spaceharrier

So my '89 just failed CA smog with high HC and very high CO, passed NO and evap sections fine. Recently had a new exhaust from manifold to tip, with new cat. I'd been wondering about unburned fuel because it smells a bit of fuel sat at idle and already has a reasonable amount of soot on the exhaust tip after only a few hundred miles.

Reading around, seems like checking the O2 sensor is one of the earlier steps, with the O2 sensor heater relay coming up as a possible culprit.

Tried measuring voltage of the orange wire at the connector for the sensor, with ignition on and engine off, got 0V. I understood that the rearmost relay of the 4-relay block next to the ignition control module (from front of engine bay toward the firewall) was the for the O2 sensor heater. Tried tapping the wire up by the relay socket using a posi-tap, read 0V there too, so I thought the relay was duff and swapped it. Still 0V.

Now I'm confused, after reading through later postings in this thread where there's some question about the correct relay, and this thread where it mentions the engine needs to be on to check the heater. I'm wondering two things:

1. Which relay on a Renix 4.0 is actually the O2 sensor heater relay?
2. Should I see 12-14V at the sensor end of the orange wire just with the ignition on, or does the engine actually need to be running?

cruiser54

http://www.naxja.org/forum/showthrea...ht=sensor+test

spaceharrier

Not to be (too much of) an ***, but is that confirmed as the right one? Seemed like the thread got a bit vague after that version was first posted.

Also, any guidance on the the other point, whether it should be ignition hot or need engine on to measure the O2 heater voltage?

(And thanks!)

DFlintstone

Yea, that's the correct relay. Just to be sure, check it running. It may be energized through the fuel pump relay.

Here is a long train-wreck of a thread...( I sort of was sabotaged). Right on top is a link to Ecomikes informative thread. >
https://www.cherokeeforum.com/f2/ren...ce-rms-133153/ (Oh! I see you found it. Yes, years ago we had an incorrect relay diagram, Cruiser had a member fix it to provide that one there)

ALSO!
There is a brittle little tube. It comes out of a rubber plug, low on the engine side of the throttle body. It runs back to the firewall, then up to your MAP sensor sort of high up there near center on the firewall. CRUCIAL!! The MAP will tell the ECU to dump fuel with less/no vacuum. Any crack or blockage and it will be pig-rich. (black smoke, fouled black plugs). You can pull the line off the MAP and feel for vacuum with your finger. If it runs!

firehawk618

Or you can do it the old school way. Disconnect the whole line, wet your finger and place it on one end. Put the other end in your mouth and put a vacuum on it and see if it holds.