High Resistance Battery to Chassis Ground

[BrokeDown]

I am having some strange electrical problems and I have been thinking that it might be a ground problem. The most recent problem is that my SeeLevel II system is getting false reading. According to SeeLevel Tech Support, it is most likely a ground problem. They said that 20 ohms between the ground wire and the battery is enough to cause problems.

I cleaned up the connections that I could find in the front of the coach/frame but that did not help. I check the battery connections and they are all good.

There are 3 heavy cables that run from the house batteries towards the center of the frame and another heavy cable running from the engine batteries. I am a fat guy so crawling around under the coach which is parked on crushed sea shells is not my idea of fun.

I went to the auto parts store and bought a couple of ground cables. Prior to installing the new cables, I took a resistance (ohm) reading from the house battery negative terminal to the chassis. The measurement fluctuated between 90 and 100 ohms. I installed the new ground cable and that brought it down to 15 to 20 ohms. I measured between the negative terminal and next to where I mounted the wire to the chassis. I expected to see 0 ohms.

Please note that I have a very good meter and I made sure that the mounting and testing surfaces were down to bare metal.

I did the same resistance check at the engine batteries and the reading was fluctuating between 5 and 7 ohms. I added the new ground and it dropped down to 3 to 4 ohms.

This has me wondering if I have some other problem in the coach.... I'll recheck the measurements after I put the coach into storage mode and unplugging the shore power. That should remove any interference that may be coming from the coach.

Can anyone think of anything else that I should check?

Thanks
John

[Stretch]

John
Glad you have a decent meter. You did not however tell us what the meter reading was between the negative terminal where you mounted the wire to the chassis.

[xrated]

You might want to check the connection between the actual sensor negative lead and the frame ground, if you haven't already done that.....you didn't mention checking that connection. And again, make sure that connection is attached to bare shiny metal and use a di-electric grease (just a dab) to help maintain the connection integrity.

[BrokeDown]

Originally Posted by Stretch

John
Glad you have a decent meter. You did not however tell us what the meter reading was between the negative terminal where you mounted the wire to the chassis.

Stretch,

I am pretty sure that I checked that and the reading was the same. I will check it again to be sure. I believe that something is putting electrical noise on the ground. Possible the inverter???

Stay tuned.

B

[BrokeDown]

Originally Posted by xrated

You might want to check the connection between the actual sensor negative lead and the frame ground, if you haven't already done that.....you didn't mention checking that connection. And again, make sure that connection is attached to bare shiny metal and use a di-electric grease (just a dab) to help maintain the connection integrity.

A few days ago, I checked the connections at the sensors. All of the connections were a mess. Whoever installed the system used wire nuts which rusted and corroded the wires. I cut the wires back and soldered them together. I thought for sure that I found the problem..... but that was not it. I will check the sensor to frame resistance and see what I find.

Thanks
-B

[nothermark]

You do have the power off and the battery cable disconnected when trying to measure the resistance between the battery connector and chassis, don't you?

Any measurements of resistance need to be done with the power off. Any power on in the circuit under test will distort the readings. If your circuit is under power measure the voltage drop between the center post of the battery and the connector and again between the center post and the frame. It should be in the low millivolt range unless you have a current monitoring shunt in the lead.

[TeJay]

This post has had some excellent information. Thanks BD and all the other posters.

I've been thinking back during all my training and little time was devoted to checking voltage drops, or even grounds. Yes back then just about the only grounds used were chassis grounds. I have schooled my students on the idea of voltage drops and the equally importance of all ground connections. We also shared several stories from personal experiences with bad grounds.

One technique, that for what ever the reason was never covered nor did I ever think of it was to do as BD did and that's to add additional ground wires. Actually one of my students who is now the shop Foreman at a local Semi Tractor trailer facility recently mentioned how he's used that simple yet very important technique.

Why do I mention this?? Today's vehicles are more susceptible to ground issues than ever before. In recent years the manufacturers are including ground schematics showing locations for the technicians.

Consider vehicles of the 70's and the electrical needs compared to what we do today with electricity. There is so much going on with all the bells and whistles the need for an even greater understanding in the area of electrical troubleshooting has increased at an astounding rate.

Just last week I talked to the Service Manager at our local Wingfoot dealer. He has had an ad for a technician out for going on 2 years and can't find anybody who can think beyond hooking up a scan tool and taking readings. He put the kid on a vehicle with dim headlights and told him to look for a bad ground. The kid didn't know what to do.

In addition, while the amperage needed to operate some of the sensors today is less so is the voltage. Most sensors (TPS, MAP, Oxygen, IAT, etc,etc) will use a 5-volt signal or less instead of 12-volts making the need for a good ground more important. Add to that mix the Hybrid vehicles and good grounds become even more significant and important.

[BrokeDown]

Originally Posted by nothermark

You do have the power off and the battery cable disconnected when trying to measure the resistance between the battery connector and chassis, don't you?

Any measurements of resistance need to be done with the power off. Any power on in the circuit under test will distort the readings. If your circuit is under power measure the voltage drop between the center post of the battery and the connector and again between the center post and the frame. It should be in the low millivolt range unless you have a current monitoring shunt in the lead.

I do not have the battery (positive) disconnected. I was a Xerox Service Tech back in the day. I know that when checking across components like a resistor, that you would need to isolate it. I did not think that would be the case when checking for continuity in the ground circuit. In essence, I am simply checking two ends of a wire.... the wire just happens to be the frame of the truck.

-B

["007"]

Did you cross the meter leads to each other to see if zeroed reading?
Grounds can be some fun here is a link to other ground discussions, getting your readings as close to 0 should serve you well just remember you have ground wires from GEN, your converter, transfer switch and your load center that need to be cleaned also.

[aether_one]

Originally Posted by BrokeDown

I do not have the battery (positive) disconnected. I was a Xerox Service Tech back in the day. I know that when checking across components like a resistor, that you would need to isolate it. I did not think that would be the case when checking for continuity in the ground circuit. In essence, I am simply checking two ends of a wire.... the wire just happens to be the frame of the truck.

-B

Isn't a high resistance ground connection a resistor? If current is flowing won't there be a voltage drop? Won't that voltage drop interfere with the voltage the meter is trying to sense?

Disconnect the power when taking resistance readings......PERIOD.

[nothermark]

Originally Posted by BrokeDown

I do not have the battery (positive) disconnected. I was a Xerox Service Tech back in the day. I know that when checking across components like a resistor, that you would need to isolate it. I did not think that would be the case when checking for continuity in the ground circuit. In essence, I am simply checking two ends of a wire.... the wire just happens to be the frame of the truck.

-B

The issue is how the resistance meter works. The general approach is a battery or other source feeding a known voltage or current to a load then measuring what happens in the meter circuit. If there is power in the device under test that adds to the power in the meter circuit and distorts the reading. You would have to know what is in both circuits to calculate the amount it changed the reading.

In addition the power from the meter can cause problems in sensitive circuits if you just go probing around. Better to disconnect the device you want to measure or check voltage drops on the live circuit.

[BrokeDown]

Thanks for the info.

-B

[wa8yxm]

Have not had to do this on the RV or any RV but have on cars.

I put a STAR washer under the ground lug, and under the screw head.. By By Problem.. This was only for wire to chassis/frame type grounds though.

[BrokeDown]

Originally Posted by wa8yxm

Have not had to do this on the RV or any RV but have on cars.

I put a STAR washer under the ground lug, and under the screw head.. By By Problem.. This was only for wire to chassis/frame type grounds though.

Most of the grounds that I have found so far have the star washers. I plan on continuing to look for and clean up the grounds.

Thanks
-B