Openai/6962ea5b-2e48-8004-9c43-6e6976669fa0

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gpt-5-2

User: 1985 Chevy Chieftain is boiling the water out of the batteries.

1985 Chevy Chieftain is boiling the water out of the batteries. What the possible cause in order of highest probability?

Assistant: On a 1985 Chevy Chieftain Chevy P-30/P-32 chassis RV, batteries boiling gassing hard, losing water is almost always an …

On a 1985 Chevy Chieftain (Chevy P-30/P-32 chassis RV), batteries boiling (gassing hard, losing water) is almost always an over-voltage problem. Here’s the probable causes ranked from most likely to least, with brief reasoning and what to check.

1️⃣ Failed Voltage Regulator (MOST LIKELY)

Why: That era Chevy uses an internal regulator in the alternator. When it fails, system voltage can jump to 15.5–18V, which will rapidly boil batteries.

Clues:

• Battery caps hissing or bubbling
• Strong sulfur smell
• Headlights unusually bright
• Multiple batteries affected

Check:

• Measure voltage at battery with engine running - Normal: 13.8–14.4V - Boiling: anything above 14.8V

✅ If high → replace alternator (or regulator if serviceable)

2️⃣ Bad or Missing Engine / Chassis Ground

Why: A poor ground can trick the regulator into “thinking” voltage is low, so it overcharges.

Common on RVs due to:

• Corrosion
• Frame extensions
• Added accessories

Check:

• Inspect ground strap from engine to frame
• Battery negative to frame
• Measure voltage drop between alternator case and battery negative - Should be <0.2V

✅ Clean, tighten, or add grounds

3️⃣ RV Converter/Charger Stuck in Boost Mode

(If this happens while plugged into shore power)

Why: Old converters sometimes fail and output 15–16V continuously instead of tapering down.

Clues:

• Batteries boil only when plugged in
• Happens even with engine OFF

Check:

• Measure voltage plugged in, engine off - Normal: 13.2–13.6V - Bad: 14.8V+

✅ Replace converter or upgrade to modern 3-stage charger

4️⃣ Wrong Battery Type or Mismatched Batteries

Why: Using:

• Starting batteries instead of deep cycle
• Mixed battery ages/types
• AGM charged like flooded (or vice versa)

Can cause overheating and gassing.

Check:

• Confirm battery type matches charging system
• All batteries same type, age, and capacity

⚠️ This usually worsens boiling, but rarely causes it alone

5️⃣ Battery With Internal Short (LESS LIKELY but possible)

Why: A shorted cell lowers system voltage → alternator overworks → overheats other batteries.

Clues:

• One battery hotter than others
• Won’t hold charge
• Low resting voltage (~10–11V)

Check:

• Load test each battery individually
• Disconnect suspect battery and recheck charging voltage

6️⃣ Temperature Compensation Failure (Rare on 1985, but possible)

Some aftermarket regulators or converters use temp sensors. If the sensor fails, voltage can skyrocket.

🔧 What to Do FIRST (Quick Diagnostic Order)

1. Measure charging voltage at the batteries
2. If >14.8V → alternator/regulator is bad
3. Check and clean all grounds
4. Test voltage plugged into shore power
5. Load-test each battery

⚠️ Important Warning

Boiling batteries:

• Can explode
• Damage wiring and paint with acid vapor
• Will destroy batteries quickly

👉 Disconnect charging sources until voltage is confirmed safe

If you want, tell me:

• Does it boil while driving, plugged in, or both?
• Chassis battery, house batteries, or all?
• Measured voltage (if you have it)

I can pinpoint it very quickly from there.