| Achieving a proper fuel mixture is vital to the | | | | The diagnostic socket inside the left fender allows us |
| performance and economy of your Mercedes 126. | | | | to observe lambda control in action. When we know |
| This article looks at the mixture-control procedure for | | | | what the controller is trying to do, we know whether |
| the early V-8 engines with K-Jetronic fuel injection. | | | | our base mixture is too rich or too lean and can |
| Many other European cars of that era follow | | | | adjust accordingly. To be specific, the voltage across |
| essentially the same method. | | | | pins 2 and 3 of the diagnostic socket shows us an |
| For cars with lambda control, it is not possible to set | | | | aggregate voltage across the frequency valve. The |
| the basic fuel mixture correctly without a properly | | | | lower the voltage we see on our voltmeter, the |
| functioning oxygen sensor. Please refer to the | | | | more the lambda controller is trying to richen the |
| companion article Mercedes 126 Repair: Tracking | | | | mixture; i.e. compensate for a lean condition. The |
| Down Vacuum Leaks for more on that subject. As | | | | higher the voltage, the more it is trying to lean the |
| with the vacuum-leak detection procedure, we need | | | | mixture out. |
| our car's oxygen sensor to be thoroughly warmed up | | | | So what should the voltage be at the diagnostic |
| before setting the base mixture. Naturally, any | | | | socket if the mixture is perfect? There will always be |
| vacuum leaks need to be rectified before the | | | | some fluctuation when the oxygen sensor is hot and |
| mixture can be set. | | | | the car is in "closed-loop" control; i.e. constantly |
| A Little Theory | | | | responding to the engine's behavior. But those |
| The voltage generated by the oxygen sensor is | | | | fluctuations will be in a narrow band around 12.2 - 12.3 |
| interpreted by the lambda control unit, which | | | | volts. (The exact center point can be found by |
| responds with a voltage of its own sent to the | | | | observing the voltage at the socket when the |
| lambda valve (frequency valve) next to the fuel | | | | oxygen sensor is disconnected.) Longer voltage |
| distributor (FD). The frequency valve is essentially a | | | | "journeys" into the 11-volt range indicate a lean |
| pulse-style fuel injector -- the only such device on a | | | | condition; up into the 13-volt range bespeaks a rich |
| car with continuous injection. It is a controlled | | | | condition. |
| "bleed-off" of fuel back to the fuel tank. The amount | | | | Our response to this intelligence is to make very fine |
| of fuel bled off by the valve affects the pressure | | | | adjustments to the mixture control screw, accessed |
| drop across the differential pressure valves within the | | | | with a 3mm allen key inserted down through the |
| FD. More fuel bled off increases that pressure drop | | | | tower directly in front of the fuel distributor. Turning |
| within the FD and increases fuel flow to the injectors, | | | | clockwise richens the mixture; anti-clockwise leans. |
| richening the mixture; less fuel bled off reduces the | | | | The screw is extremely sensitive; one should only |
| pressure drop and leans the mixture out. Fuel is bled | | | | move the screw a fraction of a turn at a time. After |
| off when the frequency valve is open, and this | | | | each adjustment, remove the allen key (so it doesn't |
| occurs when the voltage across the valve is zero. | | | | get stuck when the air flow meter plate deflects) |
| The valve is closed when battery voltage is applied. | | | | and rev the engine. Then recheck the voltage at the |
| The voltage from the lambda controller pulses rapidly | | | | socket. If the voltage stops fluctuating entirely, the |
| and can only be fully observed on an oscilloscope. | | | | oxygen sensor has cooled off and you've gone back |
| (This is known as a duty cycle.) Fortunately, | | | | to "open-loop" operation, wherein the lambda |
| however, we do not need a scope to see what the | | | | controller sends out a default 50/50 duty cycle. Hold |
| frequency valve -- and by extension, the lambda | | | | the engine at 2,000 rpm for a while to warm the |
| controller -- are doing. | | | | sensor back up. |
| A Delicate Practice | | | | |