Help...how to read .7V+/-.25 on MAS potentiometer
 

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.

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)

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

Right now the connector is disconnected.

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

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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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.

For what it's worth, I've never seen a digital multimeter that wasn't 10Mohm impedance.

Measurements below 3 Volts frequently involve sensitive circuits which have high output resistance. To measure them accurately, you want a DMM with at least 1000 MegOhms input impedance. Better DMMs automatically provide this high input impedance.

There are DMMs which let the user select the impedance. Some DMM defaults back to 10 MegOhms input impedance, potentially altering the measurement accuracy by orders of magnitude.

Have seen cases where a DMM emits pulses from its measurement terminals, potentially altering your measurement results.

What about this one ? See no impedence listed.

http://www.herbach.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HAR&Product_Code=A4-221&Category_Code=MM

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

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.

A thousand Megohms? That's a new one for me. What kind of accuracy difference is there between a 10M ohm and 1000M ohm in testing?

It'd be more convincing if it listed a lower impedence rather than not listing it at all. An internet blurb is hardly the entire Users Guide. As to the rest, I'm guessing that the meters with variable impedence aren't exactly the cheaper ones. I'm also guessing that a meter with 10M ohm impedence is accurate enough for the test in question. Or, have you seen a DMM with less than 10M ohm impedence?

In any case, it's good to have impedence information out where it might help somebody.

That may work, but I usually use a small resistor with the lead jammed in beside the pin, from the back of the connector (pin retainer snapped aside). If your meter is affecting idle, then you are either on a non-voltage range, non-d.c. range, or your meter probes are not in the right banana jacks on the meter. It sounds like you are either on a resistance or current range. Measure a flashlight battery with the meter and verify you get about 1.5 Volts, before measuring the pot. Note that the pot does not affect idle speed, but ANY false or intentional change in its output will cause a quick, temporary change in engine speed.

Steve

I use Fluke equipment.


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O.K. Steve,

I'm going back out to try it again. I'll try to jam the probe to the back of the connector, if that doesn't work, I'll use a needle through the wire (do you happend to know by chance which wire it is? Or is it pretty much self explanatory when I look at the connector?

I can measure an AA battery and get 1.3V (old battery). I probably didn't have it on correctly. I've got big clumsy hands.

I'll be back in 30min. I'll try it again.

JR