The following is transferred in from the old Archives. Sorry, they are not in any particular order and it was not practical to include the authors or posting dates. ;p
Defog cut out relay, '85 vanagon; purpose?
The relay mentioned above (Bentley manual page 97-66, current tracks 92 - 98) is giving me grief, cuting out at bad times. My questions are direct: What is its purpose, and is it necessary (can I bypass it)? A blower fan is needed in Winnipeg in January!
Any help would be appreciated.
Answer: Bless you; someone who actually reads the manual and does some preliminary checking before starting a thread! You must have read my new message board rules.
Typically the purpose is two-fold. High consumption devices typically use a relay to provide high levels of power to the appliance (heat grid) while using low power to control it. This allows a system where high amps are not run through the dash wiring and switch, but are availabe at the appliance. It also cuts down running power back & forth the vehicle length. Notice your relay power uses .5mm wire vs. l.5 for the defroster grid.
A cut-out relay is sort of the opposite. It requires a current to close and allow the rest of the system to work so that certain other conditions are consistant with the appliance. In this case, it requires the ignition be on, thus allowing power to feed the defrost, wiper & blower circuits. Otherwise, the items could be left on when the engine isn't running and kill the battery poste-haste. Sometimes a relay will have a built-in timer so the high-amp appliance shuts off after a reasonable period of time. Not in this case.
So to answer your question: Yes it's required. The cost of the relay in comparison to dead batteries and safety of having a high-amp circuit left on accidentally is worth it. Can it be bypassed, I suppose so, with those risks!
For a cost savings, remember relays are very standardized. Aftermarket sources should be much less than VW OE. There will be a cost difference between timed & untimed, but on a heating element circuit, I'd stay with timed if that was OE.
When a relay is intermittent, though, it's often not the relay, but the power supply to it or ground. Check for ignition switch controlled power. I once had my headlights go out and finally traced it to the power lead through the ignition switch that fed the headlight circuit. Required a new ignition switch (electrical, not the keyed part) which was relatively inexpensive. A lot less than the headlight switch I was ready to blame it on.