A70 Voltage loss and Fluctuation

[midnight_purple_GA70]

Ok so some of you might of read a thread or 2 of mine and know i decided to rewire the microtech in my car to fix some wiring issues etc...

long story short ive now got a couple of issues i am trying to pin point when and where they have come from or if they had always been in the car...

So when i start the car the microtech hand controller will read 14v+
after the car gets up to operating temp this tends to drop down to around 13.8v

under load the volts on the hand controller will fluctuate from 13.8 down to 13.0 13.5 13.3 13.6 13.1 etc during this time the car misses rather badly and as such does not proceed to accellerate.

now for the next random thing turning the wipers on will see voltage increase back to 14v+
as does holding the window up or down button...
the other thing that also causes volts to go back to 14v+ is turning the ignition back to the acc position or off position and allowing it to run on the turbo timer...

this has lead me to believe there is an issue somewhere with in my IG1 circuit as i am lead to believe the turbo timer runs off IG2

I am rather at a loss as to where to from here but i have been considering the idea of taking the alternator sense wire and wiring it direct to the battery as that is directly where the microtech is drawing from...
It currently follows the factory wiring back into the fuse box

cheers guys
Scott

[1JZ-Rolla]

Hey Scott, I wouldn't be too concerned about seeing 13.0 volts at full noise, its unlikely to be the cause of your misfire.

Need to get your car on the dyno to diagnose the misfire, bit hard to say on the street without appropriate monitoring equipment, or being able to see what the engine is doing at that point while the car is stationary.

Cheers

[thechuckster]

the sense wire for the factory Alt s etup will be connecting to a battery source at the fuse box. If the IGN voltage is all over the shop, then you need to go over all the connections at the batter/fuse-box/engine/chassis to fix any loose connections, dirty connectors on fuses & relays, clean and tighten earths.

re: ignition breakdown, have you cleaned up the connectors and earth(s) at the ignitor?

[midnight_purple_GA70]

microtech is the ignitor and have tested its grounding and it appears to be fine what the battery is seeing in voltage seems to be the same as what the microtech is seeing

[thechuckster]

how is the starter cranking speed? am just wondering whether the batt earth to engine block is OK? If the main charge wire to the battery and also the sense wire (it goes to the battery via a fusable link) have zero resistance back to the battery (+) terminal then maybe poor earthing might both confuse the alternator (the alt sense circuitry needs a stable earth back to the battery).

(fwiw: I have 2 earth straps in the GT4, one to the chassis and another (largest) to the gearbox right next to the starter and obviously well connected to the block)

You've probably done it, but measuring resistance from ECU earth to battery earth might be of interest.

fwiw: I have seen a couple of dodgy Alt installations where the sense pin was directly connected the larger battery charge terminal on the alternator - but problems with this setup usually manifest themselves as low battery charge conditions. And if your system is rising from 12V to 14v after engine start, the alt is generating power, albeit a bit variably.

[midnight_purple_GA70]

starter speed is good... doesnt struggle...

have added 2 large (read 64mm2) ground straps
head to chasis (battery grounding location)
intake manifold to chassis (battery grounding location)

i am starting to think as i havent recently checked while the cars been warm that the voltage loss when warm could be lost from the alternator to the battery... i think i have a 125amp cable but it isnt the thickest of cables as i figured the alt output is 100amp so wouldnt need to be any bigger if that cable can deal with 125 amp...

also havent actually checked resistance of battery earth and ecu earth however they both go to the same point on the chasis...

not 100% sure on the checking process either...