Quasi Electrical Question

callahanoffroad · Post by **callahanoffroad** » 22 Mar 2018, 07:38

I've noticed since it has become a little warmer that my radiator fan has been kicking on to cool the radiator when idling. Well this wouldn't honestly have even caught my notice if the idle hadn't dipped when this happened. I also noticed my idle dip when I switched on the cabin fan. Seems like the idle is only dipping when these two devices are switched on. Doesn't happen with indicators, radio, power mirrors, power windows, etc.

So, that leads me to my question. Where should I even begin to find out why my idle is dipping? Grounding? Battery connections? Alternator?

The entire ignition system is only 10,000 miles old. Battery is 6 months old.

Post by **abscate** » 22 Mar 2018, 10:39

Is your AC kicking on since the temperature is above freezing?

How much of a dip in rpm are you noticing?

Post by **sleddriver** » 22 Mar 2018, 13:08

I've recently run across the solution to this while researching something else. Problem is I'm unable to recall the details. It is here on MVS.

Post by **wizechatmgr** » 24 Mar 2018, 18:53

Those are two of the larger electrical loads outside of headlights. It isn't unusual for a car to take a second or less to adjust the RPM to coincide with the new load. How much of a drop in RPM are we talking? I'd say a dip of around 200 RPM is expected.

Post by **RickHaleParker** » 24 Mar 2018, 22:53

A/C compressor and Alternator loading can cause the idle speed to dip. If it does not recover one of the two may have excessive drag.

Post by **polskamafia mjl** » 24 Mar 2018, 23:01

Increasing the load on the alternator may cause a dip in RPM especially since those fans require a good deal of power. Sounds like everything is working as it should.

Post by **sleddriver** » 25 Mar 2018, 12:08

When's the last time you cleaned the idle air speed control motor? Check for air leaks in the hose as well.

Post by **MrAl** » 26 Mar 2018, 10:35

Hi,

Mine dips too, but then recovers. It takes a full second as i can see the voltage dip too as i have a fast analog meter connected at all times that i can view as i turn things on and off. The needle dips, then recovers. It's a full second though NOT really really fast down then up again. I could easily say, "One Mississippi", while it is down.

I have to say though if this was one of my designs when i worked in the power industry i would probably have been questioned about it because a power system that responds that slow in this day and age is not very well designed.
One of my (non automotive) designs had to be as fast as 1/4 cycle of the AC line, which amounts to around 4 milliseconds which is about 7 times faster than the fastest human reaction time. If the ECU could respond that fast i would just see a very quick 'blip' where the needle might dip very slightly down for an extremely short time, or might not even move at all with something that fast.
So why they made it that slow i dont know yet. My guess is that they were afraid of some instability.

Post by **mrbrian200** » 26 Mar 2018, 11:28

This wasn't unusual on older cars (idle speed dips when heavy electrical loads or the AC compressor are switched on. On these older cars, idle speed control tended to be 'reactionary' because the engine management electronics didn't see what these other electronics were doing over CANBUS like the newer ones do. So idle speed dips, the ecu has no way to know know why, then compensates (idle speed recovers).

Newer cars these items (such as the AC compressor, interior fan controls etc), ECU can usually monitor these higher current electronics over CANBUS and learn how much compensation to apply as the electronics are activated. But because they see the electronics being activated on they can compensate to raise idle speed proactively before the idle speed dips.

Not to talk specifically about the AC compressor. If the idle dip has become more pronounced over time this probably indicates either a worn AC pump, improper refrigerant charge or loss of refrigerant oil that is resulting in the AC compressor having increased internal friction which drags down the idle speed more than an system that is in ideal condition. If the idle speed doesn't recover then you might have an issue with the IAC or on newer vehicles the electronic throttle body. This also can be a symptom of a worn engine that needs to be rebuilt: lower compression due to blowby = lower torque output at idle making it more 'sensitive' to electrical loads at low rpms. Blowby has a larger effect on torque output at slower RPMs as there is more time for the compressed gasses to leak past the rings as opposed to when the engine is revving up faster.