Electrical loading of MAF sensor output...

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frenchy - 01 Aug 2006 05:33 GMT

Hello there,

I am designing an auto accessory that would respond to the MAF sensor
voltage (0-5v analog GM/Bosch style). I will therefore need to tap
into the MAF sensor signal which is being sent to the ECM. My
question is relating to the input impedance of my accessory. What
shall the minimum input impedance be such that the original MAF sensor
signal feeding the ECM is unaffected? I cannot find any specifications
that deal with this topic.

For instance would an input impedance of 90kohms be acceptable?

Any help or references would be very nice indeed,
thx,
frenchy

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Scott Buchanan - 01 Aug 2006 06:23 GMT

The obvious answer is the more impedance the better. Ask Heisenberg. I would
imagine that there is not much current in the circuit. Modern Multimeters
are 10 megaohms to minimize the effect on a circuit. You load seems a little
low but the only way to know is to test it yourself. Apply a load and
measure the voltage drop. When it is low enough it is good. I would say not
more than .05V.

> Hello there,
>
[quoted text clipped - 11 lines]
> thx,
> frenchy

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frenchy - 01 Aug 2006 14:42 GMT

> The obvious answer is the more impedance the better. Ask Heisenberg. I would
> imagine that there is not much current in the circuit. Modern Multimeters
> are 10 megaohms to minimize the effect on a circuit. You load seems a little
> low but the only way to know is to test it yourself. Apply a load and
> measure the voltage drop. When it is low enough it is good. I would say not
> more than .05V.

Scott,
Thank you for your reply. I do not have any MAF sensors here in my
design lab, which is why I am researching this. I want to simulate it
here in my lab 1st, then indeed I will test my prototype on real
vehicles. Certainly there must be some printed specs regarding MAF
sensor output impedances or a discussion on loading.

thx,
frenchy

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Scott Buchanan - 02 Aug 2006 05:33 GMT

I would be surprised if there was anything in the public domain. It is too
esoteric. The only place you may find any documentation is in the
engineering departments of auto manufactures. Not to tell you what to do,
but if you are going to properly develop your product, you will need a test
bed in your lab. Obtaining parts from a yard would be a good bet. In the
mean time, I would tap into the circuit of a car to do the testing and get
the information that you need.

If you can share, I am curious what you are doing. If you need a tester let
me know.

> > The obvious answer is the more impedance the better. Ask Heisenberg. I would
> > imagine that there is not much current in the circuit. Modern Multimeters
[quoted text clipped - 12 lines]
> thx,
> frenchy

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=?x-user-defined?Q?=AB?= Paul =?x-user-defined?Q?=BB?= - 02 Aug 2006 00:01 GMT

> Hello there,
>
[quoted text clipped - 11 lines]
> thx,
> frenchy

The ecu is ttl and cmos. I would try to get at least 10 meg ohms.
IMO, 90k would NOT be acceptable. Nor would 900k.

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Paul Hovnanian P.E. - 02 Aug 2006 03:31 GMT

> Hello there,
>
[quoted text clipped - 5 lines]
> signal feeding the ECM is unaffected? I cannot find any specifications
> that deal with this topic.

The higher, the better. 100K would be good, 1 megohm would be better. Be
careful not only of loading the sensor with a low impedance but of
conducting external interference back into the ECM through your device.

A better (but more complex) solution would be to read the OBD II data
and get the air flow value from that, scaled to pounds per second or
whatever.

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larry moe 'n curly - 03 Aug 2006 02:34 GMT

> I am designing an auto accessory that would respond to the MAF sensor
> voltage (0-5v analog GM/Bosch style). I will therefore need to tap
> into the MAF sensor signal which is being sent to the ECM.

> For instance would an input impedance of 90kohms be acceptable?

I thought that platinum MAFs had only a few hundred ohms resistance,
but according to this thread it's higher:

www.innovatemotorsports.com/forums/archive/index.php/t-1399.html

Is there any reason why your accessory's input impedance will be 90K?
Why can't you use a buffer IC and get much higher impedance/

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