Fwd: [Chrysler300] Do guages care?

[William Huff <whuff@xxxx>]

Frank,

Under no circumstances would I try to put a full 12 volts to the 
gauges. 12 volt MOPARS use a voltage regulator that provides a pulsed 
voltage to the instruments. The effect is to have a lower mean voltage, 12 
volts will fry them in short order, plus they will be inaccurate since they 
are not calibrated for that voltage.

Having a car rewired from 6 to 12 volts does not always mean the wiring 
itself was disturbed, since some sort of voltage divider might have been 
used to provide a lower voltage feed to the instruments. The starter 
doesn't really care that much, and the generator would have to be rewound 
or replaced along with the voltage regulator. The light bulbs, for the 
most part would just be replaced with their 12 volt equivalent. I don't 
know about power window or seat motors, but I suspect that since they are 
low duty cycle, they would work just fine, only a little faster.

Sounds to me like you might have a problem that is not related to the 
actual conversion. first thing I would do is crawl under the dash and try 
to ascertain if the factory wiring is basically intact. If it is, wait for 
the manual to try and troubleshoot. Since the gauges usually work by 
measuring amperage (micro-amps) through a variable resistance (the gas tank 
sender or the temperature sender) to ground, if they work at all, I would 
suspect the problem to be in the voltage feed to the instrument panel 
rather than a ground. The only ground in most instrument circuits should 
be where the sender is grounded. There are exceptions, but that is the 
norm. I would try to find a common feed for the instruments, the map light 
and the overhead lights. Not an easy task, the ability to read a schematic 
diagram would be very helpful.

You might be surprised what you find. I know in the late '60s MOPARS, if 
you turned on the wipers, the emergency flashers, pushed the turn signal to 
one side or the other and turned on the radio, everything started working, 
pulsing in time with the emergency flashers. This with the key off and in 
my pocket. The voltage was back feeding through the turn signals and 
powering the accessory bus every time the flashers turned on. You may have 
a similar situation here, where the instruments receive a voltage feed only 
under certain conditions. Sorry for rambling, but please don't connect 12 
volts direct to the instruments, whatever you do.

300ly,

Bill Huff





> > I'm trying to figure out my weird gauge problem. My c300 was rewired to 12
> > volts. The gas and temp gauges only work if the door is closed (interior
> > lights off) and the map light is on. Open either door and the gauges go
> > dead. Everyone I've talked to says its a ground problem. To make a long
> > story short, I'm thinking of taking the "gauge power" wire and going right
> > to the ignition switch, rather than trying to figure out the rats nest of
> > wires. Do the gas and temp gauges care if its 12 volts? I have no idea at
> > this point if they are getting 12 volts when they do work. I still do not
> > have a manual (one is coming) but it may not be that helpful since the car
> > is re-wired.