1994 Ford Bronco 94 Ford Bronco

First of all, this site IS free. You didn't have to make a donation for my time, and I like it that way. Second, you are risking doing major damage by removing a battery cable with the engine running. Rather than get myself worked up over this, as I usually do, here is a copy of my previous reply:

AGGGGHHHH!

DO NOT DISCONNECT ANY CABLE WITH THE ENGINE RUNNING!

Every year I did a demonstration on the generator test bench for my students to show what can happen when you do that. It was real easy for the voltage to reach over 35 volts. That WILL destroy any computer on the vehicle, the generator's internal diodes and built-in voltage regulator, and any light bulbs that are turned on.

The thinking is that if you disconnect either cable and the engine stays running, the generator must be working but a lot of them will stop working due to the voltage regulator responding to the dips in the "ripple" voltage being produced. That will make a perfectly good generator appear to be bad so that test is not valid.

If a mechanic is caught pulling this stunt he will typically get one verbal warning. For the second offense he will be fired. It's that big a deal.

Some generators respond to the high points in the ripple. That momentary higher voltage goes right back to the field winding and creates a stronger magnetic field. That stronger electromagnet creates a higher output voltage which again creates a stronger electromagnet. It's a vicious circle and voltage can keep on rising until something gives out. The main thing that smoothes out that ripple so it doesn't affect the voltage regulator or the generator is the battery.

Three things are needed to generate the output current. They are a magnet, (electromagnet, in this case), a coil of wire, and most importantly, movement between them. That's why the belt needs to make it spin. One thing that can save you from doing damage by removing a battery cable is not raising engine speed. Generators are relatively inefficient at low engine speeds and their output voltage is less likely to rise to dangerous levels, ... As long as you don't raise engine speed.

One other thing to keep in mind is batteries give off explosive hydrogen gas. Regardless if your generator is working or not there is going to be a big spark when you remove a battery cable with the engine running. Either the generator's current will be recharging the battery, and that can be up to 20 amps, or the battery is going to be supplying the car's electrical systems, and that can easily be over 30 amps. That kind of current is going to create a big spark when a connection is broken or reconnected. Small arc welders run as low as 40 - 60 amps and look at the sparks they create. The reason we don't hear about more battery explosions is because people are careful to not disconnect the cables when there is current flowing through them. It's also why there are huge warning labels on all battery chargers to be sure they are turned off before connecting or disconnecting them from the battery.

Another common generator problem is one defective diode out of the six. You will lose exactly two thirds of the generator's capacity but system voltage will remain normal or it could even be just a little high from the voltage regulator responding to the greatly increased dips in the ripple voltage.

It's always a good idea to wear safety glasses when working around car batteries, but if you still insist on removing a cable while the engine is running, a face shield makes more sense, and have plenty of water on hand to wash any acid off the vehicle's paint.

Ford used to have a really nice generator design that allowed testing right on the back of the unit. Only Chrysler alternators are easier to diagnose. Unfortunately the engineers don't really care about ease of service on GMs and many other brands.

The way you tell if the charging system is working is to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. There still could be a bad diode though. You need a professional load tester to test for that. Ripple will be very high and the most output current you will get will be one third of the generator's design value. That is not enough to meet the demands of the electrical system under all conditions so the battery will have to make up the difference, until it runs down. You still could have a generator problem. Either have it load-tested or use a home battery charger to fully charge your battery at a slow rate for an hour, then see if it is dead again the next day.

Now, if you want to do a valid test, unplug the small three-wire connector on the back of the generator while the engine is running. If something changes significantly, have the charging system professionally tested for full-load maximum current and "ripple" voltage. Those will tell us if the generator has a bad diode. I can explain that in more detail if it comes to that.

Dandy. The first and last paragraph in my reply were original. Everything in the middle was a canned reply that I post multiple times, so only take from that what is of value to you.

Second, I had a major house fire a year and a half ago, so I have to come into town and use the library's wireless internet, either in the parking lot or inside, so you'll only hear from me at certain times. Don't panic if I don't get back to you right away. Today I'm in the middle of delivering a friend's car to the repair shop, then walking five miles to the dentist, then walking in the rain a mile to the library! Grrr. From here it's another four miles to the shop to pick up the car. Hopefully it will just be cold but not raining.

There's a couple of things to consider with your vehicle. The first thing is to have the charging system tested since disconnecting the battery caused a change in operation. If you want to start the tests first, use a inexpensive digital voltmeter to measure battery voltage with the engine not running. If the battery is good and fully-charged, you'll find 12.6 volts. If it's good but discharged, you'll find closer to 12.2 volts. What is of interest here is if the battery has a shorted cell. For that the voltage will be around 11 volts, and it will need to be replaced.

If you find near 12.6 volts, the next test is to measure the battery voltage again with the engine running. It must be between 13.75 and 14.75 volts. If that is okay, the next part of the test has to be done by a mechanic. He will use a load tester to measure full-load output current and "ripple" voltage. Your vehicle uses a 95 amp generator, so under full-load it should deliver around 90 - 95 amps. If one of the six internal diodes has failed, the most you will be able to get is exactly one third of that. Thirty amps is not enough to run the entire electrical system under all conditions. Charging voltage may still be okay, or even a little high, but generators put out three-phase output, and when one diode fails, you lose one of those phases. During the time the missing phase is occurring, the output voltage drops considerably. That is where the ripple voltage comes from. It is a good indicator of whether the generator is working properly.

That ripple voltage also affects the rest of the electrical system. Computers are very sensitive to varying or pulsing supply voltages, so a generator problem can cause weird things to happen. Rough running is one of the more common symptoms. By unplugging the small three-wire connector at the back of the generator, you'll stop it from working and if it's developing high ripple voltage, that will stop too. You only need to unplug that connector long enough to see if the running problem clears up. If you have the professional load test performed, post the result here.

The next thing to look at is the EGR system. Ford has had a lot of trouble with the tubes becoming plugged with carbon. I don't know if that goes back to a '94 model, but it's worth considering. When the tube to one cylinder becomes blocked, no exhaust gas goes into that cylinder It gets only air and fresh gas, so it runs fine. The volume of EGR gas stays constant, so it is split among seven cylinders instead of eight. That means each of those seven cylinders is getting too much exhaust gas. As more and more tubes become plugged, all the exhaust gas goes into just a few cylinders. Since they're flooded with that inert gas, it leads to misfires.

The clinker with this EGR problem is there shouldn't be any EGR flow at idle, so the problem should only show up at higher speeds. Consider an EGR valve that isn't sealing properly to close at idle. You can unbolt the valve assembly, then slide in a thin steel shim to block it to see if that clears up the running problem If it does, have the EGR system checked.