I have a '75 1.8 FI car that originated in California. This car originally had an O2 sensor, but now it doesn't. I'm looking to replace that O2 sensor. I've been told that the O2 sensor hooked into some sort of relay which then ran back to the computer. First, is that correct? Second, does anyone have a photo of the O2 sensor hookup from the exhaust all the way to the computer? If there really is a relay in this scheme I could also use a picture or description of that relay too. Once I figure out what I'm looking for I'll try to find the parts. Thanks.

George

Indeed. the sensor was mounted in the Exhaust connected via a small acc. loom to a relay mounted just to the left of the R. deck latch which in turn plugged into the late main loom (w/taillights).
I dont have access to a pic but that's not the only parts involved.
Correct exhaust, CAT, EGR pipe from muffler to valve in Eng. bay. Trip meter in footwell, guage bezel with idiot lights, ect ect.
Late CA cars were *special*

Does Virginia require you to smog it?

If not, pull all the smog junk off and store it in a box in the garage.

This is close to being correct.
They did not have an O2 sensor but they did have a temperature sensor.
It would turn on a warning light on the dash if the cat got too hot.

The sensor was mounted and wired as described above.

It is not functionally necessary, these were not used by the fuel injection or any other engine controls. It was just for the temp warning light.
If you are still running a cat converter or want originality you might want one.

Jim

IIRC that sensor is heat activated, not O2
FYI I have a copy of the CAT/ERG wiring diagram and possibly a spare harness that goes from the relay (in the trunk) to the sensor

75/76 California cars:
The relay in the trunk is temp control relay which reads the thermocouple in the thermal reactor. It usually will only trip if spark plug fouls or MPS failure which puts excessive raw fuel into the reactor causing it to overheat therefor damaging it or drastically shortening it's life.

For further protection it also has fuel cutout rev limiter instead of ignition (sister board on battery tray), and decel fuel cutout in the ECU below 2K rpm with full off throttle. Very annoying in slow traffic.

Thanks everyone for your comments - as usual, very helpful. The catalytic converter is no longer on the car, so it looks like I don't need that relay. But the question remains: how does the O2 sensor hook in? Through that same relay?

And for matthepcat: no, my area of Virginia doesn't do smog checks. The reason I was looking to hook up the O2 sensor again was to see if it would solve a problem with the engine running very rich at idle. Car has all new wiring, and all the usual suspects (aux air regulator, throttle valve switch, MPS, temp sensors, etc.) and vacuum lines have been checked and/or replaced. Fuel pressure both with and without vacuum is spot on. Car runs and idles just fine. I thought that the '75 computer may be looking for an input from the O2 sensor to control the mixture at idle. My thought was to hook an O2 sensor up the tailpipe and connect it to the harness and see if that made a difference. If so, I'd permanently install an O2 sensor. Really grasping at straws here.

Thanks again.

George

Do the two red dash lights (cat / erg) light up when you turn the ignition key to position one ?

Repeat after me - No 914 has an O2 sensor, No 914 has an O2 sensor...

Its a thermocouple as mentioned.

And its not a relay back there by the tailights, its a small computer that looks like a relay that controls it.

Find the circuitry to better understand it on the last page of this document:

http://bowlsby.net/914/WiringHarnesses/Har...ssis_1975-6.pdf

Your rich at idle condition could be related to other things...have you checked?

* Leaky MPS?
* TS2 accurate?
* Leaking fuel injectors or CSV?

PS just found this photo on CL, its FS is you want it:

http://sfbay.craigslist.org/sfc/pts/5515698159.html

OK, thanks Jeff. Back to checking the usual suspects.