[sswari]

Over the last few months, I have noticed a strange rhythmic vibration while traveling at highway speeds. At first I thought it might be suspension/tires balance, but is was very rhythmic, 3 secs of vib 3 seconds not, It did not change (Much) with speed, but was really only noticeable above say 55 to 60 and up. After my trip to the JY yesterday, I have been looking at the logs, and sure enough, it somehow seems to be related to battery voltage. Is this the alternator/Regulator going out, coil going bad, any other Ideas? You can actually feel this in the truck.
https://drive.google.com/file/d/12oBSRv ... sp=sharing

[cgrey8]

It could be something rhythmically drawing the voltage down more than the alternator can respond to.

Thus it's possible the thing drawing the current could be the thing making the vibration.
Or it could be the alternator making the vibration trying desperately to keep the voltage as high as it can during those times of high current draw.

So you need to eliminate things. Is it the alternator going out or something causing the alternator to act up. This could be done a number of ways. But one of the easiest is probably to disconnect the alternator and see does the voltage still sway without the alternator in the mix. You can probably leave the belt on the alternator, just remove it from the electrical by either disconnecting its power lug OR removing the field-control wires which trigger it to generate voltage. This will tell you a few things:

First, note whether the vibration is still there or has disappeared. This will eliminate whether the alt is the source of the vibes or not.

However it doesn't indicate the alt is going bad unless the vibes are REALLY bad. Even if something else is to blame, if the alt is making excessive noise, even if only under load, that's likely an indication it's on its last leg and needs to be replaced. The question will be was it going out anyway or did something cause it go out?

If something is still rhythmically dragging the voltage down without the alternator, it'll likely be a lot more pronounced with only the battery. Find what that is as it might be what caused your alternator to go bad and could do the same to a replacement.

If the voltage and vibes have both disappeared, then the vibes are likely the alternator and you can replace it with a fairly high confidence that this will take care of it.

[sswari]

The big concern I have with trying to testing this without the Alternator is, that it only (seems to) happen at speed of 55 and up. The only real place to test this is on the freeway, so disconnecting the alternator at home is not an option, as the truck would never make it to the freeway on battery alone. I could disconnect it on the side of the road, but then would be concerned about the battery going flat on the freeway.

IT seems the search is not working today, so I can not really look up DESMAF and ISCDTY and how they work and what they do when the engine is off idle. I know they both are "At Idle" parameters, but take a look at the attached file. Seems interesting, but may be nothing of interest, but it looks interesting to me at least.
https://drive.google.com/file/d/1Stan26 ... sp=sharing