Re: High Freq MAF Problem

["Thomas Matthews" <tmatthew@stny.lrun.com>]

In the 89 165 MAF F and Y apps, they have MAF and VATS both, but I know on
the 90-92 730 F and Y that the VATS input is into where a MAF would connect
on a app with MAF.
Scott, would love to hear details about the code changes you made.
Tom
----- Original Message -----
From: "Scot Sealander" <Sealand@clarityconnect.com>
To: "gmecm" <gmecm@diy-efi.org>
Sent: Friday, January 28, 2000 7:21 PM
Subject: High Freq MAF Problem


> Well, been trying to get a high freq MAF (Buick 3800) working as
> an additional sensor input on my Turbo GP.   It just did not want
> to work!
>
> Pretty sure the added code section worked, but DIACOM would never
> display any readings for airflow.  (The MAF airflow was put in the
> DIACOM scan string as a substitute for the the normal calculated
> airflow.)
>
> Found a snag for those thinking about adding a newer MAF to the
> 730 and/or 165 ECMs.  As you are aware, the MAF on the early P4
> ECMs can be DC or freq input. There is a different input on the ECM
> for each sensor type.  Both the MAF and the VATS would use the same
> counter input. (Any application can only use either the MAF or the VATS,
> not both at same time.)  The VATS input is low freq, (about 30Hz) and
> the low freq MAF is about 30 to 145 Hz....
>
> I suppose because of the low freq, the ECM input is very heavily low
> pass filtered. The min freq of the high freq MAF (1 kHz) cannot even get
> through to the input of the counter!  With the 'scope attached to the
> input pin on the counter chip, even the VATS 30 Hz squarewave is very
> heavily rounded.  The high freq MAF just doesn't have a chance.
>
> So, if a high freq MAF is going to be added in the future, a chip cap
> change is going to have to be made to the ECM to move the LP corner freq
> much higher.
>
> Scot Sealander  (Off to change a chip cap in the '727...)
>
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