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Ol_Reggie's avatar
Ol_Reggie
Explorer
Aug 03, 2017

Repeated alternator failure

I have a 1988 Ford Econoline chasis Class C Motorhome. My wife was recently returning home in our motorhome and had all of the gauges fail, the seatbelt light came on and the engine shut off. She had the motorhome towed to a mechanic shop where they charged the battery, replaced the alternator and sent us on our way. The motorhome died in the exact same way 1.5 hours later up the highway. the next mechanic told her that the alternator was fine (even though the new alternator was only charging at 12 volts!). They replaced the battery and sent her on her way. The motorhome died yet again 1.5 hours up the road, thankfully just as she was pulling up to the house.

I took the motorhome to a local mechanic and they replaced the alternator and said that it was good to go. I just put a multimeter on the chasis battery and it is 12.4 volts with the engine off, and drops to 12.0 volts when the engine is running!!!

It seems clear that the alternator is not the problem - 2 new alternators charging at 12 volts? Any other thoughts on what the problem might be? I was thinking maybe a failed isolator? The alternator post on the isolator is reading 27 volts and the house battery post is also reading 27 volts (I disconnected the cable from the house battery post).
Class C
  • ScottG's avatar
    ScottG
    Nomad
    Aug 03, 2017
    sch911 wrote:
    With a properly functioning alternator running voltage should be 13.0 to 14.5 volts. Has the voltage regulator been replaced?


    This is the voltage you should be seeing. Anything in the 12.X range indicates the alt isn't working for some reason.
    Most likely it isn't being told to work. So V reg or computer issue.
    Or, if you have a diode based isolator it's probably the cause. Get rid of it and go to a relay.
  • YC_1's avatar
    YC_1
    Nomad
    Aug 03, 2017
    Lifting the battery wire and measuring the post on the isolator will give you a false reading. To prove this, put the positive lead of your voltmeter in one hand and touch the post on the isolator. Of course have the voltmeter negative lead grounded. You will read full voltage if you hold the probes tight.

    Take the wires off the isolator after taking a picture of them. Now bolt them together. Start it up and recheck. I bet things are working then.

    The 27 volts is NOT getting a load from the batteries. You may have an external voltage regulator and due to a bad isolator it is not seeing that voltage feedback.

    If this seems to clear things up then just change the isolator and see if all is well.

    Always fix the "obvious" component failure first and don't try to understand or diagnose all of the symptoms.

    If you had a loose/intermittent connection you could get all of the symptoms fo a bad alternator twice. Even the best technicians on their best days can easily miss something like that.
  • ron_dittmer's avatar
    ron_dittmer
    Explorer III
    Aug 03, 2017
    I agree with the other comments that it is possible your voltage regulator might be a separate component from your alternator and it could be at fault, or the wire termination connections to it could be at fault. I would start trouble-shooting by first locating your voltage regulator.

    If the regulator is integrated side the alternator, the next step would be to determine if your alternator output amperage is adequate for a motor home and not just a van. It might be too small for the job and is faltering.

    One additional consideration is that the isolator could be stuck on servicing the house batteries, so the chassis battery might not be seeing a charge from the alternator. And if the house batteries are terribly drained, they just might be pulling down the voltage reading to the numbers you shared.
  • j-d's avatar
    j-d
    Explorer II
    Aug 03, 2017
    We had a 1983 E350 that used a simple 60A alternator with external regulator mounted near the battery on the passenger side. That alternator used a nut to secure the main charging wire coming out of it.

    Ford had some alternators around then, with an internal regulator also, but the way you describe the sequence of the work, seems you have separate alternator and regulator.

    But! Ford had some alternators that used separate regulators AND had push-on connectors instead of the stud-with-nut I mentioned above, and those were troublesome. The connector was troublesome, and so was the charging lead within the wiring of that connector. That's where I'd direct my suspicion.

    I looked on RockAuto.Com and it seems your chassis might have an alternator with push-on connector. Can you verify that? Could you post a couple pix of the Alternator, and the Regulator?

    Also, verify from the Ford label in the Driver Door Jamb, that the CHASSIS is a 1988. Many coaches in a Coach Model Year are built on the previous year's Chassis. That can affect parts look-ups.
  • sch911's avatar
    sch911
    Explorer
    Aug 03, 2017
    With a properly functioning alternator running voltage should be 13.0 to 14.5 volts. Has the voltage regulator been replaced?
  • cgmartine's avatar
    cgmartine
    Explorer
    Aug 03, 2017
    I am sure someone will jump in and correct me, but I don't believe 12.0 or even 12.4 indicates a good charge. When compared to the 13.2 to 13.4 I am getting on my class C with the engine running, your numbers seem insufficient. If the voltage drops, instead of increasing, with the engine running, as you indicated, I would think there was a problem somewhere. Voltage should not go from 12.4 to 12.0 If your mechanic is telling you that it is "good to go" at 12.4, then I would challenge his qualifications.

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