Ground resistance

[DumpinTreos 10+ year member]

What is the proper way to measure resistance for electrical grounds?

From what it seems like you need to disconnect power and supposedly DMM **** at measuring low ohms so you need to build a "low ohm adapter" is this true?

http://www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

The stock electrical ground points on my car seem shotty so I was trying to find out if the strut tower bolt might be a better ground.

Click to read more...

 

[fuggles 10+ year member]

that technique is no good for measuring chassis resistance. in that setup the lead resistance would dominate the measurement.

better is try different points and measure voltage at points of interest [battery, amp etc.]. that will tell you the resistance.

i doubt the stock ground points are shotty unless something is broken.

i know nothing about accord's, but a stut tower seems dubious. maybe you have other problems.

kent

 

[AlterEgo99 5,000+ posts]

I posed this question to Perry Babin, founder and creator of http://www.bcae1.com/ and here is what he said:

"A multimeter (DMM) isn't a good way to determine if the ground is good or bad. It can tell you if the piece of metal/bolt is connected to ground but the difference between a good and bad ground may be only a few hunderd ohms and that's beyond the resolution of most meters.

Connect to the floor pan of the vehicle to a point that's clean, bare metal. Fasten with a 5/16-3/8" bolt for high current applications."

He also indicated that there really isn't an instrument that can measure a "good" ground.

 

[PaulD 10+ year member]

we have multimeters here at work that can measure accurately to like .001 ohms but I don't think that will help. The problem with "shoddy" grounds is that they are not capable of handling 100+ amps efficiently, even though they may read low on a multimeter.

 

[DumpinTreos 10+ year member]

that technique is no good for measuring chassis resistance. in that setup the lead resistance would dominate the measurement.
better is try different points and measure voltage at points of interest [battery, amp etc.]. that will tell you the resistance.

i doubt the stock ground points are shotty unless something is broken.

i know nothing about accord's, but a stut tower seems dubious. maybe you have other problems.

kent

Am I looking for voltage drop or charging voltage?

 

[fuggles 10+ year member]

Voltage drop or minimum voltage same thing. Yes. If you know the regulator output voltage (~14V or whatever), you can measure voltages at other points in the system. As you know, less drop is better. Try different points (if you want/need to) and find the one which gives the highest Vmin.

I think that cleaning the stock connections and using quality wire and terminals should be enough. But I have no idea what rig you've got. Just general guidelines.

Kent

 

[I Like Waffle]

I have grounds straight to the frame in the back going to the trunk pan, and grounds straight to the frame in the front from the block and batts. Was a PITA to drill into but i know my shit gets good current flow, my frame is like 2gauge steel. lol

 

[smoka 10+ year member]

Voltage drop or minimum voltage same thing. Yes. If you know the regulator output voltage (~14V or whatever), you can measure voltages at other points in the system. As you know, less drop is better. Try different points (if you want/need to) and find the one which gives the highest Vmin.
I think that cleaning the stock connections and using quality wire and terminals should be enough. But I have no idea what rig you've got. Just general guidelines.

Kent

This guy is right but I find it much easier to calculate resistance by measuring the grounds for voltage gain (opposite of voltage drop).

But its true, you can't accurately measure a ground unless there is current running through it. Electricians have a tool thats opposite but similar its called a megger, they can hook up a wire and run voltage through it and check for any changes of resistance, this can tell you if a wire is good or not. Vehicles won't work this way since everything is connected to chasis but in theory its the same test that is needed. So we will do the similar test as a megger, but we will use your amp to pull the power and your meter to watch the resistance. Remember that you can use voltage drop to calculate resistance. In this case, we'll use voltage gain (opposite of drop) since we are dealing with the grounds.

If you want to compare two ground points resistance, do this: Connect one side of your dmm to your neg battery post. This is your reference ground. Then with your 2nd lead you will have to use a long piece of wire as an extension, it can be a thin wire. Now connect your 2nd lead to the 1st ground location where your amp is grounded. Play some test tones or a song with constant bass to get a good current flow going through the ground. Set the meter to dc volts. If you see a voltage generate when the music is playing, that means you are getting a higher resistance at that ground point. Check the 2nd ground this same way. Whichever ground shows a lower voltage (should really be 0 volts at all times), then that is the better ground point.

