I have been having some problems with my 1978 f250 lately.

I found the flex plate is cracked and needs replaced and now a new problem.

My truck is losing voltage while running. When the electric choke is on the battery is reading 14 but when the choke comes off the battery falls from 14 to 13, 13 to 12 then 12 to 11 volts while running. When it hits 11 volts it kills the truck.

I had the alternator tested, ignition control module and battery tested and all passed.

I was checking voltage on the starter solenoid and the "S" terminal has 12 volts constant. The "I" terminal if I am on the gas will get 11-12 volts then when its just idling it drops down to about 6 volts.

I can't figure out what is wrong. It was running fine friday besides the flexplate.

The voltage regulator has not been tested mainly because Oriellys can't test it off the truck and I can't get the truck up to them. It looses its voltage.

Any help is greatly appreciated!

 

what exactly do you mean by "when the electric choke is on"? Do you mean when the choke plate is closed/paritally closed and your carb is on fast idle? The coil in the electric choke receives voltage all the time so it is "always on"...

do you have the same symptoms with the electric choke disconnected?

 

What I mean by being on the choke is it having the faster rpms the same as holding the throttle open part way. If i disconnect the choke wire it acts the same way.

 

Try getting a wire from the battery to the I terminal that should hopefully keep it running to get it to the parts store to test the voltage regulator that being said they're not that expensive and it takes seconds to replace

New thought here I'm pretty sure you're only supposed to have 7-10 volts at the I terminal so if you're getting ~12 it's probably the voltage regulator

 

The I terminal of the solenoid will only have full battery voltage when the key is in the cranking position (provides full voltage to the coil during cranking)...It would be better to measure the voltage at the battery to see how it changes from idle up to higher rpms...

 

Ok, nice to know. I didnt think about the I terminal only getting full voltage when cranking.

When at idle right after acceleration the volts are at 13 and the longer it sits idling the voltage will drop to 12 then 11. When accelerating the engine i am getting over 14 volts.

EDIT:I am actually getting 13 volts accelerating

 

Yeah, that I terminal is direct to the ignition coil + terminal. If you move your test point to the ignition + terminal, you should see the same thing.

And you should never get 12 volts at the S terminal with engine running or off, except when attempting to start truck.

 

Your saying that my readings are normal on my starter solenoid? The S pin has 10 volts with the truck engine off but key in run position. The I has 6 volts while engine is off but key in run. If so I don't know what else to check

 

What I'm saying is that the I terminal voltages seem normal.

What worries me is the S terminal voltage. Studied the schematics and there shouldn't be any voltage on the S terminal unless your starting the engine. But this circuit goes to the ignition module box and might throw off any diagnosis, so perhaps it's normal.

Did you check the battery voltage? If it's low, I would guess Voltage Regulator and/or Alternator.

If your feeling adventurous about the flow of electrons, maybe consider a 3g alternator upgrade. Even a lowly 95 amp 3g alternator is, in my opinion, better than the old 1st gen stuff.

 

What I'm saying is that the I terminal voltages seem normal.

What worries me is the S terminal voltage. Studied the schematics and there shouldn't be any voltage on the S terminal unless your starting the engine. But this circuit goes to the ignition module box and might throw off any diagnosis, so perhaps it's normal.

Did you check the battery voltage? If it's low, I would guess Voltage Regulator and/or Alternator.

If your feeling adventurous about the flow of electrons, maybe consider a 3g alternator upgrade. Even a lowly 95 amp 3g alternator is, in my opinion, better than the old 1st gen stuff.

 

The battery is reading 11-12 volts with everything off- no key in ignition.

Running it reads 14 when accelerating then will drop all the way to 11 and kill the engine.
The S terminal has 10 volts with key in run but truck engine off.
I will look into the 3g swap.

 

I have been slowly working through trying to find the source of the lost voltage but now my truck will not start.

My battery says 12 volts key off then when cranking it drops to 6 volts no matter what battery i put in. (I have several cars to swap batteries from)

I checked the spark and I am not getting even the slightest fire.

I know it has gas and have even tried pouring gas down carburetor but still wont run.

I am getting 6-8 volts going to coil all the time.

10-11 volts to the distributer.

Starter replaced sunday afternoon.

I have swapped voltage regulator, ignition module, coil and battery with known good parts but my problems still persist.

Please help! I'm stuck driving a minivan until I can get my truck running right again and I much prefer my truck over a minivan.

 

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These truck's systems won't run on anything less than 10-volts.
So if the Batt. is dropping below 9v "when-cranking" you might need a better battery.

But be sure to try my suggestions to your wiring 1'st,
Before you go out & get a new battery.
-
You need to insure all your grounds are clean & solid.
There 4 major grounds:

Battery (-) to Engine block.
Battery (-) to Chassis/Frame.
Battery (-) to Body [usually connected to radiator support.]
Engine block to Cab firewall.
-
Now, here is some things you can do to aid the truck in getting better voltage.

Add an additional 16-gauge (or) 14-gauge Ground wire from the Battery (-) terminal to the Stater-solenoid ground. (Mounting bracket bolt)

Add a 6-gauge Stater-ground (mounting bolt on starter) to Chassis/Frame.
[be sure to leave enough slack to ensure no straining. & keep away from exhaust.]

Install a
2-Guage (or) 0-gauge Positive (+) Battery to Stater-solenoid cable.
a 4-gauge (or) 2-gauge Starter-solenoid to Starter cable.

Do a battery to headlight conversion.
Where you convert the headlight's power draw from in-cab switch to drawing from the battery directly with a solenoid switch kit.
---

Now as for the 1'st Gen to 3'rd Gen Alternator conversion.

Unless your adding more stuff (load) to your tuck's electrical system, there is no need to do this conversion.
If you have a stock system, these trucks benefit better from the wiring upgrades I mentioned.

Now, if you've added "anything" extra to the electrical system.
A.K.A. extra lights, aftermarket radio, etc.
This would be a better time to do this conversion.

Of course it would be wise to get a complete conversion wiring kit.
Messing with Alt. leads & charge-wire lead size can be disastrous if done wrong.
-

Good-luck.