Are relays supposed to get hot?

[ROWSC]

I was working on another electrical issue (fog lights) when I noticed that the power windows relay was hot to the touch. I had the accessory switch on so I could listen to the radio. When I turn the ignition off it seems to be fine. I verified that one socket is continuously hot (has power) and one that comes on with the key. I'm concerned that I may have a short of some sort that is drawing enough power to heat the relay, but don't know where to even begin.

[patkeefe]

Relays are thermal switches; they get hot, and cool off. The heating and cooling makes and breaks the current flow. Usually bimetallic elements inside.

Not all relays are like this, but most automotive units are.
Pat

[vintage guy]

Actually relays are electromagnetic devices. Current through a coil of wire produces a magnetic field that opens or closes contacts which switch the usually higher current in another circuit. The current through the coil results in power that can heat the relay. Some heating is normal in a relay whose coil is constantly energized.

[steely]

+1 electromechanical - the bimetallics are suitable as thermostats or signal flashers

I don't think your window relay coil should be on all of the time - it would seem a waste especially if the power it was switching was enabled only by the ign sw. Wish I had a '79 schematic for you. There is one from an '82 on the home page here - not sure if that would apply.

[patkeefe]

Yes, they are mostly electromechanical. Turn signals in our older cars are actually thermal relays. Sorry to lead you astray. I am not sure of the switching on a power window relay, as I am unfamiliar with the operation of the motor(s). I imagine the wiring diagrams in the tech section are fairly accurate for your application.
Pat

[86 911 Targa]

Originally Posted by patkeefe

Yes, they are mostly electromechanical. Turn signals in our older cars are actually thermal relays. Sorry to lead you astray. I am not sure of the switching on a power window relay, as I am unfamiliar with the operation of the motor(s). I imagine the wiring diagrams in the tech section are fairly accurate for your application.
Pat

Pat,

With the ignition in the ON or Run position the P/W relay is activated, however, the current draw is only about 100ma until the window switch is activated at which time the motor pulls additional current.

Gerry

[patkeefe]

Gerry:
I hate to get OT, but this reminds me that I have an issue with the pass side window switch in the 968, which I never bothered with.
Pat

[JJ 911SC]

Originally Posted by patkeefe

Gerry:
I hate to get OT, but this reminds me that I have an issue with the pass side window switch in the 968, which I never bothered with.
Pat

Start suspecting one of the 3 switches... On mine, the passenger window switch on the driver door does not fully returned to the neutral position, so I have to middle it by hands. I got the replacement switches since last year... On the list of this winter project

[dw1]

Simple answer:
Warm yes, hot no.

The prior descriptions are correct - a relay coil is typically an electromagnet.

Again to simplify: Heat dissipation (i.e. temperature rise) is an indication of electrical power. Too much heat = too much resistance and/or or too much current and/or too much voltage, since P=IV & V=IR.

[911pcars]

Current flow through a relay control coil determines how hot it gets. Under normal conditions, it's relatively low current; the number of windings limits current flow to the proper design limits as well as creating the necessary electromagnetic force to close the power circuit contacts.

However a partial short circuit (a common malfunction) can bypass some of the windings which results in more current flow and thus more heat. The relay may still operate in this overheated condition for awhile until the increased temp of the windings gradually melts the insulation film and creates another short circuit path.

If the relay is hot to the touch, replace it before it let's out the smoke.

Sherwood

[ROWSC]

I switched the relay with another one to see if it was the relay. It seems to only get hot with the power windows circuit. Is it possible to measure current draw with a household multimeter? Can I use it to identify where the excess current is being drawn from? The driver's window operates considerably slower so I think I should start there once I know what I am looking for.

[911pcars]

Originally Posted by ROWSC

I switched the relay with another one to see if it was the relay. It seems to only get hot with the power windows circuit. Is it possible to measure current draw with a household multimeter? Can I use it to identify where the excess current is being drawn from? The driver's window operates considerably slower so I think I should start there once I know what I am looking for.

Current flow is dependent on the load, in this case the power window motor. Normally, power circuit in the relay (internal contacts) carry the current flow from battery to the load, not the windings.

Measure current flow at any easily accessible point in the circuit. Connect ammeter in series with circuit, then operate it. You might want to compare each window motor. Not sure there's a spec. However, if the relay is hot to the touch, there's excessive current flow through this circuit (maybe a short in the window motor). Is there a separate relay for each motor or does one relay share both motors?

Hope this helps,
Sherwood

[TechnoViking]

Originally Posted by 911pcars

Current flow is dependent on the load, in this case the power window motor. Normally, power circuit in the relay (internal contacts) carry the current flow from battery to the load, not the windings.

Measure current flow at any easily accessible point in the circuit. Connect ammeter in series with circuit, then operate it. You might want to compare each window motor. Not sure there's a spec. However, if the relay is hot to the touch, there's excessive current flow through this circuit (maybe a short in the window motor). Is there a separate relay for each motor or does one relay share both motors?

Hope this helps,
Sherwood

What he said.

Warm OK. Hot not OK. Two things can make an electromechanical relay hot. One is a coil going bad as Sherwood pointed out (or over-voltage to the coil), and the other would be contact overload (excessive current flow).

Relay contacts will eventually fail under normal use, and that *could* cause welding of the contacts. But I don't see that generating long-term heat.

[jimz993]

I am having the same issues with this relay, and wondering if is it safe to drive the car under this condition. Could the car get fire because of this?
I am getting paranoic now!!!

[lupin..the..3rd]

When you say that it's "hot" all the time, how do you define "hot"? Do you mean to say that it is constantly energised, with ever-present output voltage? Or do you mean that it is warm to the touch? Or spicy? Or perhaps it arouses you in a more intimate way? I once saw the naughty bits of a 904 racing engine, and I was unsuccessful in suppressing an erection. Yeow it was hot!

[pdtweeks]

I'd give the window switches a quick check, to make sure that one of them is not sticking in the "up" or "down" position......which may contribute to a "hot" relay.
I've seen similar scenario with power seat switches...