V).

So the pump will stay on after ignition is turned off for as long as the "T" terminal is grounded (in the case of the Microbus, because the thermoswitch is on (> 104C)) and the charge on the 470uF capacitor C2 is higher than 3 1/2 V, give or take). Ergo, time delay.

Testing

Connect 12V to "30" and "15", with the negative on "31". Ground "SL1" and check that the relay clicks, or connect a test lamp to "87", it should come on and go off again when you disconnect ground from "SL1". Now ground "T" and remove power from "15". The relay should stay on for five to ten minutes (mine's about 8 minutes).