From: bernie@metapro.DIALix.oz.au (Bernd Felsche)
Subject: Re: Vacuum Sensor at Speedo head (mpg readout from MFA wrong)
Organization: MetaPro Systems, Perth, Western Australia
Date: Sun, 13 Dec 92 12:16:21 GMT

In <hersh.723504391@xenon.lcs.mit.edu>
   hersh@expo.lcs.mit.edu (Jay Hersh) writes:

>Recently my MPG readout has been reading 99.9 so I got to figuring some
>sensor was busted. The VW shop manual says nothing about this, so when
>I had the car in for the heater caore recall I asked the dealer to diagnose
>the problem. The dealer charged me $25 to tell me the problem is 
>"the Vacuum Sensor at the Speedo head"

Check the Bentley manual. If everything else in the MFI
seems OK, then it could be that, or a faulty connection to
the tacho.

>all of which means nothing to me (i.e. no part number, nothing mortals
>understand, seems like mechanics think they're like Doctors, invent a 
>lingo that no one else understands and you can get people to pay you big $$
>for your supposed expertise, but that's a different tirade :-)

Quite naturally, they'll replace the most expensive part
first! :-(  You should be able to check the integrity of the
vacuum line from the engine to the sensor yourself. Make
sure it's not kinked.

The pressure sender senses manifold pressure so that the MFI
can map fuel input / engine speed / road speed to fuel
consumption. There are 3 electrical connections, one
(presumably) produces a voltage/current output proportional
to manifold pressure.

>Of course what does mean something to me is that they said this part costs
>$186 and is $100 (i.e. 2 hours labor??) to install....

They gotta be kidding. 2 hours? That's how long they're
supposed to R&R the whole dashboard! 1 hour if you have a
manual and do it the first time! Just remove the instrument
panel fascia and the whole instrument panel pops out
(undoing a couple of screws, cables and clips). If your's is
pre-87, then some modification is necessary (shouldn't take
10 minutes more).

>Anyone got any clues here as to what/where this is.

On the back of the instrument cluster, and it has *nothing*
to do with the speedometer, other than mounting on the back
of it!

>I'm wondering if we may just be talking bad hose or something similar,
>since my cruise control routinely fails ever since I got the car and the
>remedy is to replace the vacuum line hose which gets brittle and cracks
>every 1-3 years, so I am wondering that if this is some vacuum sensor related
>thing maybe it's potentially a similar or somehow otherwise repairable
>deal versus the dealer mentality (which says the headlight is broken
>lets replace the whole front end... :-)

Ahh, you have cruise! If it's factory, it has its own,
separate vacuum system, driven by a vacuum control motor.
This is not to rule out that the hoses to the sender unit
are stuffed; one can't say those things without looking, but
it would be lot cheaper to replace the hose than the sender.

Invest in a Bentley manual. But be careful not to tell the
mechanic how to do his job!

>--
>Hopfen und Malz, Gott erhalts

What a considerate .sig! :-)
-- 
+-----+ Bernd Felsche                     _--_|\  #include <std/disclaimer.h>
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