Heavy85

I'm using the '02 Z28 fuel tank in my swap and am working to wire it up including the pressure sensor. There are two wires I have a question about. One is blk/yel for the level gauge and one is org/blk for the pressure sensor. These are both grounds but are wired to separate inputs all the way to the PCM. Does anyone know if I can just ground these to the chassis near the tank and save running the extra wires? I'm thinking it may be a 'noisier' ground but it's not like they are tach signals or anything that really needs to be that precise.

Thanks
Cameron

G-Body

iTrader: (4)

If you ground them to the frame you may end up getting codes (check engine light). The fuel tank pressure sensor seems to be pretty sensitive so if the return path is a noisy ground it may end up pushing the signal out of the normal limits and popping a code. It won`t hurt anything permanently, but you may end up adding the wires later if you end up getting codes.

ghettocruiser

iTrader: (17)

?? I dont remember those colors on my 99 wiring... Didnt think it was different, but perhaps it is. Purple was the only wire for my level sending unit...although...now I remember there may have been a ground in there too... There was a ground that was in the 4th gen harness that goes under the rear seat. Then for the pressure/evap stuff, there were two grey wires that went up to the PCM.

The PCM uses switched grounds for a lot of its control. Its best to hook any wires that are "grounded" by the pcm, to the PCM. The PCM will be looking for them, the ground signal will be cleaner and will happen when its supposed to, and then you know its hooked up correctly.

Here is a thread I made on my tank hook ups. No one responded in it...but its how I hooked things up in my car.

https://ls1tech.com/forums/showthrea...672&highlight=

J.

EDIT: Maybe the wires you are talking about are coming right off the tank? I used the little body connector and factory wiring from the 99, so the wires could have switched colors at the fuel tank bulk head connector behind the rear seat. In any case...ide run them to the PCM and hook them to their proper locations.

EDIT AGAIN: Are you trying to make the EVAP system work? If so...are you using an aftermarket fuel gauge? Because if you are, you are going to hit a hurdle.

3.1EyeCandy

What hurdle would that be? I'm in the planning stages myself.

Heavy85

Yes I am making the EVAP system work. For now I only have the stock fuel gauge ('72 Datsun 240Z) and haven't thought yet what I'm going to use.

Cameron

G-Body

iTrader: (4)

I believe the hurdle he is talking about is the LS1 fuel sending unit. The standard GM sender reads 0-90 ohms of resistance. The LS1 reads 240 - 40 ohms, backwards and a different range from the standard.

ghettocruiser

iTrader: (17)

Well...that is one of the hurdles. The only way I was able to get an accurate gauge, was to get the programmable one. Which was worth it.

However, the hurdle I was talking about was for the EVAP system to work, you need the fuel level sending unit wire hooked up to the PCM. The EVAP system uses fuel level, as well as pressure to operate the EVAP system (as far as I can tell). This presents a problem...a win/lose problem. You can hook up that purple fuel level wire to the PCM just like stock, and have the EVAP system work like it should. But...if you do that, your fuel level gauge will not work. It will just peg, no matter what gauge you have. Because the PCM is already sending voltage through that wire...5v I believe. THe stock LS1 fuel level gauge gets its data from serial data from the PCM. You can clip the purple wire and use it just for the gauge and your gauge will work, but then you will set DTCs for "fuel level", and from what I was told, the EVAP system will not work correctly. Wont set a SES light...but it will set a code. I have a scan gauge in my car so I constanty check for codes. I believe its P0461 and P0463 that will set. One is "fuel level sensor performance" and the other is "fuel level circuit high voltage".

There is very little info on how the EVAP system works. If its out there, I hvaent found it. One of my ideas was to add a resistor that is grounded to the PCM. If I can find the "fuel level" that allows the EVAP to function, I could figure out the resistance, and hook it up so the PCM always things the tank is at xx.x level. I havent looked into this much more though...and I think that for my next build in the spring, Im going to ditch the EVAP system. Not too many great write ups for deleting the EVAP either though.. so we'll see.

J.

oange ss

iTrader: (50)

you could wire the lfuel evel wire to the pcm and tap off of it for you programmable guage...

ghettocruiser

iTrader: (17)

No..you cant. Thats what my last post described. The PCM does exactly what your aftermarket gauge does. It sends voltage down the sender wire and figures out the resistance to ground. When its wired to the PCM, your gauge and PCM fight for this voltage signal and you wont get a reading on the gauge. It just kept pegging my programmable gauge, and Im almost positive it would do the same thing to a non-programmable gauge.

