MalibuRacing

iTrader: (1)

I am experienced with wiring and electronics (I'm an engineering electronic tech with over 20 years experience), but this is got me. I'm thinking it's either the ECM is bad, VATS hasn't been disabled, or some other signal is incorrect going to the ECM.

First, let me give my setup:
'04 4.8L from Tahoe
Wait4Me harness and ECM re-program

Engine runs great when I "hot-wire" 12v to my fuel pump.

Checked fuel pump wire all the way back to ECM, OK.

Checked grounds, all OK, I even connected ECM housing to battery ground to make sure.

Tested relay, it's OK. Tried another relay.

But, the problem is the fuel pump activation wire out of my ECM is not pushing enough voltage/current to power the relay. I checked with a multi-meter, this is what I found:

Getting 6.40V out of ECM fuel pump wire when engine is running (fuel pump hot-wired to battery).

Getting 10.45V out of ECM fuel pump wire when turn key on. Stays there, doesn't change until engine is running.
(Both voltage measurements were taken with the relay disconnected, measured right from ECM wire with no load attached).

This is how I wired up the relay, just for reference:


So, is there something I could be overlooking, like:
missing ground, missing signal to ecm, VATS not disabled??

Any help would be appreciated!

Thanks,
Phillip@maliburacing.com

andrew69_04

iTrader: (148)

these computers dont switch voltage, they switch grounds only. you shouldn't have any voltage comming out of the fuel pump wire from the ecm. When the key is powered on, the fuel pump wire is grounded pulling in the relay to power the pump. it should pull in for 2 seconds and then kick out until the key is in the crank position. i think you have your relay wired wrong.

MalibuRacing

iTrader: (1)

Hmmm....

That's not what others have told me. Also, that's not what Jesse at Wait4Me said... Now I'm really confused.

andrew69_04

iTrader: (148)

well maybe im wrong. I have only wired ls1's and a handful of 5.3's. The 4.8 might be a totally different breed of its own when it comes down to the ecm so i could possibly giving you false info.

aknovaman

iTrader: (7)

LT1 PCm's are different from LS1 PCM's in fuel pump control.

Go here and scroll down to the 99-02 LS1 schematics: http://chevythunder.com/

Halfway down is the actual BCM, pcm to fuel pump relay schematics.

Noid light should light for 2-5 seconds, enabling the relay to prime the system and start the engine. as long as there are crank pulses the relay will stay engaged. Engine dies/stops, no pulses, relay shuts off pump.

LT1 operates similar but it has a switch in the oil pressure sensor to keep the relay engaged.
No oil pressure, relay shuts off.
Anthony

MalibuRacing

iTrader: (1)

That was my understanding. On previous setups (LT1/TPI, etc), the signal from the ECM is a ground. On LSx ECM's, the signal is a voltage.

aknovaman

iTrader: (7)

BINGO.

Send me your email and I can send you the specific wiring diagrams for your YR/engine size/harness if you need them.

69LT1Bird

iTrader: (1)

Look at the schematic and relay, the ECM supplies the ground to close the circuit. Check it with you meter or a test light. Unplug the relay, see if the DRK GRN/WHT wire from the ECM is grounded with the key engine off for two seconds.

MeanYellowZ

iTrader: (19)

all LS....4.8, 5.3, 5.7, 6.0 etc give power out for the fuel pump relay...opposite of old stuff and opposite of the fans and other outputs. gen III does not output to ground for fuel pump relay. Sounds to me like the ECM is faulty if you are certain you are checking the right ECM pin for output.

MalibuRacing

iTrader: (1)

It was labeled by Jesse (did the harness for me). I guess it's correct. I have some schematics, I will see if I can determine if it is the correct pin. It's a pretty bad printout, so it is difficult to read. Is there a good quality ECM pinout schematic someone can point me to?

69LT1Bird

iTrader: (1)

try this schematic

Looks like you may have 85 and 86 backward.

MalibuRacing

iTrader: (1)

It's just a relay coil, it is not polarized. It will work either way.

aknovaman

iTrader: (7)

Emailed you specific schematics for your application.

Go check your email account.

Anthony

MalibuRacing

iTrader: (1)

Thanks Anthony. I've been out in the garage and used my schematics. I had to pull out the "old man" glasses to read it, LOL.

I got it fixed.

It appears that the Jesse had the labels reversed on two wires that were next to each other on the PCM. Pin 9 of the red connector is the fuel pump activation and Pin 10 is the tach output. He had the labels on the wires reversed, so of course the fuel pump was not coming on connected to the tach output wire.

Now the pump primes and operates like it is supposed to. It's amazing how you can fix things if you actually read the documentation....

Thanks guys,
Phillip

the_merv

iTrader: (88)

^^Exactly.. and I'll back up anyone that says that the Fuel Relay Wire in the LS Harness gives it power.

Here is the F-body one.. PCM RED #9, Fuel Pump Relay Control Primary, goes to the Fuel Pump Relay. You have to Ground it to make it work properly. I know this cause I checked it when I wired up my Car.

Glad you got it worked out.

aknovaman

iTrader: (7)

In the Air Force, we had a saying like: Read the F-ing Tech Data.

Glad to help.

bczee

iTrader: (12)

General rule of old gearheads.. When all else fails, then read the instructions ! ... LOL

Glad to hear you got it up and running now..

andrew69_04

iTrader: (148)

glad to hear its running. I guess i learned something today!

Neil Anderson

Can I get the emc plugs pin out for 07 Corvette zo6.
I need to know the fuel pump wire specifically
Thanks my friend

the_merv

iTrader: (88)

From all the info here, if you have a multimeter you can figure it out.