V2Rocket

Hi,
A few years back I bought a used AEM wideband setup (digital gauge + fairly new sensor) from a Rennlister. I finally got around to installing it a few months ago, but I have been having some issues with it since then.

Car is a 1985 non-turbo 944, sensor is mounted in the stock 944NA oxygen sensor location just after the 4-2-1 collector. The gauge gets its power from the car's stock narrowband-O2 sensor heating circuit. There is no narrowband sensor installed, and the "sensor heater" is an engine-on 12v source and ground.

For a week or so after initial installation the gauge seemed to work as intended. But now, when I cold-start the car the gauge starts up and flashes its number sequence and then shows 14.7 or 14.8 for about 10 seconds. After this the displayed AFR gradually goes down to 10.0 or - - - in another 10 seconds or so.

Once it says 10.0, it will remain displaying 10.0 regardless of temperature, throttle position/load or any other situation. It will read 10.0 for 3-hour freeway rides or half-mile WOT blasts.

I occasionally do dyno-testing of my car with the dyno's own Wideband in the exhaust, showing the actual fuel ratios are matching what the car's tune is asking for, so it is not a tune issue. The car runs and drives great.

I just replaced the first sensor with another brand new Bosch replacement and the gauge behaves the same way... 14.8 for 10 seconds or so, then counting down to 10.0 and staying there.

Can you guys help with what this issue could be? Could the gauge itself be faulty, or is it a wiring issue?

fiily

WHAT IS FREE-AIR CALIBRATION?
Some of our competitors require that you frequently perform a free-air calibration procedure to calibrate their O2 sensors. This process involves removing the sensor from the bung while still connected to the controller device, then waving the sensor in the air so it can read atmospheric O2 levels. We use Bosch 4.9 LSU sensors in our wideband controllers. Our sensors are individually laboratory-calibrated by Bosch and never require free air calibration!

V2Rocket

They say that their sensors don't need Free-Air calibration.
Mine is the 4.2, not the 4.9, but...

snb13

Everything I've read says the 4.9 correctes problems within the 4.2.

raleighBahn

I have all AEM gauges - given that you replaced the sensor (hopefully with AEM replacement as opposed to Bosch direct as it is pre-calibrated), it certainly sounds like power or gauge itself - or the integrity of the wire from the sensor to the gauge.

I would pull the gauge and create a temporary circuit to your battery to see if it corrects it. If not, I'd look at and test the wire from gauge to sensor.

Once you get it sorted out, I believe you'll want to be fused for safety (not clear from your description of the circuit if you are).

V2Rocket

This is the sensor I bought as replacement...has the same Bosch PN on the body as the sensor that came with the gauge.

http://www.ebay.com/itm/231372132663


As I understand it, the NBO2 heater circuit is off the same fuse as the fuel pump so there is 12v whenever the engine (fuel pump) is running. It does not work with key on engine off, since the fuel pump does not run in "on" position"

JustinL

You got the right wire connected? There's usually one for narrow band simulation that runs 0-1v and one wideband 0-5v signal. If you are calibrated for 0-5v and only feeding it 0-1v, you would get the symptom you describe.

V2Rocket

The Wideband is not connected to the DME at all.
As the car runs currently, the DME has no oxygen sensor input at all = open loop.

The "NBO2 heater circuit" power/ground wires are separate from the O2 sensor signal wire and were added on to California-spec cars. The NBO2 signal wire is just left unconnected. The NB-sensor heater/ground are off the fuel pump circuit via the DME relay, which makes me think it should be feeding the gauge 12v since the fuel pump should be getting 12v.

The gauge gets power from the NBO2 heater circuit (only 2 wires, + and -), and then the WBO2 sensor is plugged into/powered by the gauge only, no other wires.

snb13

Just as point of clarification, I "believe" (not sure), all cars have the 2 wires and little bracket for the heater circuit. Mine does, it's originally from Boston. I worked on a buddy's, it had it too. Not sure of that car's origin, though.

V2Rocket

I think you might be right re: the 951 since that was supposed to be a "world" emissions car. Mine is early NA which still distinguished USA/ROW/California specs.

raleighBahn

Not sure on the NA, but any chance that this location is too hot for wideband and you're smoking these sensors? You can go to AEM's site and pull the manual for your gauge and see their guidance.

snb13

No, I mean all. Mine's an '83, my buddy's is an '84.

V2Rocket

Should be just fine, it's the stock NBO2 sensor bung on the pipe after the 2-1 merge.
Even Rogue_Ant said his NA has the WBO2 there without issue.
But the brand-new sensor (replaced before I started this thread, turned on the car for 1 minute to check function and just idled in the driveway) couldn't have gotten hot enough to fry.

Interesting, didn't know that !

alxdgr8

Where do you have it grounded? I've had problems with them if you don't ground them directly to the engine (I'd use the intake manifold).

V2Rocket

It's grounded by the NBO2 sensor heater circuit too. I just put a matching connector on the gauge power/ground wires and plugged it in.
Today after work I'll mess with the wiring and report back. I haven't driven the 944 in a week, or even pulled the cover off.