sub tank on 78 series

Submitted: Sunday, Mar 20, 2005 at 01:47

ThreadID: 21362 Views:8616 Replies:4 FollowUps:3

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sam_84h

just bought 78 series troopy and having probs with swapping to the sub tank. when the button is engaged the light on the dash is on but it seems that its the main tank that the fuel is going out from.

perhaps optimistically, i thought it would be a fuse, i checked all the fuses in in the driver's side foot well and found the ECU-B fuse was missing. i replaced it but still have no idea what it's for... nor do i know what the ECU fuse 2 below it is. Any Ideas?

Does this sub tank problem sound electical? are there more fuses that i'm overlooking? has anyone had a similar experience?

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Reply By: BenSpoon - Sunday, Mar 20, 2005 at 03:20

Sunday, Mar 20, 2005 at 03:20

Im not certain if it is the same setup, but in my 80 the main tank is the only one fuel gets drawn out from to to go the engine. When you hit the sub tank button, a pump fires up and transfers fuel from the sub to the main tank. You can hear this thru the filler cap having the car switched to on with the sub tank button illuminated. Due to the transferring, I also cant activate the sub tank unless the main is less than 70% full- it turns on for a few seconds then goes off.

May be worth having a listen to see if its the same in yours. Unlikely, but a possibility.

AnswerID: 103140

Reply By: Member - Geoff M (NSW) - Sunday, Mar 20, 2005 at 10:08

Sunday, Mar 20, 2005 at 10:08

On the 78 Series there is a changeover solenoid, can't remember exactly where it's located. I seem to remember somewhere around the main fuel tank.
Trace the fuel line backwards from the fuel filter and you'll find it.
If this solenoid is faulty or the wiring has come off it then no fuel will be drawn from the sub-tank.
Unlike the 80 Series, the 78 has two independent fuel tanks. In my opinion a much better setup than pumping from the sub-tank to the main, then onward to the engine.
If you hole the main tank in an 80, you get to pump the sub-tank out onto the ground too!!

Find that solenoid and start from there.

Geoff.

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Follow Up By: sam_84h - Sunday, Mar 20, 2005 at 15:30

Sunday, Mar 20, 2005 at 15:30

thanks Geoff you've been a tremendous help,
it seems thats where my problem is. i got someone in the car swapping pressing the swap over button and i was underneath on the driver's side of the main fuel tank, i heard/felt nothing when holding and listening to the 2 solenoids.

there are two wires leading to each solenoid, 1 red and 1 black, using a test lamp it would appear that there is positive current going through all 4 wires (2 red 2 black) wheather the button for the sub tank is on or not. is it supposed tobe like that?

it suprises me that there is a positive flow going through the black wires as well as the red, is this an indication that the solenoid is stuffed?
also, neither of the solenoids seem to be functioning, coincidents that they are both gone?

FollowupID: 360752

Follow Up By: sam_84h - Sunday, Mar 20, 2005 at 16:14

Sunday, Mar 20, 2005 at 16:14

Just went back to check it again, i got the plastic joiner clips apart and, again with the test lamp, checked the terminals (female, going to the solenoid) and only one of each pair had a positive flow.

but when i put the connectors back together all four wires had power. from my understanding it appears the + power goes to the solenoid and because the solenoid is stuffed the power returns via the - wire.
is this theory close to being correct?

FollowupID: 360757

Reply By: Member - Geoff M (NSW) - Sunday, Mar 20, 2005 at 22:31

Sunday, Mar 20, 2005 at 22:31

Hello Sam84_h,
What this tells me is the ground path for the two solenoids is open circuit.
In plain english, the black wires are no longer connected to the negative battery terminal.
How can this happen? Couple of ways. This is where a wiring diagram is handy.
Without the advantage of one I can only guess, probably something like a faulty double pole relay, that's one that switches both the positive and negative wires.
If you or I can get hold of a 78 Series wiring diagram I'd be more than happy to have a look and suggest some other tests.

Geoff.

Geoff,
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AnswerID: 103225