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oversteer

Batt volts reading low !

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My thunder reads batt volts low by 0.4 volts, I noticed this when i first bench powered the ECU to set it up using a lipo jump starter pack, ecu showed mid 11 volts when pack as around 12v, but didn't think much of it, it still worked.

Now i have ecu in car I was closely monitoring volts as my alt needs upgrading and does not keep up with fans, pumps etc etc, I have a billet 270amp(150 at idle) on the way. 

I have tested voltage drop across earths and power feeds from batt to alt to fuse boxes relays to engine block and ecu plug(all upgraded over factory my05 subaru wiring which is pretty good to start), there is no more than 0.1V drop across any 12v line and 0.02V across any earth, I have my Fluke MM(which is accurate and concurs with my lithium battery display and my other MM by 0.01V) probing the 12v feed and earth on loom plug A and the ECU shows 0.4-0.5V to lower, this gets worrying when batt is reading 13.2V and the ECU says 12.8/7....

Not sure its causing any issues, but its not right, is there any way of calibrating ECU ?

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My 1st thunder was the same. More annoying than anything. 2nd is about .2 out. Just make sure voltage doesn't drop when ECU internal temp rises. 

Is there a voltage difference between bat voltage and aux voltage. 

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@oversteer to do an accurate test you need to check by measuring voltage between ecu pin A5 and A34.  You need to do this with everything connected so you need to back-probe by pushing a pin or needle in from the wire side through the seal etc until it touches the terminal inside.  Have your laptop open and connected and compare voltage on volt meter to that in pc link at same time. 

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MM is clipped to the pins of a Deutsch plug used for bench powering ecu etc,  that is spliced into the A5 and A34 wires ~40mm from loom plug A.

I have been comparing the MM to laptop or dash.

Seems a bit of issue with comparator network in ecu, anyway to fix ?

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As I mentioned earlier, the correct test would be to measure by back probing at the ecu plug as that is the only way to eliminate most of the external influences.  For instance if you have a bad crimp in your deutsch or ECU plug, or a broken wire in between etc, the test you have done would not pick this up.  The ECU is tested to have a tolerance of 3% (@13.2V) on the batt voltage measurement as part of its production inspection so it at least read correctly when it left the factory.  If you do the test I suggested above and find yours is out by more than about 0.4V then contact link to get it inspected/repaired/replaced. 

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Back probing the ecu plug gave the same reading, as where i was testing before, But it seems the voltage reading low issue is with the AIM dash ! 

Having all three read outs side by side(MM, Dash, PCLink) its the dash display that's ~0.4v low, looking at the dash config the "volts" defined in the display is "internal", I assumed like everything else on the display page it was pre-defined as ECU ! Changing the dash config to ECU volts the MM, pclink and dash now read <0.1v different. Not sure why the dash would be getting that much voltage drop on ~600mm of 18g tefzel wire powered from main ignition relay circuit(just like ECU). Care factor of chasing that volt drop is low so it should be fine left alone !

On a slightly different but related note, would there be any benefit in upgrading the factory circuit/power feed that supplies Injectors, coils and ECU since I am now running low impedance injectors and higher power coils(vr38) ?

I assume the low imp injectors would draw more current than the factor ones at rpm(is there a significant difference to peak and hold vs saturated?)

And I have seen comments on vr38 coil kits for rb's and sr's etc which come with "upgraded" coil harnesses to handle higher current(although no mention of the rest of the vehicle wiring regarding higher current requirements/upgrades !)

Testing voltages at injectors and coils etc checked out good testing at idle, can't see any issues, but I would think it would need to be done at rpm and load ..... just wondering if its worth it to add another fuse, relay and wiring that would end up far shorter and have better and less plugs/contacts than the factory stuff that was i guess not designed for this performance level !?

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The difference in the current draw between a "low power" coil and a high power one is about 5amp max vs 8amp max. The output Joules seem somewhat unrelated to max input current. There was a really good video on a bunch of the common australian coil swaps floating around youtube where they tested input current & joules of output to the spark plug. Assuming COP's and sequential ignition, the factory wiring can almost certainly supply the <10amps needed to power higher end coils. CDI's are another story but I dont think that's what you are asking here.

Running wasted spark doubles the current draw through the harness (assuming shared ground and/or positive feed) so then you *might* want to look at wiring - you probably  need at least 12GA wire to support ~20 amps but a lot of factory wiring will be this size already.

Peak & hold injectors can draw more current than saturated but again its well under 10 amps per injector (usually more like 4-5) so assuming sequential injection, its probably fine on factory wiring.

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Ok thanks for your thoughts on this cj, I think the factory wiring should be fine, subaru have done  pretty good job on 2004+ models.

Just fitted VR38 coils to a mates 92 RS Lib running a M800, did some testing around the place and things were not great, 1v+ drop to injectors ! usual culprit the main relay actually tested good(been replaced couple years ago), ran another power feed from batt and 40A relay to run ECU, coils, inj and new earths to M800 and it was solid voltages all around, car was miss firing at 2.1bar+ >7200rpm on dyno(670whp, 6466 Ej2.35 E85), should go 700+hp @ 2.3bar now which is stout...owner wants 9sec 1/4 mile and 290Kmph+ on 1000m, clutch gearbox and driveshafts are on death row !

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