Thread: How much power if any should I be reading on a master disconnect switch?

Have a rescue (retired e-one ambulance) that has been recently plagued with dead batteries. I've determined that there is between 5 and 6 mV of power on the disconnected side of the switch.

1. Is this normal?

2. How can I figure out how many amps this is drawing per hour on the batteries?

Any help is appreciated.
Thanks
Craig

This is a photo of the switch:

Not sure about that, however, I think that is normal to see that amount of voltage when disconnected. I dont think that is enough to drain the battery, try leaving the battery disconnected and seeing if it still drains. Have you tried other batteries with the same outcome?

Your switch is working fine.

You need to look at current draw (amps) rather than volts. I would start at the batteries with an amp clamp.

Also check your shore power or battery charger if your truck has one.

We left the batteries disconnected for one week. Hooked up the cables and it's starts fine. The truck is set up so that when the master switch is off, the truck will not start. Nothing works without the master switch on. Is it probable that the problem is between the batteries and the switch? I DID clean all of the copper cable ends and lead battery connections with a wire brush and battery cleaner. Could this possibly be the culprit?

Really trying to figure this out.
Thanks
Craig

If you haven't already, take the unit to your battery supplier and have the batteries load tested. I had a couple batteries that drove me nuts until they finally tested bad. The batteries tested fine once, then about a week later we decided to re-test and found that both were bad. No problems at all since they were replaced. Even new batteries can sometimes be bad and drive you crazy. It's rare, but it does happen.

Good luck!

I've seen ambulances that drained the batteries with the master disconnect turned off, usually it could be traced to something installed by an upfitter after the factory. For instance, if you have a battery conditioner running off 115V that also supplies a constant low 12V current for radio/flashlight chargers on a separate circuit from the vehicle battery, it may be possible for the low current side to backfeed and drain the battery even with the master disconnect off. Take it to someone who knows ambulance or fire apparatus electrical systems.

Originally Posted by firefighter_2000

We left the batteries disconnected for one week. Hooked up the cables and it's starts fine. The truck is set up so that when the master switch is off, the truck will not start. Nothing works without the master switch on. Is it probable that the problem is between the batteries and the switch? I DID clean all of the copper cable ends and lead battery connections with a wire brush and battery cleaner. Could this possibly be the culprit?

Really trying to figure this out.
Thanks
Craig

Whoaaaa...... which master? the battery disconnect switch on the floor or the master switch for the module on the console?

...... pre 2000 something pretty much every ambulance needed the main battery disconnect on to start.... now you can start the truck, but shouldn't run anything with the battery switch off. If you mean the big "master" or "module disconnect" switch on the console, the position of that should have nothing to do with the truck starting.

When this won't start, are the batters actually dead?

The truck is 1995 e-one. Everything goes through the master switch. If its off, everything is off. With the master switch off, I'm reading about .47 amps with a clamp meter through the main cables. Now to try to trace that draw down.

Originally Posted by firefighter_2000

The truck is 1995 e-one. Everything goes through the master switch. If its off, everything is off. With the master switch off, I'm reading about .47 amps with a clamp meter through the main cables. Now to try to trace that draw down.

Again... master switch as in main battery disconnect (big rotating switch on floor), or master switch as in part of the control console.. a normal sized switch?
Again, have you 100% conform the batteries are dead?

.47 amps may mean nothing depending on lots of variables.....

Also, most ambulances are plugged in and trickle charged if parked for more than a week or so.....

Most likely there is some after market accessory that has been wired around the master.....

The truck has two master switches. The first stage is the four position cole hersee switch that disconnects the batteries. Completely. Truck won't start. Once that switch is turned to the on position, you can turn on the "module" switch which is a two position on/off cole hersee switch. The "module" switch is fed by a large jumper from the "on" side of the first switch. Batteries have been tested by two different places and our own battery tester. They're testing good each time.

Sorry bout that John. I wasn't completely clear with the other message. Let me know if this is what you were looking for.
Thanks
Craig

Originally Posted by firefighter_2000

The truck has two master switches. The first stage is the four position cole hersee switch that disconnects the batteries. Completely. Truck won't start. Once that switch is turned to the on position, you can turn on the "module" switch which is a two position on/off cole hersee switch. The "module" switch is fed by a large jumper from the "on" side of the first switch. Batteries have been tested by two different places and our own battery tester. They're testing good each time.

Sorry bout that John. I wasn't completely clear with the other message. Let me know if this is what you were looking for.
Thanks
Craig

Gotcha, thanks for the clarification. So the batteries are ok, but end up dead when the truck sits for an extended period of time....?.... and you can confirm the batteries are dead, it's not someone's user error on the battery switch?

Yes. Batteries dead or not able to start the truck.

I still stand by what I said above. Battery conditioners are wired directly to the battery so they can charge it even when the master (floor mounted) disconnect switch is off. Many of them have a secondary output to charge parasitic loads when the engine is off such as portable radios and flashlights, and this circuit gets switched to the vehicle battery when the ignition is on. This creates a low amperage bypass around the master disconnect. Try disconnecting the battery conditioner from the battery, and see if the current leakage you measured is still there.

Originally Posted by NPS Ranger

I still stand by what I said above. Battery conditioners are wired directly to the battery so they can charge it even when the master (floor mounted) disconnect switch is off. Many of them have a secondary output to charge parasitic loads when the engine is off such as portable radios and flashlights, and this circuit gets switched to the vehicle battery when the ignition is on. This creates a low amperage bypass around the master disconnect. Try disconnecting the battery conditioner from the battery, and see if the current leakage you measured is still there.

I'll give that a try.