Forum Discussion

99_Sahara_OK's avatar
99_Sahara_OK
Explorer
Apr 06, 2014

Rapid Voltage Loss while driving/idling

I have a 99 Safar Sahara (C) diesel,
I've had various problems with the electrical system, dead chassis batteries, bad isolator, etc...

Now I'm facing a new problem.

When driving (currently just around the property while de-winterizing and getting cleaned up for the season) I'm experiencing a rapid falloff of voltage (according to the dashboard gauge) from 15v to 8v in 30 seconds or so. This often results in the transmission auto-shifting to neutral and an ambiguous beeping in the cab.
Usually at this point the voltage jumps back up to 15v and I can reengage the transmission after a few more seconds.

I have also found that I can arrest the voltage drop by using the emergency/aux start switch (tie in the house batteries)

Batteries are all new, however the chassis batteries have been completely drained several times over the off season because of the other problems.

Clearly the alternator and isolator are working or it would never get running to begin with and I have had it running for as much as an hour at a time.

I have read a little about the need for an alternator to be excited, but admittedly understood little about it.

Where should I start in diagnosing the cause?



Update: 4/8/2014
***************
It'll be a few days before I can get back out to it, but I will focus my attention on the cables, the chassis batteries, and test the output directly from the alternator (and probably watch both outputs from the isolator) to see if I can catch the falloff there as well.

I'll take the opportunity to yank the batteries and the alternator to have them tested.

Thank you everyone for narrowing my focus.

One last question. I have read that low voltage can cause damage to the FICM,
but I couldn't figure out if this is an immediate consequence or caused over time.
Am I taking a risk by running this thing while troubleshooting?
Would I be taking the same/more of a risk by driving this to a shop for repairs if the problem proves to be beyond my abilities?

******************
Class C
  • carringb's avatar
    carringb
    Explorer
    Apr 08, 2014
    FICM damage is a 6.0 PSD problem.

    Main problem is it can stall if voltage drops too much while you are driving.
  • 99_Sahara_OK's avatar
    99_Sahara_OK
    Explorer
    Apr 08, 2014
    It'll be a few days before I can get back out to it, but I will focus my attention on the cables, the chassis batteries, and the output directly from the alternator (and probably watch both outputs from the isolator) to see if I can catch the falloff there as well.

    I'll take the chance to yank the batteries and Alternator to have them tested.

    Thank you everyone for narrowing my focus.

    One last question. I have read that low voltage can cause damage to the FICM,
    but I could not get a sense of how long of an exposure can cause this. Am I taking a risk by running this thing while troubleshooting?

    Would I be the same/more of a risk by driving this to a shop for repairs if the problem proves to be beyond my abilities?
  • Harvey51's avatar
    Harvey51
    Explorer
    Apr 07, 2014
    Voltage regulation? Probably internal to the alternator. A clamp meter measuring current out of the alternator might be revealing if you can catch it happening. It sounds like something loose; worth checking connections to the alternator. And maybe the computer has some control over voltage regulation - see if firming up the connectors on it make any difference.
  • ScottG's avatar
    ScottG
    Nomad
    Apr 07, 2014
    Sounds like normal grid heater or glow plug cycling. Is the engine cold when this happens?
    If so then it could just be that your batteries are worn out and cant provide the current needed during the cycling. With good batteries the voltage should not drop below 10~11V.
  • wolfe10's avatar
    wolfe10
    Explorer
    Apr 06, 2014
    Fully charge the batteries and then load test them.

    Make sure all battery connections are good-- clean and tight.

    Most diesels have two batteries. It may be that one is bad.
  • Road_Runners's avatar
    Road_Runners
    Explorer
    Apr 06, 2014
    Could it simply be a case of a bad connection at the batteries. If it were me, I would remove and clean the battery terminals to rule out that possibility.
  • carringb's avatar
    carringb
    Explorer
    Apr 06, 2014
    Sounds like it could potentially be a glow-plug problem. How cold is it where your are? Does plugging in the block heater make a difference?

    Sometimes the glow plugs can short internally, and dramatically drop system voltage.

    Also the injection control harnesses where they enter the valve covers are another common system short.

    Because your symptoms lessen when you add the house batteries, it makes suspicious the problem is on the chassis side.

    Once the battery connections have been verified, I would start looking for sources of excess draw, like the ones I mentioned.
  • Hiking_Hunter's avatar
    Hiking_Hunter
    Explorer
    Apr 06, 2014
    Suspect the chassis battery, even though it is new. Disconnect it from the circuit by removing one of the terminals, then check the voltage. Or better yet, take it to WalMart or an Auto-Zone/Advance Auto type place and let them load test the battery. While you are in there, check the terminals for corrosion or bad connection. Also check for a bad ground to the chassis (not likely the cause).

    After that suspect the alternator diodes. The same place that checks the battery can check the alternator output.
  • tenbear's avatar
    tenbear
    Explorer
    Apr 06, 2014
    I would check the battery voltage at the battery posts when the voltage has dropped. As SooperDaddy said, check all the connections.
  • SooperDaddy's avatar
    SooperDaddy
    Explorer
    Apr 06, 2014
    I'm not an electrical guru....but check all the chassis ground wires for loosness or corrosion. Maybe even replace some of the battery wires.

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