Onan 4K -12V on starter relay-why is it grounded to chassis?

[pc500]

I'm trying to hook up an ammeter and am discovering there is an additional connection from chassis to battery ground. It appears current is going from the Chassis to the onan generator starter relay back to the -12V on the battery.

Is there any reason that the genset is grounding the 12 volt to the chassis? Is this necessary?

I'm not certain but I'd imagine this would have some impact on my ammeters accuracy as their are multiple paths to ground. My ammeter was hooked up only on the main -12V going from the batter to the RV fuse block.

[pc500]

christopherglenn wrote:
I always try to put shunts (amp meters) in the positive lead, there is almost always more then one ground path. The converter 12 volt ground may be grounded through the AC ground as well. There may be leakage in the fridge and water heater circuits to ac ground also - usually these would trip a shore power gfci.

Which ammeter do you have that works?

[pc500]

I have a trailer. My understanding is most automobiles use chassis as the ground, where most trailers home-run all ground and don't use chassis. Doesn't suprise me.

tenbear wrote:
Do you have a motorhome or a trailer? I understand that trailers have some brake wiring in the negative battery lead, breakaway brake. My experience is with my Class C MH. I am also talking about the house 12v system, not the chassis 12v system.

My house 12v system uses the chassis as the negative 12v. The battery -12v is connected directly to ground with one #2 AWG wire. This is where I put my ammeter.

There is no wire running from the -12v battery post to the 12v power distribution panel. The 12v power distribution panel is connected to chassis ground. AFAIK, this is how most Class Cs are wired.

That is the way my house 12v system is wired. If yours is different, I guess I'm not much help. Yes I do have a Genny, a Generac, not an Onan.

[christopherglen]

I always try to put shunts (amp meters) in the positive lead, there is almost always more then one ground path. The converter 12 volt ground may be grounded through the AC ground as well. There may be leakage in the fridge and water heater circuits to ac ground also - usually these would trip a shore power gfci.

[tenbear]

Do you have a motorhome or a trailer? I understand that trailers have some brake wiring in the negative battery lead, breakaway brake. My experience is with my Class C MH. I am also talking about the house 12v system, not the chassis 12v system.

My house 12v system uses the chassis as the negative 12v. The battery -12v is connected directly to ground with one #2 AWG wire. This is where I put my ammeter.

There is no wire running from the -12v battery post to the 12v power distribution panel. The 12v power distribution panel is connected to chassis ground. AFAIK, this is how most Class Cs are wired.

That is the way my house 12v system is wired. If yours is different, I guess I'm not much help. Yes I do have a Genny, a Generac, not an Onan.

[pc500]

There is one wire from battery ground post to dc negative distribution block. The issue is when I disconnect it the 12 volt system continues to function.

I have isolated the reasoning for this to the fact the DC system is also grounded to chassis. This is not inherently problematic as the chassis shouldn't be grounded to battery ground. But it is.

Apparently the chassis is grounded to the battery via the -12V ground from battery to genset starter relay. The generator somehow is also grounding this to the chassis, making the missing connection.

In other words an rv without genset would not have my problem.

Now I see no reason the 12volt system on the genset should be chassis grounded (on the other hand the 120v should be). That's neither here nor there.

The problem becomes is there's now an alternative path to ground and the ammeter reading won't see all the electron flow as at least some portion will transit via the generator ground link.

( one writer noted to use +12v. Actually this would be ok as I don't care about measuring power the starter draws and it woukd be the only path for all other 12v power). However the cheap $4.00 ammeter I have uses a common ground between the shunt feed and the lcd display making it not really an option.. there is no seperate ground wire to power the lcd screen on the ammeter itself.

tenbear wrote:

pc500 wrote:
I'm trying to hook up an ammeter and am discovering there is an additional connection from chassis to battery ground. It appears current is going from the Chassis to the onan generator starter relay back to the -12V on the battery.

Is there any reason that the genset is grounding the 12 volt to the chassis? Is this necessary?

I'm not certain but I'd imagine this would have some impact on my ammeters accuracy as their are multiple paths to ground. My ammeter was hooked up only on the main -12V going from the batter to the RV fuse block.

Is there one or 2 wires from the battery post to ground? There should be only one. Your ammeter should be hooked up in this one wire. Other things are connected to ground and do not affect the ammeter.

[tenbear]

pc500 wrote:
I'm trying to hook up an ammeter and am discovering there is an additional connection from chassis to battery ground. It appears current is going from the Chassis to the onan generator starter relay back to the -12V on the battery.

Is there any reason that the genset is grounding the 12 volt to the chassis? Is this necessary?

I'm not certain but I'd imagine this would have some impact on my ammeters accuracy as their are multiple paths to ground. My ammeter was hooked up only on the main -12V going from the batter to the RV fuse block.

Is there one or 2 wires from the battery post to ground? There should be only one. Your ammeter should be hooked up in this one wire. Other things are connected to ground and do not affect the ammeter.

[tenbear]

Joe417 wrote:
It may be more accurate to install the shunt in the positive leg of the battery.

The same current flows in the negative and positive lead. It is better to put the shunt in the negative lead since puting it in the positive lead requires running 2 small wires carrying the full 12v to the meter. Of course, if you really want to, you could put a fuse in both meter leads.

My ammeter is installed in the negative lead.

[Joe417]

I hadn't thought about this until now. I installed my current shunt in the negative battery lead to the fuse box also, per meter installation instructions.

I think I remember a ground strap between the gen and the gen chassis pan which is bolted to the TC.

I haven't had the time to calibrate the meter readings so also hadn't noticed that things didn't add up. I'll have to investigate. It may be more accurate to install the shunt in the positive leg of the battery.

Also will keep up with this post to see what others have done.

As to why, I can only guess that the gen may create static noise. Connecting the gen chassis to camper chassis may help filter the noise.

[Harvard]

The only time the starter relay is conducting current through the chassis is when the starter relay coil is energized. Yes, it is necessary otherwise the coil would not energize.