Adding a gauge to your existing air–fuel ratio meter

One thing you definitely want to avoid while driving a (retrofitted) car using liquified petroleum gas is a lean mixture. If the air-fuel ratio gets far enough out of regulation, e.g. when reaching the output limit of the LPG system (pedal to the metal, lead on your feet) first the valve seats and then the exhaust valves themselves will melt sooner or later (if they are not tough enough – mine were not).

So I got myself a “lambda gauge” from ebay. Then I got shocked when I realized, that my oxygen sensor was a “wideband” type (current output) and the gauge needed a “Nernst”-type (voltage output) – but the car had both. The antiquated, but appropriate one was located “behind” the catalytic converter, responsible for the “slow” signal:


So I went unter the car to search for the proper wire (green stripe on black in this case) and looked for the same one at the control unit under the bonnet.

CIMG1433 CIMG1451

The most difficult part was getting the signal to the dashboard. The rubber dome on the right side of the (left hand drive) car, behind the glovebox was almost impossible to penetrate. Later I read about “car audi enthusiasts” using wood drills to get their power cables through, but here I used a nail. And a piece of wire mesh fence to thread the wire from the inside to the front.

CIMG1435 CIMG1437 CIMG1440 CIMG1442

Good thing this one has a strut brace, so the wire could be attached there.

CIMG1447  CIMG1450

Attaching the IDC-connector was no fun, but possible.

CIMG1452  CIMG1453

Then some more behind the radio to get power to the gauge. And of course the lambda signal. To fit it I had to dremel the storage compartment under the radio a bit – a hole in the rear side was already there for the Aux-connection.

CIMG1457 CIMG1458 CIMG1521

The result? Noise from the stepper motor inside the gauge (but not louder than the fuel pump in the tank behind the rear seats. And the mixture was getting a bit lean – as soon as the car reached 240 kph.

(Original Post:, June 6th 2009)

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