Date: Thu, 2 Sep 2004 11:26:19 -0400
Reply-To: Tom Corbitt <corbitt@ANR.MCNC.ORG>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Corbitt <corbitt@ANR.MCNC.ORG>
Subject: Re: gauge mod help
In-Reply-To: <vanagon%2004090108500716@GERRY.VANAGON.COM>
Content-Type: text/plain; charset=us-ascii; format=flowed
Mark Cumnock wrote:
>I have an aftermarket gauge, that i am wanting to make read 1/3 lower . I
>got a water temp gauge, and I am using it as a oil temp gauge. I would
>like for it to read a little lower, the upper end of the gauge the number
>are low, so the gauge almost pegs out.
>
>Do you install a resistor in line with sensor, what size resistor would it
>take.
>
>Hope someone has a clue on this ......
>
>
>
>
Do you know the behavior of your sender (does it have a negative or
positive temperature coefficient?, what is it's resistance range?) and
the characteristics of the meter you're using? I'd bet that it's a
negative temp model, meaning the resistance of the temp sensor decreases
as it's heated. As the resistance goes down, more current flows in the
circuit which moves the meter. In that case it's just a matter of
splicing in a variable resistor into the circuit in series with the
sender unit. Set the pot to 0 ohms, start the van and wait for it to
reach your max temp. The adjust the pot (increase the resistance) so
that the needle is at whatever point you want it to be at. You can then
either leave the potentiometer in the circuit or disconnect it, read the
resistance between the wiper and the leg, and then replace it with a set
resistor of the same value. You'll then have to calibrate the dial to
it's new range and either mark the face or open the meter up and create
a new dial. Most temp sensors I've seen usually go pretty low (in the
ohms department) at high heat, you'd probably be fine with a 1k Pot (you
could go with a 5k pot, you'll just have a harder time hitting the sweet
spot) I'd argue that for the amount of time you're going to spend
calibrating the dial for your range you could probably go out and pick
up a cheap oil gauge and be done with it, but what do I know?
If your system is positive or has something else going on inside it it's
going to be a different path. It's possible to build an circuit to
condition the signal from the sender to allow you to "dial in" whatever
you want on the display, probably the easiest way would be a transistor
inverter circuit or an op amp which could be designed to handle/convert
inversely-proportional and non-linear sensor inputs. As with the first
paragraph, it's really a question of what value you place on your time.
I'm assuming that you've got a voltmeter and know enough about
electronics to understand concepts like voltage/current and which leg of
a pot is which.
cheers,
tom corbitt
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