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Discussion Starter #1
Hello all,
Sorry for the long tale (I know - a bit long for my first posting)- I promise there are questions at the end...
I recently replaced both fuel pumps (and filters) in my '87 Spider Veloce because the main pump died after 20 years of good service. My friend installed the in-tank pump while I was away on a trip and he got the car running fine (idled for 10 min w/ no problems), but 2 months and 2500 miles (on a trailer) later after I moved across country, I went to drive the car and after a bit of troubleshooting, realized my new in-tank pump wasn't pushing fuel to the primary pump.
I get 12V of power at the positive terminal of the in-tank motor when the key is turned to position 2 (pre-crank position), which is what I should be getting to prime the main pump. I checked all the resistances: I have zero ohms of resistance between the ground terminal at the tank pump and the negative terminal of the battery. However, I have between 17 and 25 ohms of resistance between the positive and ground terminals of the in-tank pump assemly (measured at the leads on the tank access cover where the main feed line exits the tank). When I pull the assembly and measure the resistance of the pump motor itself (directly from the terminals of the motor), I get between 7 and 8 ohms of resistance. This is compared to the old pump which I just bench tested at between 6 and 20 ohms of resistance (depending on where on the motor terminals I touched the probes to).
So the bottom line is: what should the resistance of a healthy in-tank pump be (if any)? Will another pump solve the problem, and if so, what can happen to ruin a pump so quickly and how can I prevent it from happening again? I'm not very smart on thigs electrical, and I just don't understand how I can be getting 12V of juice to the terminals on the fuel tank and still nothing from the little motor. Help!!
 

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Hmm, I don't know what the pumps resistance should be. How did you decide the in-tank pump isn't pumping?

Do you realize the fuel pumps are only energized when the drive relay (located next to the FI computer behind the passenger seat) gets a valid signal from the coil that the engine is running (or at least cranking)? You should hear a brief 'bzzt' as the pumps are energized for a moment when the key is switched to 'on', then they are not again energized until the engine is cranking/started. Any faults in the ignition system (no coil switching for spark firing) means the drive relay doesn't get a valid signal and the pumps are not energized. Low battery voltage (less than 10.3V during cranking for example) is insufficient to wake up the ignition computer (thus no spark signal...).

Also, what is the condition of the short hose between the in-tank pump and the exit pipe? If that hose is faulty (cracked, loose) the in-tank pump can be operation OK but not actually supplying fuel out the exit pipe.

OH, BTW, the fastest way to ruin the in-tank pump is to run it dry - out of the tank & not immersed in gasoline. It needs the gas for cooling and probably lubrication. I've heard that even a few minutes of such operation can ruin the pump.
 

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When I replaced my in tank pump I purchased it from Advance and they offered 4 or 5 different brands if memory serves. My question is do you know what brand your friend installed? There has been quite a few post here of Blue primary pumps made in China that turn to Crap in short order. Maybe you have a cheap pump that has already give up the ghost or ..........;)
 

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I agree- let's not condemn the pump until you have reason to. The resistance you quote is not much, frankly, but then I don't know what scale you had you VOM set on, either.

Regardless, why not run a pressure test and tee in a fuel pressure gauge off the filter outlet? That way, you'll know if you have adequate line pressure to the manifold and injectors. IIRC, you should look for 38-45 line psi (you guys please correct that if I'm off). Use fuel injection rated hose, good quality clamps, a brass tee with hose barbs, and a gauge made for fuel injection test purposes.
 

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Discussion Starter #5
Thanks all for the replies!
As far as Eric's suggestion of the drive relay, I get the "bzzt" from the new main pump, but nothing from the in-tank pump. The drive relay can't be selectively sending a signal to one and not the other, right?
As far as the suggestion to run a pressure test, the simplest test I ran was to have the fuel line disengaged just before the primary pump and while I had the wife start the car, I hung out underneath to see if fuel was getting pushed out of the tank to the main pump (don't worry - I had the main pump power disconnected so I wouldn't burn it out by running it dry). That's what started this whole thing in the first place - after replacing the old rotted hose I wanted to make sure I saw fuel flowing to prime the main pump so I wouldn't ruin it... Here's the interesting part - the car actually started and ran for 15 seconds before I had her shut it off, and that was with no fuel coming out of the main feed line from the tank and a disconnected main pump! we did this one or two more times, and I never got any fuel coming out from the tank to the main pump.
As far as brand goes, I got it from International Auto Parts and it's American made (original one I pulled out of the tank was made in England).
Thanks again for everyone's suggestions. The big thing I still need to understand is what (electrically) could be preventing the 12 volts at my tank terminals (when the key is turned) to be reaching the actual pump motor 6 inches below, when all the connections seem good...
 

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You're sure you're getting 12V at the pump terminals and not reading 12V at the sending unit, right?

If it's all good up to that point, then it may very well be that the pump is dead, or, the connections inside the tank have come loose somehow.
 

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I replaced a faulty in-tank pump this spring. Upon further examination, once removed from the car, I noted that the ground wire from the pump to the housing had deteriorated and was not making a connection. I replaced the pump and hose anyway but you might want to check those connections with the unit out of the tank.
 
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