1990 240 Battery Drain Caused by Short in Fuel Pump Circuit?

Hello and Help!

I have been working on my own 240's for several years and am
reasonably competent with mechanics but have little experience with
electrical problems and terminology. I have searched this board
thoroughly but have not been able to find the information I am looking
for in this particular situation. I have been reading this board for
years and don't know where else to turn.

PROBLEM: I have a 1990 240. My battery drained overnight when I
recently left the glovebox door open for ~18 hours. The next day I
got a jumpstart, drove the car to work, ran some errands in the
evening (starting the car several times with no problems), and parked
it overnight. The next morning, however, the battery was almost
completely drained and the car would not turn over. Both nights the
temperature had dipped into the low thirties.

INVESTIGATION:

1) POSSIBLE BAD BATTERY: I took the battery (age unknown) to the local
auto parts store and had it charged and load tested, which resulted
"good," although I am not sure I entirely trust that considering the
battery was almost completely drained twice.

2) POSSIBLE SHORT: I connected a voltmeter/ammeter in series with the
negative lead of the battery (now fully charged), which registered
current draw of > 150 mA. The car was turned off (naturally) and the
radio and interior lights/clock fuses pulled. I don't suspect the
alternator, which puts out 14+ volts, but I disconnected it anyway to
eliminate the stuck-diode possibility with no resulting drop in the
current draw. Leaving the alternator disconnected, I pulled the
remaining fuses in the fuse box (corrosion free) one at a time with no
resulting drop in current draw. Stumped, I looked under the hood for
the problem. When I pulled the 25-amp fuse for the fuel pump circuit,
the current draw on the battery went to zero. Eureka! or so I
thought.

3) FUEL PUMP CIRCUIT: I checked the fuse and housing for signs of
corrosion (non present) as well as the relay under the passenger-side
dash (no corrosion present). With the battery connected, I metered
between the red lead (#30 at the relay)and a ground and measured
current, which doesn't seem right to me. When I completely
disconnected the fuse circuit from the battery (disconnecting the wire
from the side of the post) and disconnected the relay, leaving the
fuse intact, then checked for continuity between the relay lead and
ground, the meter shows continuity. At this point, shouldn't I
effectively be measuring the continuity of a loose wire with something
it is not attached to? It seems to me (and I may be grossly wrong)
that there has to be a short somewhere between the fuse and the relay.

QUESTIONS:

The sub-par manuals I have, Chilton and Haynes, have limited
schematics and none depicting the fuel pump circuit, so I don't know
exactly what I am looking at.

1) Should there be any power drawn by the fuel pump circuit when the
car is shut off? I probably would have read about this in other
descriptions of battery drain problems and testing procedures.

2) Are there any interuptions/junctions in the circuit between the
battery terminal lead and the under dash relay lead besides the 25-amp
fuse, or is it a straight shot? I tried to visually follow the
harness and don't see any "breaks." I have considered running a
bypass wire directly from the blade fuse to the relay in order to see
if that makes a difference, but am concerned that I may be bypassing
some hidden junction.

3) Is there something obvious that I should be taking into
consideration, or am I on the right track?

THANKS: I have almost run out of ideas and am not yet ready to swap
the battery and alternator with the hope of correcting the problem
"magically." I have to take a trip next weekend and hope to resolve
this problem before then. Otherwise, it looks like I will be pulling
the battery cable every time I park my not-so-trusty Swedish steed.
Any advice or criticisms this group has to offer will be tremendously
appreciated. As I mentioned earlier, I only have a passing familiarity
with electrics and electrical terminology, so I will be glad to
clarify anything in my description that does not make sense.

Thanks Again,

Ezekiel Woods

 

To me your method och checking sounds right.

When you did your tests, did you have the ignition key removed?

I am not sure, but I would think the fuel pump is switched via the
ignition key. Is there really +12 V at the 25A switch with the ignition
key removed?

A good battery has a capacity of about 60 Ah. If it is fully charged it
would take 30/0.150=200 hours to drain half the capacity at 150 mA. Even
if 150 mA are mysteriously drained, it should definitely not drain a
sound battery overnight.

You measured current as "more than 150mA". Was that because your
instrument was limited to 150? Could it be several Amperes being
drained? That would give you potential problems overnight. Otherwise it
sounds as if your battery is bad. Bad batteries typically give full
voltage when charged but their storage capacity is close to zero.

18 hours of glove compartment should be no problem. If there is a 5 W
bulb there, it draws 0.5 A (rougly). In 18 hrs that amounts to 9 Ah,
which would be absolutely no problem for a healthy battery. Even a 10 W
bulb for 18 hours should give no starting problems unless you live in
Alaska and it's winter.


--
Gunnar

240 Turbo Wagon '84 200 K Miles
940 Wagon '92 150 K Miles
on Swedish roads

 

Completely draining a battery twice will not necessarily kill it. Also,
a battery can test OK under the load a starter requires, yet discharge
itself over a period of time due to an internal fault. I had a battery
that would start the car fine on a cold day, provided the car had been
used recently, but was completely dead within a week if not charged,
even when disconnected.

Ordinarily you'd like to see no more than 100 mA draw total with the car
off.

The wire goes from the fuse to the relay and fuel computer directly.
This wire may be also connected to other things in the fuel system, that
don't use current until the computer grounds them. You really need a
wire diagram book to know everywhere this wire goes. The fuel computer
has memory circuits that need constant power, like the radio. The fuel
pump relay only draws current when the fuel computer grounds it. AFTER
REMOVING THE FUSE, disconnect the fuel computer from the harness, and
see if the draw changes. Do this test with the fuel pump relay, and any
other component that is connected to this wire. The only current should
be a very small amount (under 10 mA) for the computer memory.

Instead of disconnecting the battery, just pull that 25A fuse.

 

I measured +12 V between the negative terminal and ground cable. With
the car turned off and the key removed, I should not detect any
current or voltage in that circuit. Right?

Unfortunately, my meter is of the small, cheap variety. When I do the
amperage tests the meter pegs beyond 150 mA. I am guessing that it is
at least several amperes being drained.

As for the battery, I had it tested at the local parts store, and
their equipment indicated my battery was good. Is that sort of bench
test reliable? It would probably be a good idea to replace the
battery at this point because I don't know its age and it has been
drained completely twice remaining uncharged for several days in cold
weather. But, I am reluctant to install a new battery, if I have what
appears to be a short in the system that will just as likely damage a
new battery.

Do you know whether I can directly bypass the suspect wire (buried in
the harness) with a wire from the 25A fuse directly to the fuel pump
relay, or will I also be bypassing some other component or hidden
junction by doing so? I have visually traced the harness and don't
see anything that looks like a problem, but since I don't have a
wiring diagram for the circuit, I can't be sure.

Thanks, Gunnar, for your input and the brief lesson on amperage, amp
hours, and battery drain. As I said, in my initial post, I don't know
much about electricity but am trying to approach things deductively.
While I am annoyed on the practical level with my current situation in
that I need to make a long trip with the car next week, I do
appreciate the situation on another level as an opportunity to learn
something new.