Tech Tip #91: Alternators for Perkins Engine Models 4108, 4203, 4236 and 6354

Dr. Diesel
Written by Dr. Diesel

Helpful guide when considering an alternator for your Perkins engine

This Tech Tip discusses alternators on Perkins Engine Models 4108, 4203, 4236 and 6354. We are developing another Tech Tip for the Perkins Model 100 engines which we will have out shortly. This Tech Tip focuses on these earlier Perkins engines and amplifies information contained in

Perkins diesels, like most diesels, are shipped from the factory with a very weak original equipment alternator. The Perkins Model 4108 for example usually leaves Peterborough with only a 37 amp alternator. Perkins must have thought that was OK because diesels don’t have very extensive electrical systems. Weak alternators were probably fine back in the day, but it can spell trouble in a marine application. If you add to your Perkins an electric fuel pump (8-15 amps) to help the mechanical pump, a wash down pump (8 amps), a fish finder, and maybe a couple of gel cell batteries for your house system, your electrical load at idle could be easily double the 37 amps. Factor in a bad cable or connection and a 30 year old wiring harness and you may have big problems.

Here are three ideas.

  1. First things first. Test your present Perkins alternator. The easiest way is to simply remove your alternator from the engine and bring it to an electrical shop. Have them test the output but factor back in that there will be a drop in output when the alternator is back on the Perkins and running hot. If you test the alternator while still on the engine do not disconnect a battery cable with the Perkins running. The voltage spike will blow a diode or two. (This winter when your neighbor asks you to jump start his car from your new Tahoe, unless you want to risk having to buy a new alternator, you should simply point him the direction of the nearest place they sell Optima batteries.)
  2. Going all the way. Some people want to go all the way and install a new “marine” 100 to 150 amp alternator on their Perkins engines. This is a nice idea if you have an extra $1000 in your jeans. Let me break the figures down for you. Most good marine alternators are $450 or more and are “externally regulated”. This means that they do not come with a regulator but you need to purchase one for roughly $175 or so. Moreover, once you get above 100 amps, most manufacturers (e.g., Balmar) recommend that you use a double pulley drive set up. While Foley stocks dual pulley set ups for the Perkins 4108 the cost can add up. A double pulley for the water pump as well as another pulley for the crankshaft won’t leave much change from $400.
  3. Install a Foley Hytork 105 Amp Alternator. We have come up with an alternative that makes a lot of sense to us. We have a replacement alternator for your Perkins diesel that is the same GM case size, same mounting feet, same internal regulator, same “single wire” design, and will simply bolt in place of the existing Perkins alternator. These units are 105 amps and install in a matter of minutes.
  4. How to install a one wire alternator. Installing our 105 amp Perkins replacement alternator is simple. Remove the old one wire alternator and install the Foley Hytork alternator, running as before wire from the positive position the battery to the large output terminal on the alternator. Because this is a 105 amp alternator, you should consider installing a six or four gauge cable. If you are really sharp, you should consider using high quality muti-strand single gauge (1/0) welding cable.
Dr. Diesel
Written by Dr. Diesel
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