A Short Guide to Modifying and Reclocking the 3800 Accessory Belt Tensioner for Low-Mount AlternatorsWhile this isn't the most difficult of tasks, I was unable to find practically any information on how this was done on PFF. This has been done by quite a few members that are willing to share but I noticed that no pictures or detailed descriptions were readily available. Thus, I decided to snap a few pictures and share what I did to possibly help those going through this in the future. My methods here may not be the absolute best, but for those treading into unknown territory, this write-up should at least provide a good idea of what you are getting into.
There are two style tensioners and Darth Fiero claims that the later style is easier to work with. EDIT: It appears that this is actually the later style tensioner (at least later than 98).
This write up was performed on a 97 Bonneville tensioner (or at least so I was informed).
| | | quote | Originally posted by Darth Fiero:
There are two different designs of assy belt tensioners used on the 3800s. Early (~1998-earlier) and late (~1999-up). The late style is much easier to modify and work with than the earlier style...
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Let's Get Started The first step (for me) was removing the entire alternator/tensioner bracket assembly that holds the tensioner. I imagine that this job would be possible while the tensioner assembly is still bolted to the engine but taking it off obviously makes things much easier.
Next, the dust cap/cover needs to be removed. It comes off pretty easily. I found that drilling to two small holes on opposite sides of the cover lets it easily be pried up and out without warping it at all. The small holes can easily be filled later with JB weld, RTV, or your preferred weapon of choice.
I didn't get a good picture of it (and perhaps I'll later edit this post with one), but after the dust cap is removed you'll find that the rotating assembly is held to the stationary part of the tensioner by a retaining washer on the central shaft. This washer needs to be cut and removed for the unit to come apart. Under the retaining washer (shown in the next picture) is a bushing that you want to be careful not to damage which probably makes this the trickiest part of the entire job. I used a Dremel with a small cutting blade to make deep notches in the washer without actually cutting all the way through it and into the bushing, I then broke the washer apart and pulled it out. Its okay if you notch the housing for the dust cap a little as all you'll really be affecting is aesthetic value.
Now, to actually remove the rotating assembly once the retaining washer has been removed. Note that the whole arm is still spring loaded rotationally. Once the outer assembly has been removed enough for it to spin past the stop on the stationary part of the tensioner, it will do so violently. I would recommend removing the rotating assembly with a tool and not holding onto with your fingers. Also note that the large spring in the tensioner assembly also applies a small amount of upward force on the rotating arm and the retaining washer. While I haven't yet had that force be enough to pop off the tensioner assembly when the retaining washer was removed, you should nonetheless err on the side of caution. Wear safety glasses and be careful with your fingers are when you are removing the retaining washer and the tensioner arm. The next picture shows what the rotating arm looks like under the washer (and without the central shaft).
The next picture shows what you'll find beneath the rotating assembly on the stationary part of the tensioner
You can see the notch that holds the lower end of the spring. It's this notch that we'll be "relocating" to reclock the tensioner. The underside of the rotating assembly also has a means by which to hold the upper end of the spring in place but it doesn't need to be modified in any way.
Let The Changes Begin! A new hole was drilled approximately 60° to the right (thanks Dennis LaGrua) to retain the lower end of the spring. Note that this new hole is not going to be protected from the elements as well as the original. While I haven't shown it in a picture, I would recommend building a small cover for it when everything else is said and done.
Now, to reassemble the tensioner we need a way to hold it all together again. I used a M8-1.25 tap to add threads to the center shaft. The dimple in the center of the shaft makes it easy to center a new hole for this purpose. Once that's done, a M8-1.25 bolt can hold the entire unit together!
However, first we need something to replace the retaining washer that acts not only as a bushing surface, but also serves to fully seat the rotating assembly down far enough.
I saw two practical solutions to this problem. The first would be to add a spacer that sits where the retaining washer did that is large enough for the bolt to clamp down on to (a couple large washers would work as well).
The second (and probably better) solution would be to grind/lathe the upper part of the shaft such that a single washer could be used. The advantage of this is that the bolt holding everything together will also sit down lower. This is what I chose to do. A large flat washer is best (though the one pictured here is slightly larger than what can be used without alteration).
Before reassembly, I also smoothed out some of the ribbing where the stop for the rotating arm will now be moving (part of the red box). Depending on where exactly you drilled your new spring retaining hole, you can also remove the larger tab to the right of that (green box) and grind that down but I preferred to leave mine there to act as a stop for the rotating assembly. Fighting the spring over during installation is only a little more difficult. I would actually consider even adding to the return stop if you have a welder (or through similar means) so that installation and use on the engine is easier. (EDIT: Also, Dennis Lagrua added that such an addition would be helpful if the belt were to break during operation so the tensioner wouldn't swing into the supercharger pulley).
The final step is bolting it all together and putting the dust cap back on if you wish (recommended). I ground down the head of my bolt a little bit so that mine would fit down nicely and I could reuse the original cap. The smooth pulley will also have to be changed over for a ribbed one if that hasn't been done already as well. I'll add a picture later with the assembly installed.
For those of you that are knowledgeable on this, feel free to leave suggestions or corrections and I'll append the post. For everybody else, hopefully this helped somebody! Also, here's also a link that has some good information on building a low-mount alt bracket. I used a 99 Astro alternator like Dennis Lagrua suggested. It makes for a rather easy build.
https://www.fiero.nl/forum/Forum2/HTML/119994.html[This message has been edited by GraterFang (edited 07-11-2012).]
