I followed the instructions on this board about changing out the MAS potentiometer on a 1986 300E.

I outlined the old one with a blade, removed it and replaced it in the exact same spot as the old one. Now I'm trying to measure the voltage.

Instructions given were to hook up a voltmeter from the #2 pin to ground. So I put the red prong on pin #2 and ground the black prong to the strut bolt. I keep getting readings of 025.5mV?! What am I doing wrong?! I'm using a RadioShack digital multmimeter and have it set on V. I tested my battery it checks out at 13.5, I tested an AA battery and got 1.3V.

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JR
1986 Gold 300E 4Dr automatic

Has the multi meter got a 10 mg ohm resistence.....'cos if not, your gonna fry somethin'... !



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I have no idea. Just went to radioshack and got the $33 multimeter. I'm not an electrician nor am I an engineer.

There are 2 prongs for the red wire. One says +10A Max (unfused)

The other one says +V.omega.mA, CATII 600V max to gound (400mA MAX positive)

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JR
1986 Gold 300E 4Dr automatic

Am I supposed to be testing the pin with the connector on?

Right now the connector is disconnected.

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JR
1986 Gold 300E 4Dr automatic

Depends on the test....

You need to 'back probe' the connector when KOEO/KOER.
.Key On, Engine Off.
.Key On, Engine Running.

You should have a supply voltage to the MAS and a signal line to the ECU.and a ground too....Most MAS failures are due to poor grounds and dirty sensor wires/strips.

The way to check MAS operation is with a Digital Lab' Scope, usually a dual screen type.



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[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Okay...let's first find out ...what is it you are trying to accomplish.?..
The unit you seem to be refering to is the actuator...the black plastic unit on the back side of the air flap housing.....This unit changes the fuel pressure in the upper chamber to control the quantity of fuel delivered by the injectors.

It is driven by variations in mA (Mili amps ) by the ECU after it interprets the signal from the O2 sensor in the exhaust.

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[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."

Absolutely, and the engine idling (steady, low idle). The connector applies a voltage to one side of the potentiometer, and connects the other side to ground. It connects the center pin, which should have about 0.7V now, to an input of the FI computer.

The pot acts as a voltage divider - it does not generate any voltage on its own. It simply 'divides' the voltage applied to it acording to its angle of rotation.

Even though it is a variable resistor, note that any resistance measurements will be meaningless. The output is isolated with a high impedance.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

Steve...thank god you're here.

I followed your instructions on your post regarding the MAS potentiometer. I'm at the point where I'm trying to test the voltage on pin 2.

This is what I just tried a few minutes ago...I put the connector half way to expose the pin, then tried to test it (red prong to pin #2 and black prong to the strut bolt for ground). Problem is, when I put the multimeter on, the idle will suddenly rise. What am I doing wrong?

Am I supposed to put the connector all the way on then try to test pin #2 (ie. put a needle through the wire and place the red prong on that?)

Car runs but the idle is really low around 400rpm.

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JR
1986 Gold 300E 4Dr automatic

You need a digital multi meter with 10 Megohm Input Impedance.
This makes the meter invisible to the electronics.

Without this feature you can introduce voltage with will not only change the measured signal but induce noise and sometimes ruin a sensitive component.

Try this for an explaination........http://081368.tripod.com/diags/motoragejune98.htm

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"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."