Ignition voltage drop on 411 S5

[Thomas591]

I have a problem with too low ideling in City traffic. I found out that when the electric fans in front of the radiator are switched on, the voltage on the ignition coil drops to 9,5 Volt when ideling. The result is a drop of 200 rpm, making the Engine run too Slow.
To solve this problem I am thinking to activate the ignition coil ballast relay every time the fan relay is activated..

Is my idea of a solution practical?
Will I fry the ignition coil on the Long run?
Any other ideas?

Regards

Thomas

[sv8predator]

Not sure of your thinking there. The engine is running on the ballast relay all the time, anyway?

Or do you mean "bypass" the relay, rather than activate it?

Anyway, if you've measured the voltage at the coil with the fans running and it shows a substantial voltage drop when the fans are running, then that could possibly be the reason for the drop in revs. But it could also be the fans' current draw causing load on the alternator that is dropping the revs; more strain on the engine, thus the revs drop.

Find out which is causing the drop in revs, then you can devise a solution.

[Kevin H]

Hi Thomas,

I think you should identify the cause of the problem rather than creating a 'band-aid' fix, because it could mask the root cause, which may be an electrical ground problem, or a high resistance somewhere in the ignition circuit.

You have said that the input voltage at the coil is 9.5V after the fans come on, but what is it before the fans come on?

What is the voltage across the battery terminals when the fans are on?

With the fans off, and then on, what is the voltage drop between the battery positive terminal and the input to the ballast resistor from the ignition switch? (put the voltmeter positive lead on the battery positive terminal, and the negative meter lead on the ballast resistor input)

Regards,
Kevin

[David C]

My best guess is that the battery is getting a bit tired or something is amiss with the alternator / regulator. A healthy battery with a decent charge should be able to run the fans for a while without that much voltage drop even without the help of the alternator.
What is the battery voltage at idle both with and without fans and also at around 1500RPM? If the alternator isn't providing 13.5 to 14 Volts to the battery on a fast idle you might not be getting enough charge into the battery.

I just re-read the post and now not sure I fully understand. If the voltage on the coil is lower it may result in a weaker spark but that would tend to cause misfiring. If the engine is running smoothly but 200RPM lower the coil voltage may not be an issue. It's just the load on the alternator slowing the engine.
Briefly shorting across the ballast resistor will tell you if the voltage drop is your issue but your cunning plan would probably toast the coil if used for more than a few minutes.