At wit's end with this screeching noise

moroza · **Joined:** Thu Aug 04, 2022 5:56 pm **Posts:** 17

06 CRD has a screeching noise between about 1000 and 2000rpm. Possibly higher also, but definitely not at idle. With the serp belt removed, it's absent. Two experienced mechanics with stethoscopes isolated the noise to the alternator, but three alternators later, it hasn't changed. We've also replaced the turbo, both idler pulleys, the fan drive pulley, the tensioner assembly, and tried to isolate the viscous heater and AC clutch - none with any effect whatsoever. I'm running out of ideas and will soon run out of hair to tear out. Any insights?

WWDiesel · **Posted:** Thu Apr 27, 2023 12:05 am

Does your alternator have the OAD pulley? (overrunning alternator drive pulley)
Have you replaced the belt tensioner mechanism? See diagram, item No. 7

moroza · **Joined:** Thu Aug 04, 2022 5:56 pm **Posts:** 17

WWDiesel wrote:

Does your alternator have the OAD pulley? (overrunning alternator drive pulley)

All three alternators appear to have this pulley.

Quote:

Have you replaced the belt tensioner mechanism? See diagram, item No. 7

moroza wrote:

We've also replaced the ... tensioner assembly

layback40 · **Posted:** Fri Apr 28, 2023 7:47 pm

I feel your pain!
A bit of thinking outside the box.
Is your fan surround moving at those RPM's & having the fan touch it?
Is the fan hub bearing good?
Clearly its something that moves when the belt is in place.
If the belt is not tight enough it could be slipping & causing the noise.
Does turning the lights (or some other load on the alternator) on make it worse?
I would be spraying some CRC/WD40 on things & seeing when the noise changes.
Drylube on both sides of the belt may help.
Good Luck!

moroza · **Joined:** Thu Aug 04, 2022 5:56 pm **Posts:** 17

Sometimes it takes someone stating the obvious for me to be able to see it. Thank you. Unplugging the alternator silences the noise by about 95%; a faint whirr remains, well within normalcy. Three alternators do this, and the battery takes and holds a charge just fine. What can this be?

layback40 · **Posted:** Wed May 03, 2023 2:06 am

moroza wrote:

Sometimes it takes someone stating the obvious for me to be able to see it. Thank you. Unplugging the alternator silences the noise by about 95%; a faint whirr remains, well within normalcy. Three alternators do this, and the battery takes and holds a charge just fine. What can this be?

Does turning the lights on make it worse? Or turning the steering wheel? I would be trying drylube on the ribbed side of the belt.

WWDiesel · **Posted:** Wed May 03, 2023 11:46 am

This may be a rather obvious question; but have you tried a new serpentine belt, and did it make any difference? :?:

DieselJeepLuvr · **Posted:** Wed May 17, 2023 2:11 pm

Also check your output voltage with the engine running with/ without the engine running between the battery posts. If the ECM is thinking it needs to send 100 amps to the charging system it could be causing you belt to slip.

DieselJeepLuvr · **Posted:** Wed May 17, 2023 2:12 pm

Also check your output voltage with the engine running / off between the battery posts. If the ECM is thinking it needs to send 100 amps to the charging system it could be causing your belt to slip.

moroza · **Joined:** Thu Aug 04, 2022 5:56 pm **Posts:** 17

Ok, we're getting warmer... pun intended: 12.6V off, 11.9 on but not running, 14.2-14.3 running, and the alternator is medium-warm after ~30 seconds. Swapping batteries and alternators doesn't change the behavior. TBH, I never paid attention to alternator temperature, so my frame of reference is inexact... How do these datapoints square with any of yall's Jeeps (my other running ones aren't here at the moment)?

Battery temp sensor? Bad PCM?

moroza · **Joined:** Thu Aug 04, 2022 5:56 pm **Posts:** 17

The headache continues, in no small part because the FSM goes into considerable detail about the charging system that's frankly a bunch of lies. The wiring diagrams, however, appear to be correct on page 8W-2002. The BR/DG wire that plugs into the top of the radiator measured <2 ohm resistance along its full length - within the error of a dorm room fridge. Idling the engine, voltage between that wire and ground is 13.66 at the alternator and 13.20 at the Body Control Module. Before I go hacking apart the (rather buried) wiring harness ,has anyone ever had this problem and solved it by improving the conductance between the alterator and the BCM?

At wit's end with this screeching noise

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