Question for a Diesel Mechanic

My Fist Aid Squad has a 1998 Ford E350 Powerstroke Ambulance with what I believe to be the 7.3L engine. The rig has a habit of dying on calls suddenly and the batteries become fully discharged as indicated by the "clicking" that can be heard when you attempt to restart the rig. I figure there is something wrong with the alternator and voltage regulator, but I am no technician. The weird thing is that the gauges inside of the rig indicate that the charging system is working but then suddenly the system appears to stop working. I don't know if that is some sort of controller kicking (since it is an ambulance) or something is wrong.

All ideas welcome.

Have a very similar rig.
To qualify myself, I used to be a mechanic. I am not just some quack.

Having said that, Replace batteries.
There is a battery (or two) that have a damaged cell in them. (always check connections too). Because of the damage, it is drawing more current than the alt can put out. It is drawing current from the other battery (batteries) as well.

I assume it is a van chassis. (Although they build ambulances on a pick-up chassis too) The outboard battery (batteries) are beside that frame rail on the passenger side. They are a SOB without a floor jack or something.

Take that for what it is, i haven't diagnosed the vehicle in person, but I'd bet a beer on it.

It has two outboard batteries in an easily accessible compartment on the box. They are wired in parallel. The connections are tight and the terminals are astoundingly clean (probably because the rig is kept in a bay and the compartment is sealed from the elements). I am going to go over and start it up with a meter connected and see what the output is so I can rule out the voltage regulator. I did put a load meter on it and it said the charging system was bad. Could a damaged battery cause that?

Rig is one of the ones below:

Absolutely. Pull the batteries, and have them load-tested. You'll find the bad one.

So I went to go play with the rig an hour or so ago and here are the results:

- With the rig running and no excess loads it maintains just over 14VDC; with every load I can think of turned on it never drops below 13.4VDC
- With the rig running and no excess loads it maintains roughly 40 AMPS; with every load I can think of turned on it never dropped below 38AMPS

Even with the electrical appearing to be normal, the rig never adjusted its idle (which I was under the impression it should do since it has some sort of added controller) and I ran another load test with the positive clip on one of the batteries and the negative clip grounded to the rig and it continuously read that charging system is failing.

Suddenly the "OSS" light started flashing and I noticed that the charge gauge started backing from the green towards the red. There are 7 or so relays in the electrical panel marked "OSS". Does anyone know what OSS stands for and if it is what can be causing issues?

OSS is Out Side Storage. Could you have a loose compartment door that's causing a light to stay on, thereby draining the battery overnight?

Well I feel silly. I had the compartment door open for the batteries. Coincidence and I didn't know any better, but now I do.

I did not load test the batteries since I didn't have a wrench to remove the terminals from the posts. I will do that tomorrow. If they pass then the alternators are getting replaced, if not then the batteries are getting replaced in which case can determine if the alternators are bad later.

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