Current in any circuit can be read anywhere in the circuit. What you must remember is that you must place the ammeter into the circuit to read the current.

EHA current is done most easily by using a ready built harness which I don't remember the number for but can be found in the archives. I will explain a technique that could be used without harness: Remove connnector from the EHA, hook a wire from one terminal on the harness to the terminal on the EHA that it would connect to if the harness were plugged in. At this point you have an open circuit because the other side of the EHA is not hooked to anything. Now use the ammeter to bridge that open by placing one lead on the EHA and the other lead on the connector terminal that would be there if plugged in. In this position the circuit runs through the ammeter.

Now turn the key on, engine off and verify that you are reading +20ma. This is the key to verification of your hook-up. If the polarity is wrong either switch the ammeter leads or do the correction mentally.

You should not be reading 2ma with an O2 sensor pegged high. To verify lambda capability disconnect the O2 sensor EHA current should be 0ma. Ground the disconnected lead that goes to the controller (as opposed to the other side which goes to the sensor). The EHA current should slowly walk all the way to 12-13ma positive and stop. To test the other direction place one hand on that same lead and place your other hand on a 12v source. The EHA current should walk itself all the way to 12-13ma negative current. These values are constants and can help in viewing what is happening and as to whether you hook up is correct. BTW this last test can be done with the engine running and the correction in mixture will be evident in the way the engine runs.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician