Tacho Broke On Dyno, Twitches Up And Down At Idle

[jarrod83]

Hey guys, finally got my car back on the dyno for the second time yesterday, it made bugger all power which im pretty sure was caused by the active lsd playing up since it was error coding and came back with the tacho broken

Ive tried pretty much everything i can think of.. leaving me out of ideas..

The problem is with the power fc in, the tacho twitches sporadically at idle and as you revv it up past 2000rpm it drops to 0, the blitz i colour i have is also wired into the tacho wire from the ecu and displays the exact same problem.

The rpm display on the power fc comes up fine, which im pretty sure is sourced from the vct sensor? When the standard ecu is in the car, the tacho reads fine, when my power fc is in my friends car the tacho reads fine, and when my friends power fc is in my car it has the exact same problem, so it's definitely not the ecu.

Ive checked all the grounds i can find, all the fuses, removed the blitz wiring from the ecu loom and checked the speed sensors on the gearbox visually. I measured the voltage coming from the ecu on pin 7 which was flactuating with the tachometer and dropped to 0 as the tacho did as well.

The only possible cause i can think of was yesterday i had my drivers side door open when it started to rain about a 1 hour before the dyno session and got just inside the door and some of the power steering controls wet

Cheers for any suggestions

Jarrod

[jarrod83]

Ive checked the crank angle sensor outputs to the ecu, the 2 120 degree cas wires pins 41 and 51 both display 5v at the same 120 degree interval... pin 42 and 52 are the 1 degree cas signals, pin 52 is empty and pin 42 displayed between 2-3V at much smaller at intervals.. is that meant to be 5V as well?

[paulr33]

i have this issue

apexi powerfc

active lsd

apexi rsm

series 2 GTST

when driving the RPM needle goes from where it is ie 5400 rpm and then instantly flicks to 0 rpm and then back again

it keeps doing it in a loop. i eventually found the apexi RSM has a REV0 output for a relay switch and it was set to 5700rpm

so when my car went to 5700rpm the needle went spazzo, as soon as you went under, it was OK

the fix was i set the apexi RSM REV0 output to 10000rpm and the problem went away.

it might not be the same issue, but the same symtom i found in my car

active lsd and powerfc has a known compatability issue. have you applied the a-lsd fix for the powerfc?

if not, your active lsd will always fail and not work properly (A-LSD light on dash comes on randomly), the fix is the PFC FAQ in my sig

[Crans]

Jarod i also read somewhere that the PowerFC likes a clean power source while the stock ecu will deal with crappy power alot better.

Get your alternator checked and make sure she pumping out enough power after the little misshap

[jarrod83]

Jason:

when i first turn the car on the power starts at like 13.5V n quickly climbs to 14.2-14.3V, after drivin around for a while i noticed it falls down to about 13.8-13.9V, but as you know we had some alternator problems before and i think it would definitely be worth having a look at

Paul:

Yeh i thought it may of been my blitz revv meter causing it at first, but i cut that from the loom and made little difference, its strange our symptoms are the same as most the posts i have read with a faulty tacho talk about theirs hitting red line. I haven't done the A-lsd power fc mod, because my A-LSD has never ever failed before, Ive done some big burnouts and the lights never came on. It has played up where it only spins one wheel, but that generally improves greatly after i use a compressor to bleed the system. I read one of the posts towards the end that the A-LSD also gets the RPM of the vehicle sent to it so it may explain why the A-LSD has been error'ing since the tacho has been on the fritz. ill check my tps out from the ecu and if its not getting the tps out voltage ill splice it in when im fixing the wires Ive cut from the loom.

Ill be checking the alternator and dash cluster today and will get back to you's

Cheers for the info

Jarrod

[Crans]

Haha just a though here i am telling you to sort your alternator out when my own is causing me problems.

[jarrod83]

haha Jason, you'll get around to fixing it someday

I swapped the alternator with a friends and didnt make a difference to the twitching, is there anything else in the alternator circuitry that could be playing up?

does anyone know if its possible for the cas to play up in such a way that it still runs the car but the RPM will play up with a power fc? i find it strange that it was being tapped with a screw driver and hammer then 5 mins later my tacho is broken i have a late model series 2 with the plastic cas, so its a very hard part to get hold of and test

[paulr33]

it wont be alternator or cas or anything random like that

it will be whatever is tapped onto the RPM wire

so piggybacks, other systems etc

in my case with the exact symptoms you had, was the RSM was being random

it could be the RPM wire (like in yours) is either damaged, mangled, a botched RPM hack previously etc

we both have a powerfc, active lsd, series 2 GTST and the same problem, RPM need goes spazzo between 0 and real RPM

[jarrod83]

hmm the only thing i have on my rpm wire is the blitz wire which i have removed, i only have the tacho wire from the loom loosely wired in atm, i probably should add some solder to completely eliminate it, but i doubt its the problem and as far as i can tell its the only thing wired into the tachometer line. Does your car do the weird tacho thing with the standard ecu in? and does the A-LSD error whenever the tacho is playing up?

i noticed my car also uses pin out 5 which is also suppose to be a RPM out which i assumed goes to the active lsd, i wonder if pin 5 and 7 are connected and its actually a fault on the active lsd side of the circuit, ill check it later when im re-testing all my powers, grounds and wire resistivity's

Jarrod

[paulr33]

issues only occurs with powerfc and apexi rsm

when the problem occurs the hicas light flashes on as well

the a-lsd is not related to my rpm needle going spazzo

ie if i replicate the needle going spazzo active lsd is fine and it doesn't fail

when you say pin 5 and pin 7 - what are you referring to ?

[jarrod83]

the pin outs from the ecu, pin 5 is RPM and pin 7 is the tachometer speed signal that runs to the tacho in the dash

[jarrod83]

just swapped the entire instrument dash over with a friends and made no change, i also checked all the positives to the ecc and all grounds to the ecu all were fine, ive looked for anything else feeding off the tachometer wire but couldnt find anything connected to it =/

Jarrod

[jarrod83]

alright ive tracked down the problem to be the resistance on pin 7 on the ecu causing it to twitch around, when the cars off every 15 secs or so it goes from earth to discontinuity causing the blitz tacho to flick upto 500rpm n the dash tacho to slightly flicker, when the cars on and idling this resistance cycles between 900-1400, ive checked the earths on the ecu and they are all stables, has anyone got any clue what could cause this?

Cheers

Jarrod

[jarrod83]

after completely stripping the car, i found and fixed the problem, ended up cutting the boost solonoid signal from the ecu and fixed the problem

Cheers for the suggestions

Jarrod

[Guilt-Toy]

good work