Idle drop under battery load

[JetspeedCamry]

Ok, i just wanted to know firstly, what the problem is, and secondly, how this can be remedied.

Engine all warmed up nicely. Idle speed, approx 700rpm. Slight fluctuation, of about +/- 10rpm or so. When the thermo fan kicks on, idle speed drops to 640-670rpm. When thermo stops, idle returns to normal again.

First thought would be excessive current drain or bad earthing? I have 2 4GA cables running from the (-) terminal of the battery to the body. One to the engine bay side (near the airbox) and the other runs to the chassis rail, near the fuel filter area.

Thoughts? Suggestions?

Thankyou!

[slide86]

a slight drop in RPM is normal when the alternator is charging. it takes a bit of load to charge the system and get those fans spinning up. late model commodores drop in RPM a fair bit when the alternator is put under load.

you should see what the condition of your battery is, if it needs charging a lot, there may be a problem.

use a multimeter to check teh charge rate, if its round 14.0 - 14.5v then its all good.
anymore and its overcharging, which isnt good. and results in needing to change the alternator

[Cool1]

Have you got a decent earth going to the engine block?

[oldcorollas]

echo cool1... (and check bolt tightness etc)

also.. at idle. you alternator is producing not much current at all.... sayyy... 10-20% of max...

thermofans..... take HEAPS of current... like.. 10-15 amps... and start-up current is higher again!

and that 10-15amps may only be 0.3 or 0.5kw... but at idle you are not making much power at all... hardly any....
what you could do is to fit an air-con idle-up valve (elec controlled air bleed) connected to the thermofan so it auto idles up to compensate for the drop.... ish..

[Mos]

A lot of engines have idle up valves for thermos and other high electrical loads - check that you don't have any blockages in those lines and check the operation of the valve (if it has that design).

When the fans turn on the alternator increases the mechanical load on the engine that reduces the revs at idle that significantly - at higher revs you have much more torque available at the crank, hence a higher mechanical load from the alt would go unnoticed.

I would imagine that as you turn on more accessories the revs will only drop to a certain value as the limit of the alternator current output is reached and the rest of the current demand is filled from the battery (but have yet to test it).

I'm trying to think why a bad engine earth would cause a higher rev drop?
If anything, the fans would draw less current from the alternator by virtue of the higher resistance and they would draw more from the battery. Thoughts?

A single fan draws around 6-7A when running (interestingly the size of the ones I have tried doesn't seem to make a difference). The start up current (not measured accurately) is about double that, but it is so short that the engine shouldn't change revs. The 2JZ doesn't seem to budge when the twin 14s are switched on...

(Damnit all these things to experiment on to get the exact results even have the current shunts and scope,etc )...

Mos.

[MacroP]

When the thermos(Falcon twins) in my car cut in, they load up my 1UZ quite noticably, revs drop about 50rpm(from 700rpm) but the ECU does not try and compensate. They draw about 15 Amps each when running. The alternator is a 150 Amp unit but at idle it can only give out around 20 amps so with the fans running my charge voltage drops to 13.5Volts.

Seems normal to me though.

[Cool1]

I'm trying to think why a bad engine earth would cause a higher rev drop?
If anything, the fans would draw less current from the alternator by virtue of the higher resistance and they would draw more from the battery. Thoughts?

You need a good earth to your engine to enable your plugs to fire! Poor earth = poor spark!

[JetspeedCamry]

Ok thanks alot for that!... I have 2 earths going to the engine block. One 4GA that piggybacks of the end of the one near the airbox, and goes to the transmission. The other is is a similar spot over the other side, but is 8GA and goes to the side of the intake plenum. (was were the original one was).

I guess the drop is normal. It doesnt really effect drivabilty or anything, as its only 50 or so RPM. And yes, if the fans are on and I depress the brake pedal or pop my headlights on, then the idle drops another little bit...about 10-20rpm again. Pehaps a beefier alternator would do the trick?

[Hydra]

My 1G's ECU uses the tail-light power to tell when there is electrical load, well something like that. Hooking it up seems to have made the engine handle electrical load a bit better but who knows if it actually did

But a small drop is always normal.

[BrianRA23]

A stronger (bigger or higher rated) battery should provide a better buffer so that the drop in engine rpm will not be so instantaneous when it goes under load especily for the startup curent required.

The alternator will still need to compensate with additional charge, but it will be less noticable and may actually put the extra charge into the battery when the engine is running at normal operating rpm (ie when driving).

A larger alternator will not fix your problems as it just gives it the ability to put your engine under more load.

[The Witzl]

My 1G's ECU uses the tail-light power to tell when there is electrical load, well something like that. Hooking it up seems to have made the engine handle electrical load a bit better but who knows if it actually did

But a small drop is always normal.

Yep - it measures the voltage at that point and if the voltage and engine conditions meet certain criteria, it would bump up the idle using the idle-up valves.

On a 4A-G** series engine, the ELS1 and ELS2 are the sensor inputs to the ECU, and the VSV pins are the corresponding outputs to the idle up valves