truck missing after new alternator

[foreverdead]

recently after a fishing trip my 89 ford ranger 2.3l would not start battery was dead. si u out my boat battery in it and used it to start the truck and i kept the boat battery in it untill i got home. when i got home i pulled the pos. cable off the battery and the truck died so i figured it was the alternator so i replaced the alternator and battery cable ends and the same exact thing happend so i check the volts with a meter and i was getting 14.8 charging volts and 12.2 volts on the battery when not running so it is charging. well i figured it wasent a big problem until now. now my truck is starting to miss fire and run rough like its not timed right but sometimes the truck runs perfectly fine. so i checked the battery connection i got a good connection on my battery but i started to think why will my truck not run on its own power from the alternator and why does it need the battery to run. and how does it charge the battery but wont keep the truck running can someone help me figure outthis problem.

 

[Spott]

Theoretically, a engine could run entirely off the alternator. This was entirely possible with old, simple, carbureted engines. In practice, it just doesn't work with a modern engine with electronic controls.

The alternator is a little electrical generator, but it's not very well controlled. It generates power with lots of rapid uneven surges, many times a second. It can get away with this because there is a large battery, which can absorb extra power when there's a surge and supply power when there's a low, thus "buffering" the voltage and supplying much "smoother" power to the electronics.

Old engines with simple coils and points and electric pumps could tolerate such "dirty" imprecise power, but electronic and computer systems need a good steady voltage for operating reference and sensor readings. Without a battery, that dirty fluctuating voltage can cause the electronics to generate and read unpredictable digital data, get wildly varying analog signals from the sensors, and possibly even lose power entirely for tiny factions of a second. It may even get a voltage spike so large that it fries electronic parts.

So, the computer just can't control the engine under those circumstances, so it can't keep running.

Disconnecting the battery with the engine running is no longer a valid way to test for a bad alternator, and has the potential to cause damage.

As a side note, if there were some massive capacitors attached to the electrical system (guys with big stereo amps sometimes install them) then the capacitors can provide this smoothing, buffering function and the engine may be able to continue running when the battery is removed, assuming the alternator is good.

 

[foreverdead]

thanks u scott this was helpful with the alternator but this doesnt explain why my truck is missing and acting like its not getting good power wether its from the alt. or the battery. my truck will sometimes miss and act up when the battery is still attached and im getting good connection on the battery but the truck does the same thing (missing and acting like its not timed right) when u pull the battery leads off the battery. its like my wires are not getting good connection from the battery to where ever they go from there

 

[Big Jim M]

I'm wondering how old that battery is. I'd go out this morning and check the volts in the battery before starting the engine.
The battery being dead at the first writing sort of tells me that is the problem. If it runs on the boat battery that clinches my thoughts.
Big Jim

 

[Spott]

There are many reasons why an engine will run rough, and they're not all related to the battery and alternator.

There may be weak compression (valves or rings), there may be weak spark (coil, plug wires, worn plugs), there may be fuel problems (pump, filter, dirty injectors), or there may be electronic/sensor problems (TPS, O2 sensors, exhaust leak, wires, or connectors).

An engine missing or running rough is an extremely generic symptom, and can be caused by any one of dozens of different problems. Don't assume that it's really caused by the electrical/charging system until you've tested it.

I'm wondering how old that battery is. I'd go out this morning and check the volts in the battery before starting the engine.
The battery being dead at the first writing sort of tells me that is the problem. If it runs on the boat battery that clinches my thoughts.
Big Jim

Yes, why was the battery dead? If you left the lights on (or something) and that drained the battery then that's normal, and is no reason to suspect the battery or alternator.

If the battery was just dead with no reason, then you should start by investigating the battery.

Also, stop disconnecting the battery, on this or other vehicles, when the engine is running. You can damage the computer when you do that. Maybe you damaged the computer when you did that, before replacing the alternator, and that's why it's running rough now. Maybe.

 

[foreverdead]

well i know the battery is good, i know the alternator is working, but what i cant figure out is random missing and i know it is electrical. for instance this morning the truck started missing so i was sitting still and waiting for it to correct itself and it was cold so i turned the blower motor on the get some heat going and the very instant i turned the blower motor on the missing stoped. i turned the blower motor off and it started missing again. so i drove to work with my blower motor running and had no problem on the way to work. i alos cant figure out why the alternator is poducing almost 15v and its charging the battery but the truck will not run off hte alternator and an 89 ranger should run off the alternator no problem. to me its like i got a bad ground and the vibration of the motor running will cause the gorund to work and not work but i cant find any bad grounds and i cant find any bad wires. the truck has good plugs and wires i tested them the coils are good auto zone checked them. and the alternator and the battery even tho i check all them myself and already knew they was working. i know its some kinda electrical problem but i cant put my finger on it.

 

[Andy D]

Rig a jumper wire to connect the dash area metal to the negative terminal at the battery. That would prove your theory. Most cars have a ground strap from the frame to the block and from the back of the block and the fire wall. One or both of those loose. Good luck.

 

[Spott]

i alos cant figure out why the alternator is poducing almost 15v and its charging the battery but the truck will not run off hte alternator and an 89 ranger should run off the alternator no problem.

An 89 Ranger should not be able to run off the alternator, and expecting it to do so will likely cause damage. I explained exactly why this is the case in my initial response to you.

Why is your charging voltage 14.8v? That's too high; spec is 13.8v. There's a good chance that the high voltage is causing problems. Then, when you put enough electrical load (the blower) on the system it draws the voltage back down to where it should be, and the electronics work better.

Get a battery voltage measurement when it's missing and when it's running smooth to test this possibility.

The alternator (and some feedback circuitry in the dash) should regulate the output voltage to 13.8 +/- 0.2v. Find the cause of this and fix it, and that might solve your problem.

 

[enjr44]

14.8 volts does sound high; but, not outrageous, unless it stays there. Both of mine will jump to about 14.5 for the first few seconds after a cold start. And then it starts dropping as the battery is brought back up.

If it is staying there, either the battery is bad or the regulator in the alternator is bad. Or it is hooked up wrong and isn't getting feedback voltage from the battery to adjust the output voltage (the battery isn't telling the alt what to do).

So to recap:
1.The battery died for no apparent reason, so you swapped in another to get you home.
2. You pulled the battery cable off and it stopped (which it will, because the alternator needs a battery to turn it on). It has to be turned on by the voltage from the battery to the light on the dash to the alt. No voltage in - no voltage out (it is not a generator). When the alt starts making voltage the light goes out, because it has voltage on both sides of the light. Just pulling the cable off can also mess with the computer (not always) caused by a voltage spike. Pulling the cable didn't tell you the alt was bad, won't tell you it was good either!!!
3. You changed the alt and tried number 2 again and got the same results, for the same reasons.
4. It now runs rough (misfires). Except when the blower motor is running.

Questions: Was it running rough before you pulled the battery cable off the second time?
Immediately after replacing the alt. Or did it start after you pulled the cable off the second time. What is the system voltage when you turn the blower on? Does it drop down to a more reasonable 13 plus something? Did you put the right alt in there. Any terminals on the new one that was not on the old one?

IDK, but, I think the alternator is hooked up wrong, or you got a bad alt, or you have a bad fuseable link (probably not), or bad wiring or some such. Computers usually don't have intermittent problems. They work or don't! 15 volts (14.8) can mess with the sensor outputs to the computer causing weird things to happen.