lobhater

Hello, This is my first post here. Though I have been lurking since I bought the car in December. My first Lexus and I look forward to working on her. Here it goes...

2000 ES300 Platinum edition, Automatic. The owner had installed aftermarket HID headlights.

I installed an aftermarket deck keeping the OEM amp in line. There is a slight hiss. I installed loop ground isolaters. This resolved the hiss sound in the ACC position but when the car is on or it is off but the key is in the RUN position the sound is still present. I decide to find a better ground in hopes that will remove the sound. This is where the problem starts...

I hook my multimeter up to the negative terminal of my battery, I set my VM to 200 ohms and clip one lead to the negative post and touch the other lead to the negative post. Zero or darn near. Move the lead to a fender bolt. Zero or darn near. All is well with my meter and test setup. I move inside the cabin. One lead is still clipped to the negative battery post, the other lead is touched to some good looking what I think are grounds. It shows negative resistance. I scratch my head. I have never seen negative resistance before. I switch the VM over to measure 20 volts and sure enough there is voltage on ALL of my ground points in the cabin. So I may have already shown my lack of expertise and experience but I know little to no about electricity. But it seems to me something is off. How do I go about tracking down where this "short" is? (Short may not even be the right word here.) I have the Chiltons Manual sitting next to me but dont even know where to start on the diagrams. Any suggestions?

Htony

I am retired EE.First if you work on mobile 12V DC, suggest you to use an old analog type multi meter. I think what you are referring to negative resistance, your meter is in DC volt measuring setting. Digital multi meter has pretty high input impedance so it'll pick up anything. If you set the meter to Ohm's(resistance measuring range) and hold meter lead on each hand you'll see readings. Another way of testing good ground is using
test light. You should never have a ground loop when wiring shielded signal wire. Only ground one end of the shield to ground. I still keep an old Simpson 260 analog meter to work on certain things.If you use low level input to amp the sensitivity is higher to such things a s noise. Try to use high level input(using RCA connectors). I am not a good writer, hope it made some sense to you. Don't ever make a poor connection, then the joint
may act like a diode creating noise.

lobhater

When I am referring to negative resistance I had the VM set to 200Ω and saw negative numbers. I had never seen that and used VM a fair bit so I was really surprised. Jumped online and looked up negative resistance on a VM and someone said it was easy to explain. Suggested I switch to 20 DC volt setting. I did and found voltage between the negative battery post and a known factory ground

Htony

To explain on theory about negative resistance, we have to go into conjugatory numbers. Any how simply speaking one of the ground point is no good. That is not at same electrical potential(ground level at the battery post) I suggested also using test light. Good ground point will give bright light vs. poor ground will give dim light. As a side note when you drive a ground rod to ground your house electrical system, check it with a light bulb connected between 120V hot line(black wire) and ground. If ground is good bulb will light up normal brightness.

wmj259

I don't think we would enjoy talking about impedances.

Htony

Me neither, LOL!

speedkar9

Now your bringing back memories from high-school computer engineering class...