Help understand relay coil drive current

Hello,

I'm having an issue with a car relay that controls an air conditioner electromagnetic clutch. I'm hoping someone with more electrical knowledge than myself might be able to help me understand what might be at fault.

Although there is 12V across the relay coil terminal contacts in the fuse box, the relay will not turn on while inserted. If I remove the relay from the socket in the fuse box, and connect those same relay coil contacts directly to the car battery, the relay switches as expected. What I'm not understanding is why the relay will switch when connected directly to the battery, but not when connected to the socket which has the same voltage as the battery main, according to my multimeter (while the relay is not inserted).

I tried replacing the controller that supplies power to the relay coil thinking that might be the issue, but with the new controller the problem still persists. So I suspect some type of wiring issue. I just do not understand what might cause the issue, given that the voltage reading is the same.

Thanks for any insight.

Did you really measured voltage across relay coil or measured with respect to ground??

You are measuring the voltage without any current flowing. The wiring has bad resistance that causes the voltage to drop at the relay coil when current flows. Measure the voltage at the relay coil when it does not work and you will see the voltage is dropped by bad wiring. Maybe it is a rusted connection to the car frame.

You need to measure the voltage with the relay still in the socket to get a meaningful measurement. There are two possibilities:

1. The 12V side of the relay coil is good but the other side isn't near enough to 0V for the relay to operate.
2. The 12V drops while trying to drive the coil and it no longer has enough power to operate it.

Remember, it's the voltage across the coil that matters, not the voltage at one side or the other.

Brian.

The input impedence of a voltmeter is very high. So it is sensitive to leakage currents. At this situation it is better to depend on a small 12v filament lamp as a voltmeter. If the lamp perfectly glows it indicates that the full 12v is present. Use the lamp to find presence/absence of 12v supply in the relay circuit.

Hi..

Check the DC resistance of the relay coil. Then find a resistor of the same value, or close to the same value. Connect the resistor across the relay socket terminals and measure the voltage drop across the resistor. If there is no voltage drop, then the circuit is open and there is probably a problem with the wiring or elsewhere in the circuit. Otherwise, you can then calculate the current thru the resistor using ohms law and determine if it is enough to energize the relay.

eT