One of my customers asked which relays are better and if they should get hot. So I decided to bench test a few different relays I have. Two things cause heating. First is the coil power that pulls the relay closed. Next is the voltage drop measured at the base of the relay load pins. Copper (wire) increases resistance a little when it's temperature rises so I measured the resistance of the coil which does not produce any self heating. I measured the load voltage drop at 10 amps to get the resistance to compute powers at larger current.
Power dissipated (watts) is what produces heat which needs to be dissipated via the surrounding air and the wiring on the pins that also dissipate some heat to the surrounding air. The metal can relay should keep the insides a little cooler than the plastic cased relays but I don't know how much cooler. Note the stock lambda relay has a smaller case so it's case temp will be a little higher also.
OEM metal relays part number 33375D. Note this is a used relay.
Coil resistance is 77.5 ohms. Coil power at 14 volts is 2.53 watts,
NO contact resistance is 0.004 ohms. Load power at 30 amps is 3.60 watts
OEM Lambda, BOSCH part number 0 332 015 012. Note this is a used relay.
Coil resistance is 60.7 ohms. Coil power at 14 volts is 3.23 watts
NO contact resistance is 0.0069 ohms. Lambda currents are only a few amps so load heating nil.
No name relay marked 30/40 amp. New never used part.
Coil resistance is 78.8 ohms. Coil power at 14 volts is 2.49 watts,
NO contact resistance is 0.0072 ohms. Load Power at 30 amps is 6.48 watts
Tyco part number 0-1432785-1. New never used part
Coil resistance is 90.5 ohms. Coil power at 14 volts is 2.17 watts.
NO contact resistance is 0.003 ohms. Load power at 30 amps is 2.70 watts
Beuler part number BU-5083W-0000. This relay is new and marked 40/60 amps.
Coil resistance is 82.1 ohms. Coil power at 14 volts is 2.39 watts.
NO contact resistance is 0.0022 ohms. Load power at 30 amps is 1.98 watts.
To get an idea of heating just think of putting a 7 watt incandescent bulb into that small sealed case of the relay. That much power is going to heat the outside of the relay case very warm or hot. OEM fans will draw about 30 amps and the blower on fan speed 4 will draw about 25 amps. The rest of the relays pretty much conduct less than 20 amps. So it does look like that Beuler relay is a better choice for the fan and blower #4 relays if your not going to use my solid state relays.