The users of this forum have been most helpful, so here is another challenge for all of you. I have had a 97 F250HD 7.5 Turbo Diesel for 6 months now. The odometer shows about 125K, which is fine for a diesel. I am an electrical design engineer (retired) and I have a problem with the dual battery system that Ford uses. Both batteries are connected in parallel with no apparent blocking diodes. As any one who understands batteries should know, this is not good. When one battery starts to fail it will pull the other down (the other battery becomes a charging system for the failing one). This process continues to swing from one battery to the other until they are both dead. So much for bad engineering by Ford, and now to my problem. The battery voltage indicator has always shown a very low value (about 8VDC before and after I replaced the batteries). This has always concerned me so I checked the voltage of both batteries several times. They both had good float and charge voltages (12.5 float, 13.85 charge). I also checked the alternator output and it was fine. Additionally, the battery warning light is always on when I first turn on the ignition (is this normal?). A week ago, after the truck hadn’t been started for 4 weeks I tried to start it and it was dead (no power at all). I have a very sophisticated battery charger. I showed both batteries were totally sulphated and could not be revived (“bad Ford” engineers)! Therefore, off to the local parts store and after $80 and 2 hours I had the new batteries installed. However, the problem with the warning light and low gauge reading persists. The new batteries seem to be holding up (both new) but why the very low readings on the gauge and the warning light. Could this be a result of the glow plugs? Anyone know where the battery voltage gauge gets it’s reading (battery 1, battery 2, both)? What does the warning light indicate? After the engine starts, the warning light instantly goes out and after about a minute the gauge shows normal (about 13.8 volts). I hope that one of you will have a service manual witch will show the charging system. This is not a problem yet, and any comments welcome.

 

Voltage Gauge Reads Low possable causes
Charge indicator.
Damaged regulator.
Generator

have you checked for any forms of parasitic battery drain?

 

BW
Just what is a “Charge indicator”? The only indicator I know about is the voltage gage and the warning light. I suppose the voltmeter could be defective but when the truck has been running about a minute, it shows the correct value. See my original post.

As for a bad regulator or Alternator, these were checked and they are working correctly.

As to a drain. Well of course the usual. The CD player has a clock and uses a minute amount of current. I also have a cell phone charger plugged in but it only uses enough juice to light a small LED. With those devices, the batteries should last years.

Also, just a note here, the battery warning light comes on and the voltage is low on the gage even when I drive the truck, turn it off, then immediately restart it.

 

charge indicator would be voltage gauge. you say the light goes off and gauge shows normal after sometime. so this would lead me to think there is a drain on that battery big enough to pull voltage down to where it has to charge it back up everytime you start it.

now if you drive it till it goes to normal range then shut it off and restart it will it show low and turn light on again?

more info might be helpful too looks like 2 possible generators
Generator with Integral Rear Mount Voltage Regulator, Internal Fan Type (would just have a pulley)
Mitsubishi 215-Ampere Generator (would have a pulley with a fan blade visable right behind it)

does this only happen after you let it sit over night?

 

BW

"charge indicator would be voltage gauge. you say the light goes off and gauge shows normal after sometime. so this would lead me to think there is a drain on that battery big enough to pull voltage down to where it has to charge it back up everytime you start it."

I have tested and the only drain on the battery is the other battery. The truck only draws 40 MA, this is insignificant.

"now if you drive it till it goes to normal range then shut it off and restart it will it show low and turn light on again?"

Yes that is the point. With the voltage full and a full charge the gauge should show the actual voltage of the batteries. This is NOT the case. The gauge shows 8 volts but with a good voltage meter I see 12.75 at the battery. I can't determin why this is so do to the lack of any schematics.

"more info might be helpful too looks like 2 possible generators
Generator with Integral Rear Mount Voltage Regulator, Internal Fan Type (would just have a pulley) Mitsubishi 215-Ampere Generator (would have a pulley with a fan blade visable right behind it)"

I have the integral regulator. Also it is only 6 months old. Also to reiderate, I have run diagnostics on it and it is fine.

"does this only happen after you let it sit over night?"

One last time, this happens if I drive for an hour, turn the engin off, then imediatly restart it. That is what makes no sense. I am still looking for a diagram of the battery and charging system.


Still confused...

 

Charge Indication Reads Inaccurately
Circuit.
Bulb.
Voltage gauge

well if your batteries show 12 volts on a multimeter and only shows 8 volts on voltage gauge i would think something in the gauge cicuit is faulty. this could be a bad regulator which you can replace seperate from the generator. a bad voltage gauge or instrument panel. but it comes up to normal after you drive awhile. so its leading me to think bad regulator.

i sent you an email so you can send me an email then i will attatch F250 diesel charge system diagram.