Lose the connection to the PCM, and the gauge works fine. But the EVAP system throws DTCs....hence the issue with running an aftermarket gauge and keeping the EVAP system.

At first it was a mystery...then after talking with the autometer people, I finally talked to someone that KNEW that in order to install an autometer fuel level gauge in an LS1 car (swap or original) you need to cut that purple wire to the PCM, and use it for the gauge only.

I dont think it really makes a big difference. I havent noticed any ill effects since I built my car. HOwever Im almost sure the EVAP system isnt working correctly, if at all. So the issue is... why go through the trouble of keeping it and routing lines and hoses and what not, if its not even going to work. Might save some guys some time.

J.

G-Body

iTrader: (4)

I have the EVAP system still hooked up on my car, but I don`t have the sending unit tied in or enabled. When I start the car the purge valve does open to purge the charcoal canister. Guess I have to log it some more and check for the other codes. Right now the fuel level codes are set so they don`t pop a MIL light.

ghettocruiser

iTrader: (17)

Hmm... My guess is that it purges during start up regardless of tank conditions??? Ill bet it needs the fuel level input to purge during driving or after start up.

When I first built the car, I didnt have fuel level going to the PCM. I noticed that there was NO fuel smell in the evap lines. Up by the intake. I mean, I know the charcoal canister filters it or whtaever...but I expected it to atleast smell alittle. After I hooked the fuel level to the PCM, and my DTCS went away, I noticed I could smell a bit of fuel in the evap lines. And I didnt get the hiss when opening my tank to fill it.

Im convinced that adding a resistor to trick the PCM into thinking the tank is half full or something is the best solution, as long as it works. I have no tuning software or any way of logging what is going on with the system...so basically everything Im saying could be me blowing smoke.... but its what I gathered from working on my car and reading up on things here and there.

If there was a way to easily delete the EVAP, and not have a ton more fuel smell, and be SAFE...ide probably do that. I have no other emissions on the car, so this could be one less thing to do this spring on the new car.

J.

Schitzo

iTrader: (6)

I am also very interested in the way the ls1 EVAP works. Despite numerous searches, I have not found conclusive information as to how the system works. I intend to keep the EVAP in my current project somewhat functional so knowing how the system works could be useful.

From what I have read (dont know if it true or not) the EVAP has two functions, ensure that the fuel vapors purged into the intake to be burnt rather than released to the atmosphere and that there are no fuel vapors leaking into the atmosphere.

The former is accomplished via purge valve while the latter is done via a pressure test hence the need for a fuel level and pressure signals.

Im not so worried about the EVAP test but would like to retain the purge valve function. I hope the PCM does not need the fuel level and pressure signal to send the ground sig to the purge valve.

oange ss

iTrader: (50)

they way i understand it, the PCM uses fuel level and tank pressure to schedule a purge event, a full tank would disable the purge event unitl the level got low enough to be considered safe...unfortunately i dont know what the maximum fuel level is for purge to be active....if you wire a resistor inline with fuel level circuit you would be purging the canister and tank with a full tank of gas, GM obviously wanted to prevent this, so that's why the level as well as pressure is monitored....

G-Body

iTrader: (4)

I think that GM wants to prevent a purge at full tank because of how close they locate the charcoal canister to the tank on newer cars (right above the tank on the f-body). Because it is so close it would be very easy to suck liquid gas into the charcoal canister and destroy it. On a retrofit situation with the canister mounted underhood I don`t see a problem with purging no matter what the tank level is.

ghettocruiser

iTrader: (17)

Alot of guys run the LS1 tank too though. One of the most common LS swaps is into a thirdgen. And more and more of them, myself included, are just using the LS1 tank. Its all ready to go...and it fits perfectly.

Anyone know of any GM contacts that would be willing to go over the workings of it? I mean, they ought to be pretty helpful, considering the LS1 is becoming a bit older, and so many people are using it for projects and swaps.

There is plenty of info on how the system physically works...but nothing on what the PCM does inside with the info that it gets from the tank. If I dont find any more info on it by spring, Im just ditching it most likely. I spent a lot of time making the 4th gen evap lines work on my thirdgen, and relocating the evap purge solenoid, and making it all look good. So Ide LIKE to keep it. But...it is what it is.

J.

nosmakesmelaugh

i just want to use my fuel level on my sending unit but i have a bg 400 and autometer gauges do i need to hook 12 volts to the sending unit and then hook the purple wire to the gauge if so what wire needs to be powered thanks