I have a 53 plate 2.0D X type which has developed a strange quirk recently. Roughly once a week, it will struggle to start but will eventually. When it starts there is a strange whirr from behind the dash and the trip counter etc. resets itself.
I have recharged and checked the battery and checked the alternator, all seems ok, so I'm guessing it's an electrical issue. Has anyone else suffered from something similar ?

 

Honestly sounds like the battery is not holding the charge enough. How old is the battery ? I would venture to swap out the battery if it is more than 4 or 5 years old anyhow... but that's just me

 

eddy, from the sounds of things, you are probably looking at a bad set of battery cables. These are known for creating intermittent issues like you are seeing. You can do some voltage measurements to see if the cables are dropping too much voltage (anything over 1.00 VDC from end to end of the wire is too much). After that, it could be the battery, but normally, they will either be good or bad. They don't waver back and forth like you are seeing.

 

Thanks for your replies. I'm not sure whether it's the battery though, I have to say, initially that was my first thought, but after doing a few drop charge tests on it and charging it to see if all the cells worked, I'm not so sure. It's the whirring sound behind the dash when it plays up is what concerns me. Is there something electronic behind the dash that would create the noise, that is also part of the ignition maybe ? As for dodgy cables, I must admit to not having tested them, but the fact that it's an intermittent issue, doesn't flag it up to me.

 

My car (2.5L gas) was doing exactly what you're describing; turned out to be a bad negative battery cable- easy fix. I have no clue what the whirring noise is; it's very disconcerting.

 

Maybe I'm being a bit naive, but why would a battery negative lead go bad and what exactly does bad mean ? Also, if the lead was duff, would it not play up all the time, like whilst driving ?

 

eddy, the battery cables are open to the environment. Simple things like salt that gets applied to the roads and stuff of that sort can be trapped between the wiring and the surrounding insulation. This will cause the copper to corrode and in some cases actually "rust" away. It can also get into the battery connector and start decreasing the connectivity there. So, in short, your battery cables will get a very slight increase in cable resistance. Something as simple as a 0.02 ohm change (smaller than is measurable by most multimeters) can potentially result in a 2 volt drop under a 100 amp draw. You get more resistance in there, you can get the same drop with less current draw or a higher voltage drop with the same current draw.

So, what happens is as things turn on and off in your car, it is changing the current flowing through the car. In most cases, the ECM is going to compensate for this. But, if it doesn't respond fast enough, the computers will start seeing a changing voltage and this causes the references that it is using to base signals off of to change, which then makes it look like the engine parameters are changing when it reality they are not, just the reference voltage is changing. This can cause all sorts of random things to happen depending on how bad the voltage changes.

Keep in mind that when you start your car, the starter can pull up to around 150 amps. You add that minute amount of resistance and you start to suddenly send only 8 volts to the starter vice say 11 volts. That makes a big difference to how fast it can roll the engine. The cable wiggles a little bit, suddenly you can get better connectivity and you don't have that large drop again. This can be from something as silly as hitting bumps and having the wiring wiggle some.

As for while you are driving, keep in mind that most of your power is going from your alternator straight to the fuse boxes of the car. The battery is only there to help smooth out the voltage to make it more of a flat line DC instead of a pulsating DC. But, when starting, 100% of the load is on the battery.

 

Thermo, thank you for your in depth explanation of the cables for me. I will spend some time checking these out over the weekend to see if I can see any bad areas anywhere near them.

 

eddy: The negative battery cable bolts to the chassis beneath the battery, between the headlight and wheel well. Try unbolting it and cleaning up all the electrical contact surfaces with some emery cloth or extra fine sand paper, including the underside of the bolt head and both sides of the washer. A thin film of oxide on these surfaces is sometimes all it takes to trick you into believing the cable is bad.