frankvan

Does anyone have a wiring diagram for the circuit off fuse 34? All it says on the fuse box cover is Fuel pump ande O2 sensor.
Having replaced my fuel pump I yet again blew the fuse today (luckily right in front of Tim Horton's). Replaced the fuse immediately and it would blow again when the engine was started. Now here is the interesting part - replaced the fuse with the engine off, shorted out the DME relay so the fuel pump would run and the fuse didn't blow!
The wife suggested disconnecting the "oxygen sensor thingy" (her words) and see if that would work. Engine ran fine and made it all the way back home. ACtually, it seemed as if the idle was alot smoother too....
Is it possible for an O2 sensor to short out? I notice there are 3 wires on the sensor - I get a resistance of 7 to 9 ohms (at idle - as engine warms up the resistance goes up) between the two female connections on the sensor. What is the third wire for?

JasonAndreas

Yes it is possible for the sensor to short, as well the heater circuit (powered by the 3rd wire?) on the O2 sensor gets its power from terminal 87b of the DME relay which is protected by fuse 34. And instead of measuring resistance you want to read 0.1 - 0.9 volts DC once everything is warmed up. What model year is your 964 and do you have access to a Hammer or Doug's software & interface?

frankvan

I have the diag software and it showed 24- oxygen sensor failure when i had it disconnected. Other than that no errors. However, if it is a problem with the heater circuit it might not show up as an error (the software only looks at the sensing function?). When hooked up to the software the O2 Sensor field reads between 50 and 90 (millivolts?)

Upon inspection of the oxygen sensor the inner heat shield appears to have corroded one of its mounts and shifted up a bit so as to rub against the outer housing of the sensor. Could that cause a short? The rubbing was enough to polish the heat shield in that one area... I have since bent the heat shield away so that the sensor is not touching anything again. Not sure if I want to remove that heat shield altogether though as the lower spark plug wires are quite close to it. Also noticed searching the forum that there are pictures of a small shield around the sensor which mine doesn't have. (maybe that would have prevented this all from happening!)

Oh, and car is an august 89 build (C4).

Thanks

Frank

JasonAndreas

Right, the diagnostic function for the O2 sensor is very simple. The software checks for an under voltage level of 0.02v and an over voltage of 0.88v for several ignition cycles and if it sees either it will throw fault code 24. It also checks for a non-changing voltage level and if it sees that it will throw fault code 23.

I'm not sure if the O2 readings in scantool are correct, they don't match the readings from a Bosch hammer. I think the O2 sensor reads lambda at between 0.33volts and 0.38 volts. Above 0.38 and the sensor is reading rich and below 0.33 means lean. Actually the A/D conversion of the sensor response is probably non-linear and the voltage levels I posted are linear. I probably should build a correction table so that I have the correct voltage levels.

Did bending the shield help?

frankvan

Thanks Jason,

i haven't had a chance to drive the car yet - haven't had the courage! I'm going to try and get out this afternoon if and when the rain stops and the roads dry up. I'm also going to see if I can get an ammeter (fused) hooked up and find out how much current is being used during operation with and without the o2 sensor connected.
Using a bench ammeter the fuel pump showed a draw of only 1 amp with no load and peaked at around 2 amps with a load. That shouldn't blow a fuse...(rated at 15 amps).
Also took the cover off the DME relay - it's just two solenoids! The top one appears to be the one for the fuel pump. What does the lower one connect to? (The O2 sensor shouldn't require a relay, should it?)