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· Registered
'92 F-250 SCLB, '93 F-350 RCFB, both 4WD AT 7.5 gassers
35 Posts
Discussion Starter · #1 ·
Problem solving a weak charging system on an '92 OBS F250 (but the basics apply to all cars)
I had a new Western MVP 3 plow installed on an old '92 F250 (460 gasser) at the start of the season to use as my new plow truck (I plow a 1 1/2 mile forest service road and the drives and parking areas around my house). The battery and alternator were not keeping up with the electrical loads and after plowing for an hour or two the battery would be dead, esp. when running the heater fan, lights and using the plow functions a lot. I was carrying a jump pack and spare battery to jump start the truck when it would die.

Charging system fundamentals
  • The alternator replenishes amps taken out of the battery to operate the truck and plow. The battery and the alternator work together: you can add a second battery and run a bit longer but if the alternator isn't replacing all the amps used by the truck the batteries will still lose charge and eventually die (unless the battery is put on a charger every night which is what I've done with a weak system).
  • The electrons move from the alternator to the battery and to the loads (plow, lights, heater fan, etc.) through wires which are connected at terminals. If the wire isn't big enough to carry the load or there's corrosion in the wire and/or at the terminals the plow, lights and fan won't get the amps needed and they'll slow down or dim. Also, the battery won't receive the full charge sent its way. Wires and terminals have to be adequately sized and not corroded. There's no point replacing batteries, alternators, etc. if the wiring isn't in great shape.
  • Snow plows take HUGE amounts of current to operate. At full load the Western MVP 3 uses about 250 amps - that's roughly like cranking the engine every time the plow is raised, tilted, etc.
  • Wires eventually corrode just sitting there so wires on a 30 year old truck may have less than half the capacity when they were new. The battery cables are the easiest place to see corrosion near the terminals.
  • Wire terminals at the engine block, battery, starter, etc. all corrode too and need to be maintained (cleaned and covered with dielectric grease or other anti-corrosion stuff).
  • Eventually, wires and terminals that may have been marginal when new have lost the capacity to handle the electrical loads of the truck, esp. when adding a plow that adds a HUGE electrical load and lights dim when operating the plow and eventually the battery doesn't get charged.

Problems that needed to be solved on this truck
  • The cheap FLAPS alternator was rated for 75 amps but even with full loads the most I saw it produce was about 50 amps (using a Fluke amp clamp) so it couldn't keep the battery charged when running heavy loads.
  • The alternator/battery/starter/ground wires were old, undersized, corroded and generally a mess adding resistance to the circuits.
  • The above two meant the plow and starter motors would run slowly, lights would dim, the wiring was running hot and eventually the battery voltage would be in the low 12s/high 11s and I'd have to jump start it eventually.

1) Correct the wiring: There's no use throwing alternators and batteries at it if the wiring is corroded and/or undersized so I replaced the wiring: 2awg wire from the battery to the starter, 6 awg from the alternator to the solenoid and solenoid to the battery. Polished all the points of contact on the positive and negative terminals, etc.
2) Replace the alternator: Next was replacing the alternator with a 150 amp Powermaster alternator. I don't have the box handy but it came with a tag noting test results at idle speed and 2,400 and the actual performance validates those results (it pushes 56 amps at about 750 rpm). This may have been overkill but I'm a firm believer that enough is never enough.
3) Add a Volt meter: I added a volt meter with the old alt. and would watch the voltage start in the high 13s at the start of a plowing session and drop to the low 12s showing the the alternator/battery combo were no match for the loads of a plow truck. With the above improvements it shows voltage in the high 13s/low 14s all the time showing that the alt/battery are keeping up.

End result: 56 amps at idle with all the lights and heater blower on (I didn't have an assistant to add the plow as a load). Getting so much out of the alternator at 750 rpm is huge factor in keeping the battery charged.
Fluid Motor vehicle Vehicle Bottle Automotive tire

The shiny Powermaster alternator (yes, the belt needs to be replaced). That damage was caused by the Western Plow bracket used to relocate the charcoal canister (I removed the bracket).
Automotive tire Motor vehicle Synthetic rubber Tread Wood

Updated wiring: 2 awg from the battery to the starter, the rest is 6 awg.
  • The high output alt. only needs the positive cable but I added a negative cable to avoid relying on the alt. case to bracket as a ground (I believe the alt. is clear coated as well). The OE Ford wiring harness to the alt. is unused which is fine because the wires definitely seem undersized to me. Also, this means the alternator light on the dash doesn't work which is another reason to have the volt meter.
  • The skinny wire to the pos. terminal is for the volt meter inside the cab - I wanted to measure voltage directly at the battery.
Motor vehicle Vehicle Electrical wiring Gas Cable

I made all the cables using a hydraulic crimper (something I had from a more extensive wiring project) and leftover wiring from that project. Below are Terminals and crimps on 2 awg wire.
Line Plumbing Gas Pipe Electric blue

I've also used a manual crimper like this
Wood Machine Metal Musical instrument Creative arts

14.6 volts at the battery at about 700 rpm. Here's a link to the voltmeter. The Fluke says it's accurate. I didn't need the clock or interior/exterior thermometers but it switches from green to red and has been working great. I added a switch to turn it off so it doesn't drain the starter battery (it has a lithium battery inside to maintain the time).

Speedometer Vehicle Odometer Car Steering wheel

New plow, old truck
Tire Wheel Land vehicle Vehicle Snow

That's what I've got on this.

Feel free to add or correct anything here.

· Registered
'92 F-250 SCLB, '93 F-350 RCFB, both 4WD AT 7.5 gassers
35 Posts
Discussion Starter · #3 ·
Nice work on your upgrades. Agreed on the battery clamps. These are what I was able to get locally and they're definitely a weak point. I replaced the positive clamp once already because the one of the bolts stripped out and it all loosened up. Those mil-spec clamps look like a nice improvement.

· Registered
'92 F-250 SCLB, '93 F-350 RCFB, both 4WD AT 7.5 gassers
35 Posts
Discussion Starter · #5 ·
OP, your up grades are the first thing everyone with a standard snow plow should do. One thing you may consider is adding another battery. I added a second to my 460. ( De-smogged mine, that helped make room)
View attachment 265312
Adding a second battery is next on the list if this arrangement doesn't keep the battery fully charged but so far so good.

That's a beautiful engine bay. I enjoyed reading your build thread a few months back - the Bronco looks like an ideal plow rig.
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