300E Airflow Sensor Potentiometer; TVS

[joel]

89 300E

Been reading MB manuals, so my brain is bleeding. Will someone be kind enough to provide me the voltage and resistance of a good working airflow sensor and throttle valve switch?

Thanks for any help.

[sbourg]

Resistance measurements on the pot are worthless, because series resistances internal to the connection tabs will mask any problems. If you measure voltages while idling and connected, you will see wildly fluctuating voltages if the unit is bad - but since the idle will be fluctuating at the same time, this isn't useful either. Best test is to mark position, remove, and inspect the pot contact area. This will reveal badly worn contact areas.

Throttle switch is just that - should measure infinity open, and zero Ohms closed.

Steve

[joel]

Originally Posted by sbourg

Resistance measurements on the pot are worthless, because series resistances internal to the connection tabs will mask any problems. If you measure voltages while idling and connected, you will see wildly fluctuating voltages if the unit is bad - but since the idle will be fluctuating at the same time, this isn't useful either. Best test is to mark position, remove, and inspect the pot contact area. This will reveal badly worn contact areas.

Throttle switch is just that - should measure infinity open, and zero Ohms closed.

Steve

Hi Steve, I have read your write-up re R&R of the pot. A project for the future for me since this car is my daily driver.

What I am trying to do is to set the air/fuel ratio mix properly. The values set for percentages at idle, deflection of throttle plate, and open throttle are given standard, before any adjustments are to be performed. http:\www.landiss.com.

[sbourg]

Joel, the pot has virtually nothing to do with the steady-state mixture. It simply signals when enrichment is necessary by delivering fast rate-of-change information to the ECU. I.e., acceleration enrichment. Poor contact fools the ECU into believing there is a rapid fluctuation in airflow, and it then falsely dithers the EHA, screwing up the idle.

Steve

[joel]

Originally Posted by sbourg

Joel, the pot has virtually nothing to do with the steady-state mixture. It simply signals when enrichment is necessary by delivering fast rate-of-change information to the ECU. I.e., acceleration enrichment. Poor contact fools the ECU into believing there is a rapid fluctuation in airflow, and it then falsely dithers the EHA, screwing up the idle.

Steve

Thanks for your reply. The reason I am checking the AFP is because the values or percentages are off the prescribed numbers. The manual says in such eventuality check integrity of AFP and/or TVS.

[cc260E]

throttle valve switch : on connector, full contact between center pole and one of the other side when full gas or at idle, in between no contact. (MB experts can confirm)

airmass flow meter voltage I have read somewhere in this forum 0.7 volts