Technical > Wiring & Sensors

Ground loop problem?

(1/2) > >>

dnb:
I have an annoying little problem with laptop serial comms.  I'm sure it was never a problem before, but it is now.  (I have a slightly different, but related problem on standard ECU'd Imprezas too, so it's not all the fault of VEMS)

When I connect the laptop to the ECU all is well when the laptop is running on internal batteries.  (When the laptop is not connected at all, all is well too, but that's what anyone would expect...)

However, when I use my 12v to 20v boost converter (bought from Maplin) to power the laptop, the CLT jumps straight to 101C (but sometimes drops back a few degrees briefly) and the IAT jumps up by 10 degrees or so.  Other sensors may be affected, but I haven't looked that hard.

I'm sure this didn't happen before the car doors (where the fag lighter socket is) were retrimmed & the wiring in them repaired.  But then there's no accounting for TVR wiring!!

Any bright ideas as to how to stop this?    Short of optoisolating or making the serial port into a bluetooth port, I am out of ideas - A perfect ground plane for the whole car is just not going to happen...

Sounds like a good time to look at these:  http://www.brainboxes.com/product/bluetooth/BL-730.aspx

hilly:
It sounds like the connection in your VEMS loom between GND and GND5 is higher impeadance than optimal.

The RS232 port uses GND (as do the ADC references) and when you hook it up to the laptop it links it to the PC ground.
When you are running on batteriues it is not a problem as the laptop is islolated.
As soon as you plug in the power cord the RS232 ground is linked to the power cord ground.
As the laptop is pulling a fair bit of current to charge the batteries there will be voltage drop in the power cord and associated TVR earth wiring (being a fibre glass car).
This voltage drop added to the RS232 ground (and ADC ground) will effectively add and offset to the input signal.

If you hook the laptop power cord to the same earth point as the VEMS do the offsets lessen ?

Try measuring the resistance between GND and GND5, is should ideally be less than 0.5 ohm.

Hilly

dnb:
If I ground the laptop from the block I'm sure the problem would at least reduce if not go away.  However this isn't the most practical of solutions...

Oh the joys of a fibreglass car! ;)

rob@vems.co.uk:
Are all your grounds joined 10 to 15cm from the EC36?
I ground the RS232 shield to the join point.

dnb:
They're joined 20cm from the EC36.  Best I can do in the space... 

Navigation

[0] Message Index

[#] Next page