(negative) battery cutoff not working

[sandsjh]

See bullet list made by AI for a summary.

Hello. We obtained this fire truck (Chevy K30/M100 from another department and have had draining issues with it from the beginning. Since putting a cutoff switch on the firewall battery positive did not resolve the issue, we put a battery cutoff on front battery negative. All I've done in car stereo and upfitting for 30 years, this should definitely stop all draw and stop everything from working with the battery not having ground. Well, that didn't work. There is continuity on the cutoff switch whether engaged or not. We took one side off the cutoff switch and the cutoff switch works correctly.

Which post do the 12-volt only wires go on? How in the world is everything still working with the ground switched off? note: the small wire on the front battery negative was disconnected.

 

[TechnoWeenie]

Did you remove the radio capacitor on the firewall 24V bus bar? It's a common issue with aged capacitors, they act more like high-value resistors. It joins the 24V+ and ground. So you could be getting a ground path through the rear battery.

 

[dmetalmiki]

"Chump Method"!... Remove a battery terminal. (Still on?), Remove Another battery Terminal. (Still On?,). Remove Another Battery Terminal..(Still on?..)..Keep going (I don't know how many batteries you have..). HA!, Off!, no power at last. (That's were cut out switches go). OR, ADD switches until power is On. That's how I did my Ambulance, (4 Batteries). 12 & 24 volt supplies.

 

[sandsjh]

Did you remove the radio capacitor on the firewall 24V bus bar? It's a common issue with aged capacitors, they act more like high-value resistors. It joins the 24V+ and ground. So you could be getting a ground path through the rear battery.

Not sure where that is, but here are some 50 megapixel wide area pics and pics of the bus bar.

Cucvpics - Google Drive

drive.google.com

 

[Barrman]

Unplug the 2 wire plug on the driver side alternator. If you have already done all that is listed above you are down to regular 40 year old truck things no matter what voltage it is.

 

[deank]

I have battery disconnects on both batteries on my M1031. One disconnects between front battery + and rear battery - . The other disconnects the front battery negative to chassis ground.

Radio capacitor

 

[TechnoWeenie]

here are some 50 megapixel wide area pics and pics of the bus bar.

Cucvpics - Google Drive

drive.google.com

It's still connected.

Also, what's that red wire? That's not a factory yellow connector.

 

[deank]

If it is what I think, trace it back. It may go to the 24 v solargizer. Little black box that also runs to the small solar panel on the hood, if yours has it. It will have a matching black wire on the negative bus bar.

I would remove the capacitor in the red circle unless you intend to run military radios.

 

[TechnoWeenie]

If it is what I think, trace it back. It may go to the 24 v solargizer. Little black box that also runs to the small solar panel on the hood, if yours has it. It will have a matching black wire on the negative bus bar.

I would remove the capacitor in the red circle unless you intend to run military radios.

I'm not the OP - I was pointing out where the resistor was in his pics.... and the oddball electrical connector.

 

[Skinny]

I would recommend not going this battery disconnect route and troubleshoot where the parasitic draw is. This is a band aid and not a fix.

These trucks are very basic and you can leave batteries in them for months without risking a draw. You got something as mentioned pulling power and disconnecting it is just not fixing the issue.

 

[nyoffroad]

I would recommend not going this battery disconnect route and troubleshoot where the parasitic draw is. This is a band aid and not a fix.

These trucks are very basic and you can leave batteries in them for months without risking a draw. You got something as mentioned pulling power and disconnecting it is just not fixing the issue.

Two possible draw items are a faulty alternator and the black out lights on unnoticed.