Dear all, 

 

I have been investigating some issues lately, that are propably raised by multiple causes, and they may or may not be related to each other. Since this is very specific, i think this deserves a separate thread. 

 

I have searched on Czech SKODAHOME forum to find some answers (I am from Czechia myself), and despite it is a very frequented forum, I was not able to get much feedback. 

 

I have a 2008 Fabia Combi mk2 1.4 16V (BXW) since it was brand new from the factory. I have driven 335000 kilometers so far with standard maintenance only and without any issues. 

 

About some time ago, an ABS light started to sporadically come on during the drive, while disappearing after next engine start. Originally, I wasn't paying much attention to it, because it came on very rarely. During the winter, the ABS light didn't come on at all for several months. This summer though, when the temperatures have risen, the occurence of this fault have rapidly increased. 

 

The VCDS had shown Sporadic fault - implausibe signal coming from rear left sensor. 

 

First of all, my mechanic had replaced both ABS sensors along with the rear wheel bearings. That, regretfully, didn't help. 

 

As an engineer, I've started investigating the issue. I have noticed, that the fault appears only, when the A/C is on and the compressor is under heavier load. 

 

From that, I thought this might be an electrical issue. I've started continuous measuring of the voltage at the alternator, battery and 12V outlet in the cockpit. 

 

I have observed, that, with the engine idling, the voltage is normally above 14 Volts, but by adding more current draw (turning on A/C, lights etc.), the voltage drops down to 12 Volts.

 

My idea was, that the voltage drop might be the reason, why the ABS sensor sometimes cannot provide correct measurements. 

 

We have found out that the engine isn't perfectly grounded to the frame, since there was about 700mV "leak" between mass of the engine and the frame or negative pole of the battery. We have completely re-grounded the car and decreased the difference to only 100mV, which is pretty normal. The overall voltage has slightly increased, but the voltage drops remain. 

 

The alternator was still original and ruprisingly in a pretty good shape, but i decided to replace it (just to be sure) with a new one of the exact same type. It came also with new voltage regulator. That again increased the voltage slightly, but did not solve the voltage drop completely. 

 

After further investigation, i found out that is not the A/C, but the radiator fan, that drops the voltage. No matter what else is drawing current, the voltage drops from 14.3 to 12.0 volts in the moment, when radiator fan starts to spin. Then, when the fan reaches target (full) speed, the voltage gradually increases to about 13.6V, while the fan is still running. When the fan turns off, the voltage is back at 14.3V.

 

The fan itself can be easily turned by hand, but anyway, If it was drawing too much current, it would have blown some fuse, i believe. 

 

All fuses - both standard and metal stripes related to ABS and cooling system haven't been blown or damaged, but I have replaced them anyway. Both those by the battery and those under the steering wheel. 

 

The battery is about 1.5 years old. When the car is off and cold, it has between 12.6 to 12.8 volts. 

 

My questions are the following: (before I start to replace any other expensive parts) 

 

- Is it normal for the radiator fan to draw so much current, that it drops the voltage by 2 whole volts? It is on a 30A fuse, so even if the draw was as high as 30A, should it decrease the voltage this much? 

 

- The radiator fan seems to be running only on high speed. Is it normal behavior for the situations, when the A/C is on and the car is not moving? 

 

- Is there a possible link between the voltage drop and ABS fault, or are those two completely independent faults? 

 

Thank you in advance for any thoughts and ideas. And please pardon my non-native english. 

 

Regards! 

I think the low speed resistor within your radiator fan has burnt out.

Test by unplugging the fan from car and measuring between the connector pins with red wires. You should be able to measure a small resistance between these pins, I think you will see no connection. 

 

Replace fan, or repair with an external resistor, but at your odometer reading,  it may be best to replace. 

 

I think there may be more wrong than just this, but it should help both temperature and voltage stability if you fix this.

Thanks for your reply. 

 

I have already ordered a new radiator fan control unit, it is not very pricey and i was suspecting some problems there, since it seemed strange to me for the fan to turn on and off every 20 seconds at full speed, instead of running longer at low speed. 

 

Hopefully I can manage to remove and replace it from the top of the engine bay, since my options to lift the car are momentarily very limited to me. There are only two screws holding it from the top, hopefully I will be able to reach the plugs, that are attached at the bottom of the unit. 

 

If the low speed is still not working, i will try to replace the fan itself. Is it possible to replace it without having to remove the bumper? 

Edited 24 July, 20241 yr by Emppu
Addition

I would be quite doubtful that it is worth replacing the fan control unit. The resistor in the fan is a much more common failure.

I think the fan can be removed from above as long as you can unplug the connection, and reach the three Torx screws that hold the plastic 'tripod' that it mounts onto.

My idea was, that one of the relays in the control unit may be broken, and also that it may be source of some unwanted resistance causing higher current draw.

 

But if the fan resistor is a common issue on these cars, i will try to replace it as well. I am a bit unsure that i will be able to reach the bottom torx, but we will see 🙂

 

Also, I will try to borrow a stronger battery, to test if it would compensate the voltage drops better. Mine is "only" 44Ah, while some people say their cars with the same engine came out of the factory with 50 or 60Ah ones. Also, some batteries do not handle long-term short trips very well, so my battery could be dying, despite it is not that old. 

 

 

1 hour ago, Emppu said:

 unwanted resistance causing higher current draw.

 

 

Resistance causes lower current draw.

Sorry, that happens when you think about one thing while writing about another. 

 

I meant short circuit, but my mind was still somewhere else, wondering about resistor failures. 

 

 

A little update:

ABS issues have disappeared after recharging battery. Propably one of the battery blocks is damaged, because the battery keeps voltage, but provides insufficient current. The starting gets worse after few days from the last recharge. I ordered a new battery and keep the old one charged until the new one arrives. 

Another issue has raised with the radiator fan though. This morning, I parked the car, stopped the engine and locked it. When I was leaving the car, the fan was not running. I came back to the car a few hours later; it wouldn't unlock remotely. Turns out the radiator fan turned itself on when i was gone, draining the battery down to 5 volts. 

It looks like there is indeed something wrong with the fan control unit (a sticked relay), or it may be a faulty temperature sensor? It seems a bit strange, since the fan must have been running for hours...