frank5079

I am totally vexed!!! Ever since I finished my 351 swap, I have had this belt squeal problem. I have new idler and tensioner pulleys, smog pump eliminator pulley, new water pump, new power steering pump and pulley, plus a new belt. When I start the engine, the belt screeches and the alternator doesn't show a charge. It'll read 10.2 volts. Now I can pull the alternator plug and the screech goes away. Or I can use my battery charger and while the engine is running, move the selector switch up two clicks from 2 amps/trickle charge to 40 amps and the belt screeching stops and the voltmeter will read 14.6 volts. I have replaced the alternator three times(new units, not reman) and this time I even tried replacing the voltage regulator to a soft start unit and still have the same belt screech problem.....I'm really lost at this point..please help!!!!

BLDTruth

Does the alternator ever turn on? Is it a 3G alternator? Year and model of pickup?

frank5079

1995 XLT RC/LB with a 351 swap/M5OD....all three alternators I used were new, not reman 3G units(the first two were 130 amp units for a '95 E350 with a 351 V8, and I reverted back to a 90 amp on this last one, just to rule out that the drag from the previous two was greater than on a stock 90 amp unit).

BLDTruth

I guess what I am asking is - does the alternator inevitably turn on and start charging the system, or does it fail to turn on at all? You said you were seeing 10.2v.....is that with a Digital multi-meter?

I ask this because there is specific wiring within the system to turn the alternator on and off - so maybe it is the wiring to the alternator and not the alternator - you have replaced it three times so the chances of it being the alternator are astronomically low at this point. Check the wire running from the charging port on the alternator to the starter relay on the fender - that wire is both how the system gets charged and how the alternator gets turned on. There is a fusible link in that wire as well that can go bad.

The screeching only happening when the system is undercharged vexes me as well - generally screeching is because of mis-aligned pulleys. I would double check your smog pump eliminator pulley and make sure you are aligned properly. There is a reasonable chance the alternator/smog pump pulley bracket is bent or cracked.

frank5079

Alternator does not come on when I start the engine. That's also when the belt is screeching and the multi-meter reads 10.2 volts. If I hook up my battery charger/jumper to the battery and move the amp selector from 2 amps to 40 amps then the belt stops screeching and the multi-meter reads 14.6 volts. Where can I find the fusible link? That has to be my problem ....

mmk

I do believe your alternator is trying to charge the battery. The screech is there when the system voltage is low, but goes away when you unplug the alternator => the alternator load goes away, causing the screech to stop. When you charge the battery (at 40A!!) the screech goes away, meaning the alternator is no longer trying to put a lot of current back into the battery. It sounds like a wiring issue or possibly a bad battery.

BLDTruth

It's inside the factory alternator harness bundle. It is kind of important to understand how that alternator harness works: The cable runs from the fender relay into a fusible link - on the other side of the fusible link is the charge cable and the origin point of the wire that runs into the 3 prong plug on the 3G alternator. That wire which originates right after the fusible link is what turns on the alternator. There is a second wire that runs from the plug to the tiny plug on the alternator, and then another which runs back through the harness and has a single wire plug connection, right near the battery tray - this wire is what turns the alternator off.

Just out of curiosity, what does your battery read when the system is off? It almost sounds like you have a crap battery.

mmk

As I think about it, it does sound like the alternator is actually working to drive down voltage, which would mean its wired into the system incorrectly. I don't believe your truck will start with a battery voltage of 10.2V. If the system voltage is higher at rest than running, then the alternator is wired in incorrectly. I would absolutely verify the alternator is wired correctly before subjecting the battery to the bad effects of reverse polarity from the alternator.

Eric Fullerton

Have you tried a little bigger pulley on the alternator? I actually went the other way, the alternator I got had an over sized pulley part of an under-drive setup. I traded it out with another found around the shop. that would give you more contact area with the belt, + a mechanical advantage as the ratio between the drive pulley on the crank and the driven pulley on the alternator would change to make turning the alternator less work..

Steve83

Actually, the single gray wire at the starter relay IS the alternator fusible link.


(phone app link)That small Y/Wh wire is constant power to the voltage regulator - it's always ON.Not really. The VR is normally OFF, except when it detects 12V on the LG/R wire from the instrument cluster indicator light. That's what tells the VR to turn on the rotor, which is what generates the magnetic field that allows the stator to output charging current (when the engine/belt/alt spins). The dash light is not powered when the key is OFF, turned back to ACCY, or in START.


(phone app link)

The voltage is low during the screeching because the belt is slipping and the alt isn't generating the requested voltage. The screech comes from the alternator load on the belt, but it may NOT be coming FROM the alternator pulley. The load on the belt can cause other sections of the belt to become slightly tighter or slacker, which can make OTHER pulleys squeal. Use a mechanic's stethoscope with the hard probe removed (open tube) to pinpoint the source of the noise. DO NOT allow loose clothing, fingers, or the stethoscope to become entangled in the belt, pulleys, or fan.


(phone app link)

A hard tube extension (metal or plastic) reduces the chance of it being grabbed by the belt; and if it's barely inserted into the stethoscope's soft hose, it can be easily pulled out (without snatching the scope out of your ears, or jerking your skull down into the fan) if it DOES become entangled.