Temp gauge goes up when I turn my lights on. Put an aftermarket gauge in and it still does it. Still using stock sending unit cause I didn't have an adapter for the one I bought. I used all new wiring for the gauge. Could this be a ground issue?
I might have misread you but you can't use the stock sender for an aftermarket gauge.
The sender compatibility aside, as you noted, it's likely a ground issue. Don't overlook the sender itself (ohm out the body to the neg battery post - thread sealants can interfere with sender grounding).
A bad ground between the engine and body is the cause of your problem.
Grounds
Grounds are important to any electrical system, and especially to computer controlled engines. In an automobile, the ground is the return path for power to get back to the alternator and battery.
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 as much current, so it needs to be 4 gauge too.
The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.
The battery common ground is a 10 gauge pigtail with the computer ground attached to it. Picture courtesy timewarped1972
Correct negative battery ground cable.
3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to its proximity to the battery, it may become corroded by acid fumes from the battery.
In 86-90 model cars, it is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.
In 91-95 model cars it is a black cylinder about 2 1/2" long by 1" diameter with a black/white 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.
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.
I wasn't sure if the stock sender would work but I didn't have an adapter for the new one so I tried it.the motor to body ground was going to be my next guess. Thanks for reinforcing my theory. I will ohm it out and we will see what happens.
Ohming low resistance connection like grounds does not work well. I could tell you why, but unless you were an electrician or electronics tech, it wouldn't make much sense. You will need to use the voltage drop test method as described in my post and http://assets.fluke.com/appnotes/automotive/beatbook.pdf.
Joe, your above post is obviously true with high-current circuits. Now in the scenario I noted in post 2 (which I think is what you were commenting upon), one would ohm out the sender-body to the battery ground (simply to ensure that the sender is properly grounded), I see no reason to do much else in terms of testing for this purpose. What exactly would you do?
You are correct about using a regular ohmmeter test to measure the resistance between the sender body and the engine block.
The sender to block connection isn't where I would use the voltage drop test. The primary problem seems to occur at the engine to body ground which causes a voltage offset between the gauge and the sender.