Battery/ECU voltage query

Matt82

Matt82

Matt82

Soldato
Joined
13 Mar 2004
Posts
16,649
Hi there

Anyone here have much experience with battery voltage vs reported ecu voltage?

Our golf is acting up. when sat in traffic for a while it will pick up a horrible hesitation when you get on the throttle, it feel slike it will stall

ive been monitoring loads of ecu things, nothing obvious is going on (intake temp seems to vary a bit wildly though) but i have noticed ECU voltage dropping to 12.9-13.0 volts (with headlights, wipers, ac on)

Anyone had experience before of ecu volts causing running issues (obviously 0 volts will cause issues, so i thought id get that funny reply out the way for you)
 
I'd expect the voltage to drop with everything switched on, I'm not sure that 12.9v is enough to cause concern. Try monitoring the voltage with a multimeter via the cig/aux socket - and see if that matches what the ECU is getting.

How wildly is the IAT sensor varying - that could be your issue (sudden swings causing fuel correction mistakes). Of course, the two things could be connected and the low supply voltage could be messing up the reference voltage for the IAT - but as I say, I don't think 12.9v is enough to do this. Might be worth cleaning up the connections on the IAT and any earthing points that the ECU uses.
 
12.9 volts is still way enough voltage for everything to work just fine, under 11v and I might start to worry. The only cars that I've come across as having very sensitive ecus are Alfas.

The only thing for me to suggest is try a known good battery on it, unless of course you've already tested the one fitted, and check all the connections like battery, alternator, ecu feed/earth, ignition etc etc.

First things first: What's the voltage using a multimeter, with all the electric gubbins switched on, on at idle/1500 RPM?
 
the iat. in traffic it will go as high as 45* (ambient was 9*) and it had only been running 10 minutes so hadnt been stationary for long.

once you moved, if you went WOT for a couple seconds it would loose almost 10* and go back up 10* almost instantly. once moving it would drop on WOT and then go back up to the original level, but all within seconds

ill get the DMM out when i get a chance next to see what the battery is doing. im sure the voltage would never drop much from the high 13s

i couldnt get the car to repeat the hesitation whilst it was being monitored though

typical ANNOYING intermitent problem. basically i think ill have to wait until something breaks lol
 
Doesn't sound like the IAT is the issue, the inlet will get heat soaked in traffic and giving it WOT will blast cool air through.

You need to get it to do it whilst connected, could be any number of things. CTS / MAP spring to mind, but it's daft to go randomly changing bits.
 
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