1977.5 F250 400 C6 4WD

Solenoid chatters no start

Ground cable replaced as it looked original and and beat up. Truck started put a muti meter on the battery with engine running @ 2000 rpm the battery voltage was dropping slowly. I removed the positive cable from the battery post, truck wants to die.

Should'nt the engine continue to run ?

Turn the truck off, attempt to re start solenoid chatters..... Battery two years old, 61 amp Alternator one month old, voltage regulator two years old.

Connecting the battery charger set to 50 amp jump results in no start just chatter.

 

Never disconnect the battery from a running vehicle, as this can make the alternator output skyrocket and fry any onboard electronics, like the ignition module.

If this started somewhat recently, I would have the alternator tested, regardless of how new it is. Also check the fusible link in between the alternator output and the battery for continuity.

You should also load test the battery, because if the alternator really is the problem, it still doesn't explain why the truck won't start after a charge. A chattering relay means the battery voltage is very low, and the battery cannot source current.

 

If you unhook the battery while the engine is running and it starts to die, you are not getting power from the alt. I would pull it and have it checked. If it is good, see if they can check the regulator and load trest the battery.


I see fmc400 was posting at the same time and along the same lines of thinking.

 

Check your conections on the solinoid the one closest to the postive cable make sure its tight enough take sand paper and clean it off then put it back on. Unbolt the postive cable from the solinoid and do the same to the o ring on the cable and to the post. If that doesn't change anything un hook the cable from the battery clean the post off and the inside of the cable terminal. Once it starts hook the voltemeter up at idle and look at your voltage. Its should be around 13.4 or if its lower than that it should show it picking up. Now if its still draining hook the postive lead on the battery cable to the alternator to see if its charging. Even though your parts are newer they could be deffective that quick. If you have a way to get the alternator to your local autoparts store like autozone, advance auto parts etc they have free alternator testing. but from the sounds of what you were saying it sounds like the alternator isn't charging. Depending on where you got your alternator if they got your phone number then it has a warrenty switch it out for a new one. It will be free.

 

A big X2, even on our pre-computer era trucks when the engine is running and you disconnect the battery, the voltage regulator senses very low, actually no voltage, and tells the alternator to produce as much electricity as it possibly can, instead of the voltage being regulated at around a maximum 14.7 volts like normal, it shoots up to around 18 volts, all the alternator can produce, that fries bulbs, voltage regulators, ignition modules and so on.

 

Thanks to all for the replies, and the tip about voltage spiking the alternator.

I'll view the issue as two seperate problems then.

1. No start

This is resolved, substituted a battery from my 1969 Comet. The F250 now starts every time hot or cold. The warranty on the napa battery is not worth much after 18 months so a replacement is in order.

2. Truck running, voltage at battery falling

Still need to repair this the fusable link that FMC 400 referenced appears to have been replaced at some point. Will confirm the alternator is good with a trip to autozone.

 

I don't believe the voltage regulator "senses" any voltage from the battery, all voltage to the regulator comes form the alternator. The voltage regulator converts varying input voltage from the alternator and produces a constant regulated output voltage to the battery. It doesn't matter if the battery has 0 volts or 13 volts.

While I disagree with the "don't disconnect the battery while running" theory I have done it countless times on pre 79's and have yet to fry anything. It is a very simple way to test if the alternator is or isn't putting out.

 

This isn't quite true. The battery, alternator output, and "A" terminal of the voltage regulator are all at the same potential. There aren't two different voltages between the battery and the alternator output; that's electrically impossible because they're connected with a dead short.

As this voltage decreases, the regulator increases the field current to the alternator, which causes it to produce more current. This increased current output from the alternator raises the system voltage. As the voltage increases, the regulator decreases the field current to the alternator, which lowers the alternator output to equilibrium, hence the term "regulator." If a higher load is placed on the electrical system, such as turning on the heater, the regulator must react by increasing the field current so that the system may maintain a predetermined voltage (13 to 14 volts).

The voltage regulator doesn't "convert" voltage in any way; it simply adjusts field current into the alternator. It directly controls the alternator's current output through negative feedback. It doesn't act like a pressure regulator where there's an "in" and an "out" - all control is relayed to the alternator through the field terminal. The voltage regulator in an automotive charging system works very differently than voltage regulators in electronic power supplies (which do act like you describe).

The battery serves to sink current from the alternator. With this load gone, the alternator output jumps sky-high because there's nothing to sink the alternator current. Disconnecting the battery from a running vehicle is a major no-no; regardless if you specifically have not had issues with it.

 

Appreciate your opinion on the battery cable procedure.

Could my voltage regulator be the offending component then or does disconnecting the cable and the result (engine quits) directly implicate the alternator ?

 

You do so at your own peril, it only takes one time to fry your electrical system, but irregardless of that fact it's not a valid "test" anyway, an alternator may be making enough amperage to keep the truck running but not enough to keep the battery charged, e.g. a 100 amp alternator that is only making 5 amps, enough to keep the truck running but certainly not enough to charge the battery plus run all of the accessories.

 

This also resolved, Alternator was faulty. Thanks to all for your input.