Re: Boost Guage Accuracy

["Scott Mo." <scottmo@teleport.com>]

Douglas,

> The latest discussion is w.r.t Colin Hames' problem. Once again,
> it's asserted that the guage is inaccurate. Ok. But where is the
> problem?
> 
> The ECU measures the boost using the pressure transducer which
> is mounted on the ECU analog board, right? 

Yes, this is correct.

>And the ECU samples
> this and then writes some value to the boost guage.

No, actually it is much simpler than that, the analog signal from the
pressure transducer gets split and sent off to the boost gauge. An op
amp is used to buffer the signal. If you remove the ckt. boards from the
metal frame, you can easily trace out the circuitry because only two
layer boards are used. The gain of the op amp can be changed to rescale
the boost gauge readout. This is normally done in the case when you
change out the stock pressure transducer with the 2.5 bar Motorola
MPX4250 unit. IC2 on the analog board is the quad op amp used for this.

There is a uP/controller in the dash assembly (dash computer) that is
used to convert and display this analog boost signal on the LCD. The
response of this display is VERY slow. You can change the analog voltage
input to the display and it takes several seconds before it changes to
the correct value. This would make for a good project for someone, to
sort out why it responds so slowly. 
> 
> Does anyone know of an ideal pressure transducer for this
> application? If not, I guess I'll have to dig out the latest DigKey
> catalog and get to work...

Motorola MPX4250 0-250kPa, absolute, designed specifically for turbo
charged vehicles.$20-35 depending on quantities and package style
selected.

HTH
Scott Mo.