FAST voltage problem

[JeffB]

i'm having trouble with the amount of voltage that my ecu is seeing... just idling or driving normally the FAST voltage sensor reads about 13.2V... the power distribution block in the engine compartment reads about 13.6V... that's not that big of a deal, the problem comes when i'm under full load... the FAST voltage sensor drops down to around 11.5V... meanwhile, a voltmeter on the power distribution block (measured while on the dyno) still reads 13.4-13.5V... my limited electronics knowledge tells me that my ECU is not being fed with thick enough power and ground wires... I'm running low impedance 75lb/hr injectors, anyone know what the current draw would be at 80% duty cycle? i am using a fast track performance splice-in harness that reuses some of the factory wiring... i suspecct that i need to run new dedicated (and thicker gauge) power and ground wires directly to the ecu... any suggestions on a) what size wire and b) which pins need this supply? i know there is a BATT+, BATT- and switched 12V... are there any others that are voltage sensistive? which pin gets the main power to actually run the injectors and what not?

thanks!

 

[TurboJim]

Your FAST is supposed to be terminated at the battery. (At least that what I was instructed to do) which would rule out any factory wiring problems. A lot of cars have a lot of voltage drop thru the firewall bulkhead. I dont know how yours is wired, but maybe you should try connecting the main power/gnd to the battery if not already?

 

[JeffB]

i'm sure my setup isn't ideal... like i said, the harness i got is not the normal FAST harness... it is a splice in harness made to work with my year/model car... so basically i'd like supplement this harness with a better power/ground feed... i just wanted to know which pins and how thick since i don't have the stock FAST harness to use as a reference...

 

[TurboJim]

Probably best to contavt your dealer. If I had a wiring diagram I would be glad to try to help, but I dont, sorry

 

[JeffB]

I've been told by several people now that the switched ignition pin on the speedpro is the one that measures the voltage that is reported by the "Battery" sensor in the logging/dashboard... but i still want to know which pins will be drawing the main current to run the injectors? i know i need to run a dedicated thicker power line to the switched ignition pin, but i don't know if i need to do the same with the BATT+, BATT-, ANA RTN, DIG RTN, etc. etc...

if the sw ign pin measures the voltage and is the main power feed then that should solve all my problems... however, if its just an on/off switch and the main power comes in on the BATT+/- pins then i need to deal with them as well...

thanks,
Jeff

 

[Craig Smith]

The injectors should be powered from a separate relay. Of course, a decent sized wire (12 or 10 gauge) would be optimal. The power wires need to be big because they run multiple injectors.

For ground wires to each injector out of the main harness, some decent 16 gauge wire should do.

I would use some fairly heavy wire for the BATT- wire, but the ANA RTN< DIG RTN, etc. can be done with 16 gauge wire. Don't spend a lot of time trying to increase the wire size on the switched ignition pin. As long as the wire is connected to a point in the car where you don't have a million other things connected that draw the voltage down, it should be OK. The ECU itself only draws about 200 milliamps. The injectors and everything else are what really pull the current.

 

[JeffB]

thanks bud! you guys should receive my computer tomorrow for its firmware upgrade... i'm anxious to get it back so i can get this all worked out!

 

[JeffB]

ok, i still don't have the ECU back so i'm "bench racing" here... trying to diagnose an electrical problem with having the system powered is interesting ...

in looking at the wiring i don't really see how the switched ignition feed would be inadequate, especially at those low current levels... but the ECU does sink the full current for the injectors that it is grounding, right? so this will be 10amps or more (4 low impedance injectors at a time on a bank to bank, right?)... could it be that my ignition voltage input is high enough, but my ground plain is raising up? in other words, if i measured the voltage between the IGN pin and a chassis ground i would see 13.5V for example, but if i measured it between IGN and the ECU ground i might see that 11.XV that i'm logging... so then the question would be, which *pins* on the ECU would need to have a beefed up ground? i know you said check the BATT-, but i don't have the actual FAST harness... looking at the pinout, i see ANA RTN, DIG RTN, PGND2, PGND1, PGND3... i'm assuming some of these are connected internally? the harness i have doesn't appear to connect anything to the ANA RTN and DIG RTN, but it does have the 3 PGNDx pins hooked up...

thanks again,
Jeff

 

[JeffB]

got my ECU back today... first things first: BIG thanks to the crew at FAST for turning this thing around so quickly... the new firmware seems awesome... the car started better and ran better... excellent!!

now, back to my voltage problem

i ran two 16 gauge wires from the engine compartment into the passenger compartment... the first wire was connected to the main engine ground, the other to the main power distribution block... inside the car i spliced a wire into the IGN pin so that i could read the voltage... i used the engine ground wire as my ground reference for the voltmeter... the voltmeter reported 13.2-13.3 V while the FAST battery log showed 12.5V (under WOT)... i then disconnected the IGN wire and connected it directly to the wire running to the power distribution block... repeated the test... this time the voltmeter showed 13.5-13.6V and the FAST still logged 12.6V... note, when i'm NOT under load, the two readings only differ by about .2 V (13.5V on the meter, 13.3 in the log), however as the load goes up and the injector duty cycle goes up the two readings differ by more and more... this seems to prove my theory that the ground plane is rising on the ECU and therefore even though the IGN pin is getting 13.6V, the ECU only sees a voltage difference (between that pin and ground) of 12.6V... so the question is, which pin on the ECU is responsible for sinking all the current for these injectors? i'm gonna run a 10 gauge for that sucker! let me know if you think i'm crazy here, but all evidence seems to point to this being the issue...

thanks!

 

[JeffB]

how many times can a guy reply to his own post!?

ok, tonight i redid the grounds... i tied pins Y1, Y2, and Y3 together and connected them to a 10awg wire right at the ECU... i run this directly to the main engine ground... now when i'm WOT the FAST log shows the voltage dropping from 13.4 to 13.0... this is much improved over where i was before the wiring and firmware upgrade (it was registering 11.2V at WOT!)... anyway, i believe this fixes my problem... as another confirmation, i'm running much richer than i was before... this would seem to indicate that i'm actually able to drive the injectors open more, right? now i can get down to some real tuning without fighting the corrections...

thanks!