Alternator/electrical system question

[athens7]

While chasing my little green turn signal indicator light problem, I find that when the car is at hot idle, turning on the headlights will shut down the radiator fan. As soon as I cut the headlights off (parking lights still on), the fan turns back on. This was replicable 100% of the time. Is it part of the car's design to do this, or is this a sign of a problem? It would seem to me that the car should be able to run both lights and fan simultaneously while running, especially at a hot idle when the car needs the fan the most.

 

I am wondering if my alternator is having a problem. I have heard that as Banner batteries die, they can take the alternator with them, perhaps as the result of constantly trying to charge the battery. What test might I perform to check the alternator, and what other issues might cause the car to not be able to power all of it's electrical systems while the engine is running?

 

Brad

 

2005 SV SVT(st170) Zetec

[allen]

Seems odd that the car can 'decide' to turn something off because you put the lights on. Unless this is a function of the ecu, which I've not come across before.

 

I'd be inclined to guess at an earthing fault?

 

Allen

[tbird]

How to test Aternator here

basically if you measure voltage across battery terminals at fast idle it should read around 14.5 volts.

its certainly strange that fan would switch in and out with headlights, if you get 14.5 volts at battery at fast hot idle, try switching on headlights and see what Voltage does,

wouldnt expect it to drop much but cannot think why fan would cut out, may slow down if you had an Alternator problem and weak battery, unless, as suggested, there was something in ecu prioritising circuits

 

CSR 200 Aztec and Black

[Bricol]

I'd second guess an earthing problem.

 

Bri

[Bob_Rich]

Hi

 

Not sure what car engine option U have but 4 what follows I am assuming it has an ECU of some form. Switching on the headlights at idle may make the voltage change just a little. This may make the reference voltage that decided whether to enable (or dissable) the fan change just a tad and so the fan drops out. If you have a DVM ( volt meter) see how much the battery voltage changes when you switch on the headlamps at idle. The alternator will have minimum output at idle and when hot (close to exhaust?) it output may be a bit lower. It may pay to check the voltage to the ECU as due to wiring this may vary more than the battery voltage. I did think that a number of ECU's regulated the voltage used as the reference for the variuos "decisions" the ECU has to make. Of course if the fan is operated directly via a stat then all I have just written is of no help at all!!

 

Hope it helps anyway

 

best of luck

 

bob

[athens7]

After checking the alternator (13.8 volts with a 63 amp load) and battery, my mechanic checked the ground at the front right of the car (at the brake line into the car from the right front wheel). He found no obvious corrosion or other contaminant, but cleaned it anyway. He then checked other grounds under the dash, and removed/inspected/replaced the flasher relay at the fuse block. There was no evidence of any problem with connections or condition of any items. When I took the car in, it was misbehaving in an easily replicable fashion. After the mechanic was done, he was unable to duplicate the problem, and I have now driven the car around 50 miles with no recurrence of either problem (glowing light or headlight/fan conflict). I guess it's fixed... 🤔

 

Brad

 

2005 SV SVT(st170) Zetec