O2 relay won't click on. what gives?

[BennyM]

So, I noticed that the O2 heater wasn't getting any voltage. Relay was bad. I tried three other working relays, but none of them will send power to the heater. Allconnections are good. Motronic sends a solid 12v to activate the relay, but it still doesnt close for the heater. What am I missing?

[yaofeng]

The function of the relay is to heat the O2 sensor before the engine gets to the operating temperature as I understand. Once the engine is warm the voltage to heat the O2 sensor is not used or not needed. The temperature from the exhaust gas then excites the O2 sensor to provide feedback to the DME. So the more important aspect is if the O2 sensor is functioning correctly at temperature.

As for the relay you can physically test it unplugged by using battery power.

[DaveVoorhis]

Motronic sends a solid 12v to activate the relay, but it still doesnt close for the heater. What am I missing?

So... You're saying you're measuring the right relay activation voltage, but relay activation doesn't occur?

[BennyM]

The relay terminals are powered properly with ignition on, but when I stick the relay in, it doesn't close (or it seems like it may close for only a split second and then re-open). All other connections are good. There are no bad connections and the heater has continuity. If I take the relay out and test it with a separate 12v source, it clicks closed like it should and pins 30 and 87 have continuity.

It seems like the DME must sense some resistance in the relay and doesn't want to close the circuit. Perhaps Ross is on to something. It must think the engine is too hot. I guess I'm back to testing the temp sensors. I must have missed something the first time.

So... You're saying you're measuring the right relay activation voltage, but relay activation doesn't occur?

[Ross]

Actually the DME goes about it's business by supplying the ground to operate various devices. The 12 v at the relay is there any time the ign is on.
The ground to close the relay should be applied from pin # 38 of the DME which corresponds to pin 4 (a brn/grn wire) in the relay socket, pin 85 on the relay itself. Perhaps this wire has been pushed out from the relay connector or another cause for this circuit to be open.
I think the criteria for the relay being energized would be cold temperature so you might want to check temp sensors. Any cold running issues?