Ford F250 3G Alternator Upgrade

As a veteran owner of used vehicles, I am more than a little used to encountering automotive problems. When my Ford's charging system idiot light began to flash at low RPMs, I knew I had a serious problem coming my way. When it picked up to doing it on a regular basis, I realized that the problem had arrived, and unless I wanted to be buying a pair of deep-cycle batteries, I should find a solution, probably in the form of an alternator replacement. After doing a little research, I decided to install a later-model Ford alternator.

The idea was first proposed to me by my friend Rob, who has an older OBS ("Old Body Style") F350 with the same 7.3 liter International-Navistar diesel, and who was also losing his alternator. These engines are designed to be run at low RPM, but the alternators put out all their power at higher speeds. While you can install an overdrive pulley, this results in high alternator speeds at higher engine RPMs. A better solution is to install a better alternator, and later-model Ford vehicles have them. But I found a nice article on upgrading a 6.9l diesel truck to a 3g alternator, and my engine is just a higher-displacement version of that one.

As it turns out, the easiest install involves the alternator from a 1994 or later 3.8 liter Taurus. These vehicles were offered with a variety of options including electrically heated windshields and heated seats, so they need an alternator which can produce a lot of power at low RPMs, e.g. while idling in the driveway and in stop-and-go surface street traffic. Because of their design, they happen to have an alternator with big "ears" on it, which is to say mounting tabs. The alternator therefore bolts directly on to these diesel pickups, with some amazingly simple wiring modifications. You can identify these high-powered (130 amp!) alternators by the two large holes on the case front; one of the links above provides illustrative photos.

To wit: There is a harness across the front of the engine which handles the various engine-front sensors and so on, which includes (on appropriately-equipped vehicles) the "throttle" position sensor for cruise control and transmission control1, A/C clutch energizing lead, and the tachometer sensor. It also includes the leads from the alternator. You rip out the alternator wiring up to and including the voltage regulator, and also including the hot lead from the alternator, with its fusible link. This leaves you with a need to hook a fused hot lead to one of the terminals on the starter relay2, and green and yellow wires which need to be connected to the alternator.

At this point, you have two choices; one is to buy connectors and wire them. The center lead on the three-prong plug on the alternator jumps back to the single spade connector; you can get both connectors on eBay. Alternately, you can pull the wiring from an LTD of about the same generation as the Taurus; see the link above about modding the 6.9 liter diesel. I bought connectors, and made heat-shrink-tubing-protected soldered connections of all wires, including the excessive but guaranteed noise-free 4 gauge cable I used for a hot lead. This cable was purchased from the battery section of the local Kragen's, and cut to accomodate the inline AMG fuse I sourced from eBay (along with my connectors.) I then used a torch to solder (sans lead) copper tabs onto the ends of the copper wire. Everything got wrapped up in conduit for protection from fluids and abrasion, and has a little bit of extra wire for proper behavior in a crash.

The fuse is mounted as close as conveniently possible to the starter main relay, the solenoid switch on the fender. I installed 130 amp fuses because they came with the very inexpensive and high quality fuse blocks I sourced. You really want at least 10 amps over your maximum planned draw, but I did this not to get more from the alternator but to replace my failing alternator with something capable of charging batteries at low RPM. If I have a fuse problem, or if I run across a fuse, I will install a 150 amp.

There is one pain in this project, when it comes time to play with the pulleys. Because diesels don't produce any vacuum to speak of in ideal conditions, and turbo diesels produce even less, these engines have a vacuum pump belt which runs solely off the double pulley on the alternator. This means you basically have to use your original pulley, which is only a problem because you're going to need an impact wrench to take it off3, and to put it back on... and because you need to grind the case of the alternator to clear the pulley. This is less horrific than it sounds, as the case is made of a fairly soft alloy. We used a cut-off wheel as a grinder, with great success. I suggest a cone-shaped die on an air grinder. We taped up the holes on our alternators before doing any grinding; even before that, I took a steel wire brush to it, and then used compressed air to blow everything out of the alternator. These things live under your hood and are designed to run a bit dirty... but not as dirty as they were when we pulled them out of the junkyard.

The total cost of this project was around $100 for two vehicles; we did a slightly less pretty but none the less functional job on Rob's F350. Picking parts from eBay will help you keep your costs low, and I already owned every tool I needed to accomplish the work as well as many of the more trivial consumables like conduit, electrical tape, and zip ties. Both trucks are now charging happily, with brighter headlights than ever before!

F250 Alternator Upgrade - Wiring
  • 1. Actually the FIPL or Fuel Injection Pump Lever
  • 2. Commonly improperly referred to as the "starter solenoid", which is mounted to the starter itself, because there is often another, lower-powered starter relay
  • 3. I have heard of some people having luck using an oil filter wrench to hold the pulley.