Automotive Relay Checkup

I always thought that relays are the last things to check in malfunctioning electronic/electrical equipment including particularly the automobiles. From my little experience, the biggest culprit could be the relay coil; the resistance of the coil decreases over time from the typical 70-110 Ohms (according to several internet sources).

I've lately been busy trying to fix the malfunctioning aircon in my 2008 Tata Indigo GLS in which the previous technicians put an after-market circuit breaker and a relay (Siemens V23134-B52-X127). I checked the relay's energizing/input (coil and parallel resistor; terminal 85 and 86) resistance and found it to be 19-20 Ohms. However the datasheet Bosch-Mini-Relays-F4.pdf (I've no longer been able to find it on internet!) indicates that it should be around 80 Ohms!

Fig: A V23134-B52-X127 by Tyco (Left) and pinout/circuitry (Right).

So, I recommend not to overlook the relays, anymore.

How to check the resistance between the energizing terminals (coil plus parallel resistor).

1. Get the pinout information for that particular relay (from datasheets or on it's body) and identify the two terminals for the coil; otherwise if you don't have this information, make trial and error (using step 2 below) for relays are not that complicated.

2. Using an Ohmmeter measure the resistance between two energizing terminals; for V23134-B52-X127 it should be around 80 Ohms, otherwise it is bad.

Another example: 2111 12V- 30A B365

This relay (see Fig below) is labelled 2111 12V- 30A B365. I measured the energizing/input terminals resistance and got 70 Ohms which is within the typical range specified above. Note that here the input terminals resistance is not referred to as coil resistance because this particular relay contains a built in resistor in parallel with the coil; the resistor is a quarter Watt 470 Ohms one.

Fig: Relay 2111 12V- 30A B365, similar to the one I measured.