Or you can save yourself the trouble and just run a 1/0 ground from the front along with your chassis point. That usually takes care of any problems.

 

[Mike465 10+ year member]

If you are looking for a better gronding point without going to the frame rails of the car take your DMM and set it to continuity. Take your ground lead on the DMM and take a 18guage wire and attach it to the negative terminal of the battery. Now attach the wire to the wire to the negative lead on your DMM. Place the red lead on the potential ground spot and if you get continuity then you have a good ground path.

You can also take your DMM and set it to resistence and take your red lead on your DMM and connect it to the postive terminal of the battery using a wire extension. Use the black lead on the DMM and probe around for potential grounding spots. A good ground will have a resistance of 0-.02 ohms.

 

[DammitJames 10+ year member]

measuring resistance is actually very easy all you need to do is a "voltage drop" test with a dmm. you power the circut up lets say you wanted to test the resistance in the ground from the battery to the chassis. place your dmm on dc voltage and place one lead on the battery negitive post and the other lead on the chassis next to the connection of the ground cable. is there is resistance you will see a voltage. usally you will see below a volt on minor reistances or up to 12 volts on massive resistance. this is the most commonly used method for checking resistannce in a circut by automotive technicians. if you have any more questions feel free to pm me. if you think im full of sh1t check my credentials.

 

[smoka 10+ year member]

Sorry, you guys are both misguided in measuring the resistance of the ground circuit. If there is no load, then the readings are useless (unless there is an obvious problem).

(V)Voltage drop = (I)current running through the wire x ®the resistance of the wire

So if you have no load, you can falsely assume you have no resistance.

Here is an example using this voltage drop calculator:

http://www.stealth316.com/2-wire-resistance.htm

1/0ga wire at 20' (the example is using power wire since the chassis equivalent is near impossible to guess, but I've heard that testing has shown that the avg chassis is near equivalent to 1/0 wire)

If there is no load, (I'll assume the amp is only using 1a of current in standby)

Voltage drop of 0.002 volts

If there is a 250a load,

Voltage drop of 0.5 volts

So even if there was a major problem and the grounds were in a bad spot, say if we multiply the resistance by 10. You would barely notice the 0.02v drop but you would definitely run into problems with a 5v drop.

See why you need to 'load' the circuit to get correct results?

 

[DammitJames 10+ year member]

measuring resistance is actually very easy all you need to do is a "voltage drop" test with a dmm. you power the circut up

i do beleive i said it needs to be im pretty sure i know how to do one seeing that i do it daily

 

[DammitJames 10+ year member]

Sorry, you guys are both misguided in measuring the resistance of the ground circuit. If there is no load, then the readings are useless (unless there is an obvious problem).
(V)Voltage drop = (I)current running through the wire x ®the resistance of the wire

So if you have no load, you can falsely assume you have no resistance.

Here is an example using this voltage drop calculator:

http://www.stealth316.com/2-wire-resistance.htm

1/0ga wire at 20' (the example is using power wire since the chassis equivalent is near impossible to guess, but I've heard that testing has shown that the avg chassis is near equivalent to 1/0 wire)

If there is no load, (I'll assume the amp is only using 1a of current in standby)

Voltage drop of 0.002 volts

If there is a 250a load,

Voltage drop of 0.5 volts

So even if there was a major problem and the grounds were in a bad spot, say if we multiply the resistance by 10. You would barely notice the 0.02v drop but you would definitely run into problems with a 5v drop.

See why you need to 'load' the circuit to get correct results?

yeah keep readin ur websites on how to work on stuff ur gettin real far

 

[smoka 10+ year member]

I guess I'm not very smart since I'm not *** certified like you. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[fuggles 10+ year member]

Now. Now. There is some good info here. Let's not get personal //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif. The key is measuring the voltage of a *loaded* system. Otherwise the data is misleading. And don't forget "resistance measurements are useless".

I prefer to measure vdd-gnd. In many areas, such as the trunk, you can't get a good gnd-to-gnd voltage measurement with out a 15' lead on your dmm. And if a capacitor is involved the ground bounce doesn't mean as much because the cap will force the vdd side up as the ground moves up.

A volt of drop on the ground side is huge. I would look for problems if I saw that much gnd bounce.

Kent