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.

[Julian Caples]

You really do need to diagnose the problem first

If your car has a ballast resistor in the ignition circuit, the current to the coil goes through the resistor after starting (ie when the engine is running). It is bypassed only when the starter motor is operating (ie during the few seconds it takes to start the car).

This means that the voltage on the coil (when running) will be less than the nominal 12V from the battery (by design) and the coil is specifically rated for that lower voltage - usually (most Lucas designs) have 6v drop over the resistor and a coil that is nominally rated for a 6v system.
During starting the resistor is bypassed (bridged out) and the full battery voltage is applied to the coil to aid starting (ie the coil is overdriven for a short time). This is fine as long as the car starts qucikly - the coil can stand the over voltage for a short time.

Assuming that the resistor is working as designed, and the coil is correctly specified, a couple of things come to mind - 9.5v on the coil looks fine for a ballasted system (actually a bit high, I'd expect 7-8v, but that depends on the actual resistor).
If you bypass the resistor when the fans are on, you will burn out the coil relatively quickly.

Does your car have the chrysler electronic ignition (my early 412 did, and the late 411s are very similar) - if so you should check that out, they are rather prone to failure in unusual ways (and are getting very old).

Bottom line - the coil voltage is unlikely to be the culprit. Your suggest fix will cause the car to fail to proceed.
Something else is happening here.

Hope this is helpful

Regards
Julian