SortaFlush

Yeah, try a new main relay for sure. It's a simple fix, it'll cause the symptoms you had. The solder they used back in the day was cheap and it just comes apart easily.

ChokolAwt

okay... well, i walked the 1/2 mile to my car... rechecked fuses, grounds and tried a new main relay. the led on the ecu is still solid, and im not hearing the fuel pump prime.


could a faulty fuel pump cause the solid led on the ecu? that's whats puzzling me... i wanna get a hold of a pm6 or a pm7 and try another ecu...


EDIT:
im running through the events of last night up to the point where my car died...
-car started misfiring throughout the driving range\
-my o-meter was displaying afr all the way into the 18.5's... when the car was shuddering... (it's not just a light show... i actually have an lc-1)
-erratic idle from 500rpm up to 2000rpm


could a bad fuel pump cause this?

SortaFlush

You sure you replaced the main relay?

Have you swapped the car recently? Are all the grounds tight?

ChokolAwt

i swapped the engine from the a6 to a dohc zc over a year ago...

and yea, i definitely replaced the main relay haha... i've already resoldered one of them before... but i bought a spare just in case. i just swapped in the spare one...


could a faulty fuel pump cause this?

ChokolAwt

ok, i just checked the main relay... and that checks out...

but for some reason, im either getting 0v or 6.3v to the fuel pump. shouldn't it be gettign somewhere near battery voltage?

83Stang5.0

I dont think volts maintain to a fuel pump, they should go up and down depending on the resistance of fuel flow.

Like, once your fuel pressure is up, it shouldn't have to use a full 12V because the system is primed and for the most part, the fuel intake itself would pull from the forward part of the system. The pump gets the system primed, and keeps the fuel flowing forward under load.

Even if you had it wired straight to a 12V source(battery), it wouldn't use it the full 12V unless it was an unprimed system already under load(first start).

Fabrik8

No. The relay design is poor and the solder joints are prone to mechanical fatigue over time from the weight of the relay and the force on the terminals. This doesn't have anything to do with the solder used. I know that's splitting hairs, but it's the truth. It's a common problem with a lot of older electromechanical devices that don't have proper strain relief internally. Solder is inherently soft and isn't supposed to take stress, so if it does it will eventually get cracked and will then oxidize and lose electrical contact. No matter what type of solder is used, the relay will eventually fail the same way unless the design has been changed. I'm not sure if they have changed the design since those cars were new, but if they haven't any new relay you buy will fail the same way after however many years.

Those relays can be fixed by having someone reflow the solder joints internally. It's kind of a pain in the ass but it can be done by someone who knows what they're doing.

SortaFlush

Learn something new everyday I guess i've been told different.

Fabrik8

Yes. You should be getting battery voltage with the car off. This would obviously mean that the ignition is on but the fuel pump is unplugged. You may see something different while the pump is working, but it definitely shouldn't be 6.3 volts. So is this 6.3V while the pump is running or not?

The reason for the change is that the wires have a resistance, and when you pass a current through a resistance, it causes a voltage drop. This is Ohm's Law, which can be basically stated as V=I*R, which is voltage (in this case voltage drop) = current x resistance. So in order for you to be getting 6.3V at the fuel pump while it is running, you either have to have a lot of resistance somewhere (maybe a corroded terminal or bad ground, etc., or the fuel pump is drawing a LOT of current. Basically, if you have 12V at the battery, and 6.3V at the pump, you have a voltage drop (somewhere) of 5.7 volts..

Given the age of the car, I would go for the high resistance theory and assume that there is a problem with the wiring or terminals somewhere, or in the fuel pump relay.

You have no idea how electricity works. You don't really use voltage for a motor, you use current. The power that a given motor can output is based on the voltage it gets, as is the current it will draw. A difference in voltage from battery to pump is just a byproduct of that. I won't go any deeper into that relationship because it's complex and it won't add anything to this discussion.

The pump will need a certain amount of current depending on the pressure and flow that it is pumping. Higher pressures make a higher load for the pump, so it draws more current. Higher current will mean higher voltage drop at the pump, because of wire and terminal resistance (see my above explanation). This is why larger wires are better for higher current, because their resistance is lower because of their larger cross sectional area. Less resistance means better current flow, so less voltage drop.

So to get the best power out of a motor, you want the most voltage that you have available, and the least resistance to current flow.

Fuel pumps are directly connected to the battery, and a fuse and a relay only add a very small amount of resistance into the circuit (just like the wire does).

83Stang5.0

Maybe I'm just having trouble explaining it in a way he can understand.

Breaking stuff down for people isn't easy for me.

I have been in school for computer electronics engineering, I know all that you said, I wasn't going into the detail you did though.


Assume I do know, then re-read my statement. Listen to Fabrik8 though, he deff knows his stuff