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  #1  
Old 06-12-2003, 04:45 PM
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Air Mass Potentiometer, DIYer?

I have already posted similar things but have not recieved any good responses. I am planning to replace the AMP (Air Mass Potentiometer) on my father's '87 300E. It has 3 symptoms that I have read about and seem to all be related to the AMP. They are as follows:
1: Car starts then dies immediately, will restart right away. Will not happen if it is given a little gas.
2: Has a rough idle. I have cleaned ICV at least 3 times with carb cleaner and still no fix. Would it have to be replaced?
3: Sometimes has a hesitation when taking off from stoplight, etc.
Can I just put the new AMP on the side of the air mass sensor? or do I need to calibrate something? Any help would be ever so greatly appreciated. I have read the thread with all the pics but I just want some clear answers on what I need.
Thanks
David
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Old 06-12-2003, 05:00 PM
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The air mass sensor is an all-in-one unit. The amp is not seperate.

That said, removal and installation is a matter of loosening the clamps at each end and swapping out. Takes about five minutes total...easy DIY.

The engine management system will eventually recalibrate for the new sensor.

The sensor is a pricey unit (over $200)...and it sounds more like a fuel related issue to me...like the fuel accumulator...
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  #3  
Old 06-12-2003, 05:06 PM
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This thread explains all that I am trying to say.

300E: test sticks on 10% duty cycle

The peice I am trying to change is a little housing piece that goes on the side of it.
Thanks
David
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  #4  
Old 06-12-2003, 05:12 PM
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Here are some pics that may better explain it. They are taken from a previous thread.

Quote:
The first pic shows the potentiometer housing mounted on the air meter. To remove the housing, put the tip of a knife in the middle of the cap (1) and pry out.
Then remove the four screws with a T15 torx driver. -right pair shown in (2)- .


If stevebfl will confirm, I think the cap on the outside with the trim screw underneath is (3). The rubber cap did not come out intact on a 15yr old version, hopefully it will on a new one.
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Old 06-12-2003, 05:14 PM
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This image was also taken from a previous thread. Sorry if I am not giving the rigth people the proper credit.
This image shows an over-all appearance of the air mass sensor and the Air Mass Potentiometer.

Note that it says "very hard to calibrate".
Thanks
David
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  #6  
Old 06-12-2003, 05:19 PM
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Look in the same link

Good homework, if you read towards the end, you will see it was succesfully installed and calibrated. There is another post from sbourg that explains the same thing. It has the link to the online parts store. I wonder if Bill at parts shop will start selling the potentiometer by itself.
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Old 06-12-2003, 05:27 PM
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I read at the end where it was successful but I want to know exact instructions for a rookie. I have the multimeter, I just don't know what they talk about when speaking of pin 2 to pin 5, etc. What pins? Where are the pins? What reading am I looking for? Those are the kinds of questions that I need answered. If it is too difficult then how much would it cost in professional labor at an idependant shop?
Thanks
David
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  #8  
Old 06-12-2003, 10:07 PM
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can anyone help me with instructions on how to calibrate the new part?
Thanks
David
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  #9  
Old 06-13-2003, 10:15 AM
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I haven't put mine in, and will likely not for awhile. The big problem is, the ECU will use two sensor inputs to adjust the mixture via the EHA. From other posts and hints by Stevebfl, I believe the pot is the first line of information on change-of-demand by the engine, with fine-tuning at running speed by the lambda sensor. This will make adjustment difficult by a DIY without the Bosch recommended procedure in hand. I would count on having a final adjust done by a pro, if no one offers a suggestion up on this site.

As a first line of attack, however, it seems to me that IF you can get the pot set very close to the correct value at normal idle, you will likely not need any further adjustment. It is vital to put a reference scribe mark on the airflow meter body BEFORE removing the pot for any reason. Careful measurements with a caliper of any mechanical difference between the circuit board alignment on the new and old parts should allow a compensating rotation to take care of this issue.

Adjustment of the trimpot is a trickier issue. Although it can easily be just set to the same value as the old one, this might not be correct. The resistance element is a complicated multi-tapped design, and my experience with deposition techniques is that variance from sample to sample is highly likely. This is what the trimpot is designed to compensate.

My method will likely put a test voltage across the old and new pots, and measuring specifically at the geometric spot where the wear marks are worst (idle?), adjust for similar voltages at the element.

But best, maybe someone can post the actual in-situe procedure?

Steve
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  #10  
Old 06-13-2003, 12:36 PM
inspector1
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I am nto sure if this is what you are talking about as you are using some terminology of which i am not familiar, but to set the air/fuel mixture for the AIR FLOW SENSOR:

1. Disconnect the O2 sensor at the plug. Observe volt meter reading and place a piece of tape on VM dial to mark position of needle. This is the 50% duty cycle position
2. Reconnect O2 sensor. The needle will fluctuate somewhat but should be centered around the 50% mark of the VM ( 50% =/- 10%) if it doesnot, then adjustment is nessary
3.Remove the plug from the mixture control uniut ( between the fuel distibutor and the A/F sensor funnel) insert a 3mm allen head wrench and adjust A/F mixture screw by pushibg down until the spring loaded adjuster engages with the adjuster screw in the air flow sensor arm. After each adjustment, rev the engine a little to allow the sensor plate to normalize before checking the meter reading
4. Adjust until the reading is centered around the 50% mark on the dial
5.Remove test equipment and replace plug on AF sensor

Hope this is what you wanted.
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  #11  
Old 06-13-2003, 09:47 PM
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No, this adjustment and procedure is for mechanical tracking of the air flow meter. The adjustment in question would be with a small-tipped screwdriver on the sensor pot trimpot, mounted to the side of the airflow meter body. Thanks for posting that procedure, though.

Steve
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