Heater Fans Bog with Engine

[monkeyodeath]

This isn't really a problem, but a curiosity...

When my engine revs are low, like when I'm getting going from a complete stop, I notice that the heater fans actually slow down too. Anyone else notice this? Anyone else have an idea as to why this happens? Just curious.

[ClickClickBoom]

Alternator is the primary source of amps. If you instrumented the electrical system you would see the voltage drop at lower rpms due to the reduced alternator output. In the realm of things the fans are a relatively high amp draw item, at low rpms the battery becomes the power supply and the voltage drops.
If your fans are older, this might be a sign that the internal lube is drying out. Different kettle of fish though.

[monkeyodeath]

Makes sense. Just never experienced that before in a car. I guess the 911 fans draw a lot of current -- probably helps that there are like 20 of them!

[Bob Kontak]

Originally Posted by ClickClickBoom

Alternator is the primary source of amps. If you instrumented the electrical system you would see the voltage drop at lower rpms due to the reduced alternator output.

Just for discussion. I usually see a 13.9V-ish charge at idle with no accessory draw. If battery has been sitting for a while without a tender, it will go into the low 14's.

That said, a happy battery is at 12.8V with engine at rest so alternator is providing the extra bonus voltage.

If I rev it up, I do not see a jump in alt output. It's pretty steady.

If I start turning goodies on, I watch the voltage drop. AC, headlights, blower. A couple of tenths each until I can get it down to about 13.3V with all gizmos on.

What I don't think I did was rev it up under full load to see if the voltage would increase.

I do not recall the fans and such dropping off after a good warm up drive and dropping down to idle after higher RPM's.

I will confirm in the AM. I can forget things. :-)

Subscribed.

[jlex]

Same thing happens to my '88. I'm thinking my current battery is marginal. Will be changing it in the Spring so I'll get my answer then.

[ClickClickBoom]

Bosch Automotive Electrics and Automotive Electronics will provide all the info you never wanted to know about automotive electrical systems. eBooks are available on iTunes and Google 30-50 bucks......

Quick and dirty:
http://www.aa1car.com/library/charging_checks.htm