Bench testing Lambda ECU

[Horsebox]

I think I just got to the bottom of a fuelling problem today. I did the battery test on the lambda ECU and got the following results:

O2 sensor unplugged - 50% duty steady (off the orange wire)
WOT switch pressed - 60% duty steady
O2 sensor grounded to block - duty increased to about 97% and stayed there
1.5V on O2 sensor - duty started decreasing and got as far as about 30% before it increased back to around 50% again and stayed there.

I think my ECU is shot based on the last test above. I reset the fuelling and took it for a decent spin and it drives like a champ again so I'm tempted to just leave it as is and run open-loop.

Is identifying and replacing the failed components inside the ECU something that a mere mortal without oscilloscopes can do?

[Bitsyncmaster]

I think I just got to the bottom of a fuelling problem today. I did the battery test on the lambda ECU and got the following results:

O2 sensor unplugged - 50% duty steady (off the orange wire)
WOT switch pressed - 60% duty steady
O2 sensor grounded to block - duty increased to about 97% and stayed there
1.5V on O2 sensor - duty started decreasing and got as far as about 30% before it increased back to around 50% again and stayed there.

I think my ECU is shot based on the last test above. I reset the fuelling and took it for a decent spin and it drives like a champ again so I'm tempted to just leave it as is and run open-loop.

Is identifying and replacing the failed components inside the ECU something that a mere mortal without oscilloscopes can do?

Yes that last test suggests the ECU has a problem. It should drop way lower than 30% and stay there. Since the ECU is basically working, I would guess you may have a bad capacitor. You can often find bad diodes, resistors and transistors using an ohm meter and a good ECU to compair the readings. But a bad capacitor and bad ICs won't show up bad unless there is a hard short. There is only one IC in the lambda ECU but many capacitors.

[David T]

Yes that last test suggests the ECU has a problem. It should drop way lower than 30% and stay there. Since the ECU is basically working, I would guess you may have a bad capacitor. You can often find bad diodes, resistors and transistors using an ohm meter and a good ECU to compair the readings. But a bad capacitor and bad ICs won't show up bad unless there is a hard short. There is only one IC in the lambda ECU but many capacitors.

While the test you did points out that the ECU is not functioning as it should you have not ruled out the wiring harness. Because of the small voltages for the O2 sensor circuit a bad connection can make a big difference. Before condemning the ECU as the culprit rule out any bad connections first. Also verify you had a constant 12 volts feeding the ECU during the test. A fluctuating supply voltage can cause the ECU to malfunction. Running "Open Loop" is not a great idea because to do it you have to set the mixture richer than if it was to run in "Closed Loop". Also hard to pass emissions testing and you can burn out your catalytic convertor. One other possibility is the battery you used for your 1.5 volt source is bad and did not maintain a full 1.5 volts during the test. I point out all of these external possibilities because it is rare the ECU goes bad. Not impossible but rare.

[Horsebox]

Yes that last test suggests the ECU has a problem. It should drop way lower than 30% and stay there.

I did the same test 18 months or so ago and it was fine. Dropped down to 10% or so and stayed there. Something has changed...

Hmmmm.

[Horsebox]

While the test you did points out that the ECU is not functioning as it should you have not ruled out the wiring harness. Because of the small voltages for the O2 sensor circuit a bad connection can make a big difference. Before condemning the ECU as the culprit rule out any bad connections first. Also verify you had a constant 12 volts feeding the ECU during the test. A fluctuating supply voltage can cause the ECU to malfunction. Running "Open Loop" is not a great idea because to do it you have to set the mixture richer than if it was to run in "Closed Loop". Also hard to pass emissions testing and you can burn out your catalytic convertor. One other possibility is the battery you used for your 1.5 volt source is bad and did not maintain a full 1.5 volts during the test. I point out all of these external possibilities because it is rare the ECU goes bad. Not impossible but rare.

I'll look into the connections etc tomorrow, when it's not dark and 0C, but light and 0C!

I did richen it up a tad, to about 1.5% CO. No problems for the CAT as there isn't one and I can run to 4% CO before failing emissions fortunately.

Thanks guys, useful stuff to check.

[Horsebox]

I found the problem. Bad lambda ECU ground connection at the bulkhead connector. I cleaned it up and now it passes the battery tests. I tried to get it to run closed loop but the O2 sensor looks like it is toast.

Thanks for the help, yet another electrical problem chalked up to bad connections.

[David T]

I found the problem. Bad lambda ECU ground connection at the bulkhead connector. I cleaned it up and now it passes the battery tests. I tried to get it to run closed loop but the O2 sensor looks like it is toast.

Thanks for the help, yet another electrical problem chalked up to bad connections.

I have managed in the past to "rejuvenate" the O2 sensor by removing it and heating it up with a propane torch. It burns off some of the contamination. Doesn't always work and even when it does the sensor isn't that great but it may get it going a little bit. At least to test things out but don't make final mixture adjustments. Always install with some Never Seize on the threads but be careful not to get any on the probe. Many replacement sensors will require cutting off the wire from the old one and splicing it onto the new one because the wire is too short and has the wrong end connector.

[cis6409]

I found the problem. Bad lambda ECU ground connection at the bulkhead connector. I cleaned it up and now it passes the battery tests. I tried to get it to run closed loop but the O2 sensor looks like it is toast.

Thanks for the help, yet another electrical problem chalked up to bad connections.

Hi
Which bulkhead connector was that you cleaned ? I thought the lambda ecu earth was that single wire that connects to the side of the inlet manifold just above spark plug 1 ..
Is there another one on the bulkhead connectors ??

Cheers
Shane

[Horsebox]

You're right but it heads into the car via the bulkhead connector I guess. It was the 7 pin white connector, the one hardest to get at closest to the bulkhead. Can't remember which pin, it's on your circuit diagram or I can look it up for you later. Cleaning it did the trick anyway!