Cummins 6.7 engine squeal/squeak on startup

Even if the batteries tested at 12.7, the voltage regulator on the alternator is set around 13.7. So it is going to charge until it sees 13.7.

The way to minimize this issue and prolong the life of the alternator is to try if possible not to start with discharged batteries. Use the shorepower to charge them, or crank the gennie if you must get on the road quickly with discharged batteries.

I know, I know, the alternator is SUPPOSED to charge the house batteries. It will, it's just harder on it.

Mine has been doing the same thing since new. I had it back to Cummins and they replaced the Serpentine belt, as well as using a special laser belt alignment tool. Unfortunately neither helped. It is now more than 100K miles, and it still does the same exact thing. It always starts up squeaking, but it goes away after a few miles. It is annoying, but I have learned to live with it. I carry a spare serp belt just in case. 2010 Discovery

Knock the glaze off the belt and pulleys with fine sand paper/scotch bright and the noise will go away. Liquid spray works temporarily but it will return.

So, I went today to fill with gas before our next trip. When I started the MH it had the squeak. I drove it for about 35 minutes on freeway / city - and pulled over in a parking lot to listen. The noise was gone as in the other times I've heard it. I know I heard this squeak when the rig was new ( but it stopped doing it ).

So it does go away in short order after it warms up. What I'm thinking is, I did turn off the shore power ( as I do once a month) and let the rig sit to test the batteries. I let it sit for 24 hours before testing. All the batteries tested good ( before charging or adding water). Starting batteries 12.7 x 2. Chassis 6.4 x 6. Starting batteries are sealed. Chassis batteries I have a battery watering system attached. I then top off the chassis batteries with distilled water and turn on shore power.

So maybe there was something with the chassis batteries and the added water maybe dropping the voltage a little? Maybe there is a topping off draw at engine startup that cause extra draw on the alternator causing this ( as someone mentioned). It does go away.

When I started the engine I watched the voltage gauge. It was just under 13v for a very short time and then went to about 14.2 or a little higher and remained there. All battery switches were on.

If it is not a bearing, your belt may have become glazed and you MAY be able to stop the noise with some belt dressing spray.

Do you have an ammeter? If so look at the output of the alterator when you here the noise. If the batteries are down there is huge load on the alternator when first starting the coach. The alternator load can cause the belt to squeal.

I would do what Charles and Brett suggested on spinning all the rotating pulleys in the system.

Safest thing to do is remove belt and spin each accessory and tensioner pulley by hand.

That will help identify if one is tight/a bearing failing.

Better to do this in your driveway and/or at your convenience rather than on the side of the road.

I would remove the serpentine belt and carefully feel/spin every pulley, alternator, etc, to get an idea if one is failing. Move them around checking for radial and axial play also. Depending on the age of the belt you might replace it.

Charles