Auto Service World
Feature   December 1, 2008   by Chuck Carman, Curriculum Developer CARS


We have all heard it, that easy to recognize generator whine. Almost all generators make noise -- which is normal while operating. Normal operating noise is caused by the magnetic pulses from the gene...

We have all heard it, that easy to recognize generator whine. Almost all generators make noise — which is normal while operating. Normal operating noise is caused by the magnetic pulses from the generator. Eliminating the magnetic field-stopping the production of current by unplugging the generator field connector-will stop this noise. It is best, when possible, to compare this “normal” noise to a known good vehicle to be sure the noises are similar.

However, when the noises are not similar and it is determined that the vehicle you are addressing has an objectionable noise, this is where it becomes interesting.

If the bearings are failing, they will create a whining noise, this will progress into a growl or even a grinding noise if left too long. Quite often, disconnecting the generator will eliminate or reduce the noise since this takes much of the operating load off the bearings.

Easy enough to correct — no one repairs generator internal components anymore so-replace the generator assembly, maybe even up-sell (recommend) a new tensioner and belt for good measure.

And the noise returns immediately. This is where that sick feeling hits you. You did everything right, yet the problem is still there; or did you do everything? Now there are three things whining, the generator, the customer and you, the technician.

If this vehicle happens to be one of certain GM models equipped with a 3.1L, 3.4L or 3.8L engine, I’m going to send you in a slightly different direction, one where normal diagnostics might not lead you to immediately, if at all.

Particular models have a condition described by the customer and technicians where there is a noise similar to a generator whine or moan, very much like a fuel pump screaming. Most technicians are sure it is the generator and replace the generator. However it may actually be the regulator, the fuel pressure regulator.


This is where, while working as a technical consultant, I would get the “deer in the headlight” look.

Here is what to look for. Using an electronic stethoscope, place the sensor on the fuel pressure regulator to see if the noise is the loudest there. If you do not have a set of electronic ears, disconnect the regulator’s vacuum line. If, after unplugging the vacuum source, the noise goes away, there may be a fuel line grounding out against the body or somewhere along the chassis-but most likely the regulator has failed. Replacing the regulator rather than the generator will correct the concern.

The cause of the noise is when the diaphragm within the regulator housing begins to oscillate at a very high frequency; the noise it creates transmits throughout the entire fuel rail. This is partly why the technician does not hear the noise coming from the fuel regulator alone. Disconnecting the generator changes the vehicle’s operating voltage level, since it is now only drawing from the stable battery voltage supply. This causes a change in the operating fuel pressure produced by the fuel pump-stopping the vibrations and stopping the noise, just like disconnecting the regulator’s vacuum supply line.

So, how does an average technician combat these types of issues? First, always check for product service bulletins. However, this particular condition has not yet made it to a bulletin. Being well trained and up to date is always a good idea. Also consider using one of the many technical assistance centers as they have a huge data base of multiple cases to pool from. Their fee (if there is one) may be worth its weight in gold, especially if you have already replaced a component which did not correct the problem as you had hoped. They may have already experienced your concern and will steer you in the right direction.

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