Rear window defogger condenser?

[Fairtax4me]

Looking at the wiring diagram for the rear window defogger in my car and I see this thing called the rear defogger condenser. Any ideas what it's for?

I'm guessing it's just a capacitor, but can't imagine why it would need it.

 

Let me see if I can upload a copy of the diagram...

Edited November 19, 2009 by Fairtax4me

[nipper]

Looks more like a capacitor to me. Maybe to have a quick heatup of the defroster.

 

 

 

nipper

[Fairtax4me]

I'll see if I can find it in the trunk and see what all is involved with the thing.

 

I have finally found the time to get in there and see what the deal is with the "slow" defogger. With 13.7 volts at the battery with the defroster on, I'm only getting 12.5 volts at the connector on the window, engine off. That was when I started looking for the diagram. I'd like to track the voltage drop through the whole journey from battery to window and maybe figure out why it's losing so much. There are a few connectors it has to travel through on the way there, plenty of room for resistance, corrosion, and general poor connections.

I'm also gonna pick up an ammeter and see if it's pulling enough amperage to get it to the rated 160W that the FSM says the rear defogger should be.

[robm]

"Condenser" is an old-fashioned term for "capacitor".

 

It is there to stop the contacts in the switch from burning out when you break the circuit (turn it off). The name is a holdover from the days when a capacitor, called a condenser, was used across the ignition points to stop them from arcing when they open. At 160 W, that is over 13 A of current, so that has a lot of "momentum," so to speak, when the switch is opened. The capacitor provides a place for it to go, rather than jumping the contacts and burning them.

 

Rob.

Edited November 19, 2009 by robm

[Fairtax4me]

Thanks for the info Rob.

 

So if the condenser goes bad, the contacts in the relay might burn, which would prevent "full flow" to the defogger grid. Any way I could test the condenser to see if it's working correctly? Probably need something I don't have.

I think I'll hunt down the relay out see what I get for V in vs V out when it's activated.

[OB99W]

[...]With 13.7 volts at the battery with the defroster on, I'm only getting 12.5 volts at the connector on the window, engine off. [...]

Can we assume that you meant to say ''engine on''? (Otherwise, a 13.7 volt reading at the battery doesn't make sense.)

 

 

''Condenser'' is an old-fashioned term for ''capacitor''.

 

It is there to stop the contacts in the switch from burning out when you break the circuit (turn it off). [...] The capacitor provides a place for it to go, rather than jumping the contacts and burning them.

Typically, to suppress an arc the capacitor would be placed across the switch (relay) contacts, not across the load (defroster grid). Placing it across the grid would tend to minimize the switching arc being ''broadcasted'' (causing RF interference), since the grid can act as a transmitting antenna for the transient.

[OB99W]

If the condenser/capacitor became electrically leaky, it could draw additional current beyond what the grid draws and cause more of a voltage drop than normal (you can expect there to be some drop). If that were the case, the capacitor would become warm or hot (under normal conditions it should be at ambient temp). Given its circuit placement, if the capacitor failed ''open circuit'', all you'd probably notice is a ''pop'' in the audio when the defroster was switched. Checking voltage drop across the relay contacts is still a good idea.

[Fairtax4me]

Can we assume that you meant to say ''engine on''? (Otherwise, a 13.7 volt reading at the battery doesn't make sense.)[/Quote]

 

No it was engine off. But I think that it may not be correct. I tried to take a few measurements today and kept getting jumpy readings from the voltmeter. I opened it up and found the battery had started to leak. Not bad but just enough to cause some light corrosion. Picked up a new one but I have to clean up the inerds of the voltmeter and I'll see if that fixes it tomorrow. I think that battery has been in there about 6 or 7 years. As for now I'm just gonna throw the previous measurements out the window. :-\

 

 

So you think the capacitor is there for radio noise suppression? Never noticed any popping in the speakers, but I usually listen to Cds in the car. I'll check for it in the morning. And see what I can find.

[OB99W]

[...] As for now I'm just gonna throw the previous measurements out the window. :-\

There's something to consider when determining the voltage drop -- if the battery has been recently charged (even if just from the engine running), it will take on a ''surface charge''. The voltage can read higher than the typical 12.6-12.7 volts of a battery in good condition unless it's loaded for a few minutes. So if you measure a substantially lower voltage at the defogger than at the battery, remeasure the battery to verify it's still as high as any initial measurement.