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[TestOmega]

I was having issues with my Cadillac Catera 2001 3.0L V6 which is a version of the equivalent Omega.

I checked performance curves for Bosch for my alternator by duly reviewing the pulley ratios for crankshaft and alternator pulley. The standard Idle on my engine without Aircon on is 500RPM and with Aircon on it is 625RPM.

As can be seen with the attached chart the Alternator/Generator delivers virtually no amps and if you have idle at 500RPM and got your accessories and it is a hotter day you will see the voltage start dropping on extended idle or stop and go.

I installed a 1.7" pulley (nominal 1 3/4") versus the standard 2.21" (nominal 2 1/4") and the curve shows how it will help stabilize the electrical load versus generation.  The chart Axis are Y-Axis Amps output of alternator and x-Axis is the Engine RPM.  To not confuse I have converted x-axis to engine RPM whereas Bosch charts show the Alternator RPM.

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It clearly shows that for my Idle conditions will improve as follows,

Idle no-Aircon at 500RPM ---> From 0 Amp to almost 35 AMP+
Idle with Aircon at 625RPM --> From ~ 40 AMP to ~80 Amp

Any comments as I believe it may rob a little bit of power (haven't noticed any and car is very happy).

Hope it is of help or generates ideas.

PS:  To compensate for smaller pulley I used a slighly shorter Auxiliary Drive Belt to ensure good tension with the auto tensioner and the following part number,

Gates Part Number:       K060790
Interchange Part Number: 6PK2005

[Abiton]

Interesting stuff. 

I guess it's only really significant if you do spend an unfortunate amount of time idling in traffic, but I suppose that's only going to get more common. 

I wouldn't worry about any extra load on the engine above idle, because the alt only supplies the current that is demanded (according to battery voltage) so it makes no difference as far as I can see, except a tiny increase in rotational/friction losses in the alt due to the higher speed it will be running.  May wear the bearings out fractionally sooner too I guess, if you like to 'use the rev range'. 

[Andy H]

I seem to recall a recommended maximum rpm for the old Lucas ACR alternators of 15,000 rpm. I am not sure what rpm the Bosch alternator is safe for  :-/

How big is the crankshaft pulley? What does maximum engine rpm translate to at the alternator?

When I changed my battery last year I checked the voltage at idle with everything switched on and the voltage was high enough to indicate that the battery was being charged (not discharged).

[TestOmega]

The dimensions are and ratios are,

      Crankshaft Pulley= 5.75 in
      Alternator Std=      2.21 in
      Alternator New=      1.70 in


Original Alternator RPM to Crank RPM Ratio =                  2.601809955
New Alternator RPM to Crank RPM Ratio =                  3.382352941

This is about 30% increase in Alternator RPM.

Bosch specification states, "The Bosch alternator can easily be operated up to 18,000 alternator rpm".

I never drive above 5000 RPM and mostly a maximum of 4000 when the auto box shifts and my steady highway is about 3000 RPM at 100 KM/hr.

The redline max on my car is I believe 7000 RPM and that will definitely not be good with new pulley as it will exceed 18,000 alternator RPM.

Hope above clarifies.

PS:  Where I was running into difficulties last year was rear window defrost and front window defrost on causing all fans to run full speed (cooling, blower) as well as electrical loads from rear window heater.   The car voltage was nearing the red zone, and it was stop and go on the highway.

[Broomies Mate]

Very odd.

The more load you put on the battery, the more the Alternator compensates.

I dont see the benefit of reducing the diameter of the pulley.  Afterall, if in traffic for a substantial amount of time, the cooling fans cut in, therefore drawing more current, and the alternator does it's job.

I get 14.2v at the battery on idle with everything off, and 14.4v at the battery with everything switched on.

[TestOmega]

There is a minimum RPM before the alternator does anything.  This is very close and marginally around the idle RPM.  In this area the alternator basically is not doing much if anything.  Apparently unknown to the driver, the ECU tries to increase RPMs in spurts but in such a way that it is not too noticeable to the driver (very cute) to rectify this situation.

In my case my voltage in driving, idle, appliances on is very steady now with this mod.

Regards.

[Andy H]

I think the general experience with the Bosch alternator is that it is perfectly capable of matching the full electrical load at idle as long as it is good condition.

There is a spring loaded connector inside the alternator that corrodes and causes a poor connection and the voltage regulator is prone to failure (there might be 'cause and effect' here but I am not certain).

Also the crimped connections on the big leads to the battery (both positive and ground connections) are prone to breaking up and causing voltage problems.

HTH

[TestOmega]

Andy H thanks your suggestions noted and I been watching the cable ends for corrosion.  The alternator was replaced on the car under extended warranty and voltage symptoms were not much better for it.

On a good day in the near future I am going to install a 2/0 welders cable for the positive cable to the alternator.  I had the dealer replace the harness once and it was not cheap and they claimed the old cable was almost totally fried.

I am going to leave the mod in for now and if I have less than positive experiences along the way I will flag for others but so far everything is a OK.

Regards.

[Andy H]

Good man.

Have you seen this maintenance guide repairing the alternator ?

[TestOmega]

Good man.

Have you seen this maintenance guide repairing the alternator ?

Yup I been reading guides on this excellent site as though I am going back to school.

Again great site and thanks.