1970s 1978 Seville: Fuel Pump Power

[Gary Christensen CLC # 21112]

I have been working to resolve a problem with the in-tank fuel pump on my 1978, which is why I've been asking all of the wiring questions.  A while back the in-tank pump quit, but I determined that it was because power had stopped getting to it.  I confirmed so by running a separate wire outside of the car from the battery to the connector at the tank and voila the pump fired up and runs.  That's the only way I've been able to operate the car.

I figured that somewhere a wiring gotcha had happened and I did in fact find a spot under the dash where a quick splice on the wire to the tank had come apart.  I fixed that (better than with a quick splice) but still don't have power to the pump or on any of the wires that were involved in the  original quick splice.  Based on the wiring diagrams, I can't figure out how the setup I discovered ever worked but that's not my real problem at this point.

Nevertheless, I found an empty connector with a wire that looks to go directly to the fuse panel.  It has no voltage when the ignition is off and 12V when the ignition is on.  Perfect!  I connected the original pump wire to that connector, turned the key, and nothing.  The pump doesn't come on even though there's 12V on the feed.  When I connect my special wire from the battery to the original under-dash wire to the pump, the pump runs just fine.

I'm stumped.  How can the pump run using a wire connected to the battery but not run when connected to a line with 12V on it?  The pump has to be grounded otherwise it wouldn't run on my makeshift wire, right?  There's something I just don't understand.

I thought maybe the problem has to do with voltage -- my wire direct to the battery puts out 12.25 volts and the feed under the dash puts out 12 volts.  Maybe my pump is right on the edge.  When I get the car running (using my makeshift wire), the under dash wire in question puts out 14V.  So I tried starting it with my makeshift wire, then swapping it to the other wire (putting out 14V) and the car ran until the bowl ran dry, proving that the pump had gone off when I disconnected it from my makeshift feed.

Can anyone explain this behavior?

Gary

[bcroe]

You have 2 pumps which are fed 12V together, if
you connect direct to one, both should run.  The
high pressure pump in the frame is the most important. 

The pump power goes thru the ECU (on the trans
hump), shutting off a second after the crank
stops turning for safety.  This circuit often
burns out, and somnetimes a jumper gets wired
in, unsafe in an equipment failure.  I am
attaching a drawing of how yours is wired, and
how an external pump relay may be added to
avoid this problem, or even cure it.  If the
circuit inside the ECU has failed, It also
needs to be repaired, which I have been doing
on 70s EFI for many years.  I also have a kit
to add the relay.  More issues, contact me at
bcroe@juno.com
Bruce Roe CLC #14630

[bcroe]

For maintaining 70s Cad EFI cars I highly recommend having the blue &
white book: CADILLAC ELECTRONIC FUEL INJECTION DIAGNOSIS
Manual 1099984 / S-1453. Its usually available on EBay, currently eBay
item number: 165136935730 and others.  Get a paper book, not a CD.
I can return your ECU with the blue and white book ($40 USD) if you
send it for repair.  Bruce Roe

[Gary Christensen CLC # 21112]

Hi Bruce,

Thanks for the response.  My car was converted to be carbureted, so I'm not dealing with EFI.  The high-pressure pump is no longer on the car.  It operates only on the low-pressure, in-tank pump which works just fine when there's power to the circuit.

Someone suggested that there's not enough amperage through the line that I'm trying to pull the power from as an alternative the the wiring that was there before, even though there's 12V on that line.  It's the connector that should be on the emergency brake for the telltale, so that could be so.

I'm getting ready to give up on debugging the wiring that used to work and just pull power from someplace else.

The silver lining of this whole thing is that I now have identified almost all of the bits and pieces under the dash and understand how most things work.

Gary

[bcroe]

Hard to tell what you have now, but the original wiring
for a big and a little pump should have no problem
with just the little pump.  There is a safety issue
originally handled by the ECU, pump power cut off
shortly after engine rotation stopped (broken fuel
line, etc).  One way that has been done is to use an
oil pressure switch that opens with loss of pressure,
this must be bypassed when starting by a diode or
other means.  Trace that out and see what you have. 

My own low pressure pump was ineffective when the
38 year old special submersible hose broke. 
good luck, Bruce Roe

[Seville Life]

Gary - mine is converted too. The fuel pump, a Carter mounted where the original EFI pump was mounted takes it's power off the rear Level Ride power wire. This wire is live only with the ignition OFF, so good.

I have known of another Seville running off the in-tank pump? What carb are you using then?

[Gary Christensen CLC # 21112]

It's a Rochester.

I have sorted everything out and I'm back to running smoothly!  My issue was staring me in the face the whole time: I missed the pin for accessories on the fuse panel.  The connector for the optional accessories on my car had pulled out.  So stupid.  My only excuse is that I couldn't really get my trifocals to focus at the contorted angle needed to see the fuse panel.  I literally didn't see the pin!

Of course once I saw it, everything made sense.  Before that I simply couldn't understand how it ever worked and I knew that I was reading the wiring diagrams properly.  Ultimately that nagging thought brought me back to the simple solution.

[Seville Life]

Great. I'm pleased you're back up and running and out there. The '76/'79 Sevilles are a glorious automobile in every respect.

When alterations are done, like our EFI being removed it can throw folk downline. Paul