I have a 1999 6.5. I have to replace the 105 amp alternator and am wondering if the 124 or 140 amp alternators are all the same size and mounting configuration.


My local parts store only has the 105 amp unit so I can't line them up to see if they will all fit (the parts guy doesn't know if they are interchangeable). I have to order the 124 or 140 amp units.


Does anyone have any experience with this?


Thx

Stay away from the parts houses. Find a local Alternator rebuilder, and ask them for suggestions.

<DIV>I have done some tests on the alternator ratings. As you know, the alternator current output is low at idle and high at cruising rpms. If your driving involves a lot of idle periods, check the engine idle current from your battery to the rest of the vehicle with all the resonably possible consumers turned on, including your trailer/caravan/etc. At the same time check the current between the alternator and the battery. The two figures must be the same. If the battery figure is higher, you'll be having problems with your battery.</DIV>
<DIV>Repeat the test with the engine at cruising rpm: the alternator rating should be at least 10% higher than the total current from the battery with all reasonably possible consumers turned on. If the alternator rating is not higher, if it is even lower, you'll be having both battery &amp; alternator problems.</DIV>

hatv


Interesting idea, but what are you using to make these current tests?


I have lost 2 alternators while towing and using an electronic cooler in the truck so I realy want to do this test.

<DIV>Just an ordinary current clamp with an electronic read-out; it can be bought in any of the electronic shops. Here in Australia they come for about AUD$50.</DIV>
<DIV>I run a small company specialising in the electrical systems design &amp; testing, and in our experience the undersized alternators are the most common source of electrical problems in many cars and trucks; and quite often, nay, mostly, other things are getting the blame.</DIV>
<DIV>Mind you, if you discover that a larger alternator is needed you may have to upgrade the adjacent wiring as well.</DIV>

I tried replacing the factory 105-amp alternator with the 140-amp alternator. It bolted up, however the connector on the back of the alternator was different, so it ended up not fitting.


I later had an alternator shop rebuild the 105-amp alternator to produce 160 amps. To make up for the lower idle charge they put on a smaller pulley. An interesting side effect of this was that the tachometer read a couple hundred rpms higher on the highway, so I believe the factory tach works through the alternator. That alternator then blew up on me after about 150 miles (fault of the rebuilder), so I switched back to a factory 105-amp unit and stuck with that. Really, you only need the higher amp alternator if you're going to be running extras like a stereo, driving lights, etc.

The late GM alternators are 400hz constant frequency variable duty cycle regulator - meaning they will supply near full output at idle.


Not noticeable with the EFI Diesel, as PCM varys idle inj to each cylinder to maintain constant rpm under any loading conditions.


If idle load is more than 105amps, larger alt is required.


And yes- tach rpm is based on pulley diameter, ac signal taken off one of three phase windings.


Diesel pulley is specific, different diameter than gasser pulley, listed as such in GM parts data.