is my autometer lying to me

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Where did you put the sender for the autometer gauge. If it is not in the location of the stock sender you may get false readings.

carbed87 said:

hey guys. ever since i got my 86 coupe, the stock gauges were always innacurate. the temp would at times randomly peg out, leading me to belive my cars overheating when it's really not. so finally i said screw it, friday i bought and installed autometer's C2 series electric temp gauge using their own coolant temp sensor. well lately this gauge has been showing that my car is running anywhere between 220 and 240. WTF! i would frequently pull over to find absolutely no sighns of the car overheating. the coolant isn't discolored, nor the oil, no smoking or boiling has ever occured, and i just flushed the entire system via machine in october, so i know the coolant is good and balanced fine. so what could it be. honestly im really stumped here, i have no clue. when i get back from my trip i plan on changing the thermostat, but i really dont think thats the reason. any help?

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If you use electric gauges and have bad or missing grounds, you will get false readings. The same bad grounds will mess up the readings on your stock gauges.

Grounds are important to any electrical system, and especially to
computers.

1.) The main power ground is from engine block to battery: it is
the power ground for the starter & alternator.

2.) The secondary power ground is between the back of the
intake manifold and the driver's side firewall. It is often missing or
loose. It supplies ground for the alternator, A/C compressor
clutch and other electrical accessories such as the gauges.
Any car that has a 3G or high output current alternator needs
a 4 gauge ground wire running from the block to the chassis
ground where the battery pigtail ground connects.
The 3G has a 130 amp capacity, so you wire the power side
with 4 gauge wire. It stands to reason that the ground side
handles just a much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery & alternator

Picture courtesy timewarped1972

3.) The computer has its own dedicated power ground that
comes off the ground pigtail on the battery ground wire. Due to
it's proximity to the battery, it may become corroded by acid
fumes from the battery. It is a black cylinder about 2 1/2" long
by 1" diameter with a black/lt green wire. You'll find it up next to
the starter solenoid where the wire goes into the wiring harness


4.) All the sensors have a common separate ground. This
includes the TPS, ACT, EGE, BAP, & VSS sensors.

5.) The O2 sensor heaters have their own ground (HEGO ground)
coming from the computer. This is different and separate from
the O2 sensor ground. It is an orange wire with a ring terminal
on it. It is located in the fuel injector wiring harness and comes out
under the throttle body. It gets connected to a manifold or bolt on
back of the cylinder head.

6.) The TFI module has 2 grounds: one for the foil shield around
the wires and another for the module itself. The TFI module
ground terminates inside the computer.

7.) The computer takes the shield ground for the TFI module and
runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from
the battery ground wire) uses pins 40 & 60 for all the things it
controls internally.

See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103#volt
for help troubleshooting voltage drops across grounds

Extra grounds are like the reserve parachute for a sky diver.
If the main one fails, there is always your reserve.

The best plan is to have all the grounds meet at one central spot
and connect together there. That eliminates any voltage drops
from grounds connected at different places. A voltage drop
between the computer ground and the alternator power ground will effectively
reduce the voltage available to the computer by the amount of the drop.

The engine secondary ground is the critical ground here. As I said it is often damaged or missing. The stock ground is a 8 gauge or larger wire from the back of the head or manifold to the firewall. Sometimes the voltmeter reading will wander at the same time as the temp gauge does.

At any rate, a spare heavy duty ground is good insurance to keep your electrical system working at top efficency.