Have a look again at your wiring diagram. Like I said, the one that
I'm looking at for a 1988 S-10 shows the oil pressure switch in
"parallel" with the fuel pump relay. If you can tell me how an open
switch in "parallel" with the relay would shut the engine off, that
would be great. In "series" I could see it....
I don't see a "grey" wire, the orange wire is 12 volts to the relay, the
"tan-white" wire is the wire that goes to the fuel pump and then
branches off to become a "prime" wire. Which means you can power
up the "tan-white" wire and bypass the relay and apply 12 volts directly
to the fuel pump. Meanwhile, the orange wire to relay has a splice
that leads off to the pressure switch and the "tan-white" wire also
has a splice that leads off to the pressure switch. If you take the
pressure switch right out of the equation (open).....that will not
affect the orange power wire to the relay, nor the "tan-white" wire
from the relay to the pump.
I'm looking at Mitchell on Demand wiring diagrams....I'll have to
dig up some factory manuals, but I'd be surprised if there is
Your diagrams seem to agree with mine.
Yea my degrees are in electrical engineering. More pertinent to this
activity is that I also have a 2 year AAS-EET degree in electrical
technology. Most of engineering school is all mathematical - so there
is some truth in the engineers having very little 'technical' skills
for hands-on tasks. But I'm here working on this stuff - so even though
you may choose to find fault in my fault isolation skills, I do have
them. There are careful ways to proceed and less careful ways - and
I have worked on TVs and Tube based amplifiers, etc. The advice someone
offered about using a little wire rather than the charger to avoid
over-jucing the computer was good - and accepted. The most important
engineering skill I know is to not be affraid to stop, think, and
accept inputs from others. The most common problem solution in the
engineering community comes from the cold observer who says something
simple without really looking at the problem at hand (i.e.: "did you
measure the ground", etc.).
You know, I am sorry I made that comment, You seem to be genuinely trying to
either answer the question to the best of your ability or to make the best
educated guess. I can appreciate that. What you do to your own stuff is
your business, just be careful answering queries on this group. I am a
Well, lets all go to the same diagram. This one will do
Scroll down to diagram 6. This is a later model with a hot fuel handling
module, but it will do. The relay is shown "power off", or NO position.
The connection between terminal C and E allows the fuel pump test point
to operate. When the relay is energized, connection is made between
terminals A and E, and this, as we agree, is in parallel with the oil
Now, I think the problem we are running into is when is power supplied
to the green/white wire to operate the relay. This is the 2 to 10 sec
prime signal, sent by the computer. Correct me if I'm wrong, but after
the prime signal is done, the green/white goes low. The fuel pump
relay is only on when the truck primes. The only path to power the fuel
pump is through the oil switch, since the E to A connection is gone.
Well, in the case of this diagram, the hot fuel module will power it 20
secs longer to help with vapor lock/hot start issues.
As this article and diagram mention, the grey wire I was referring to
can also be either tan or tan/white. My truck is grey.
I think that you are incorrect about the fuel pump relay only working
for the two second prime. It only is switched on for two seconds
if it doesn't see a "running" engine. Once the engine is running/cranking,
it's back on again. I know for a fact that I've never encountered
a GM vehicle that wouldn't run because the oil pressure switch isn't
working. In some ways, it would not be a bad idea, probably save
some engines, but with the high rate of sending unit failures, it would
probably just irritate the customer un-necessarily.
Anyway, this is not my area of expertise, but I've heard this mentioned
so often on the newsgroups, ie: oil pressure switch will shut off the
fuel pump if it's not working, and I think it's incorrect, at least as far
as GM vehicles go. I'll do some more digging and see if I can
come up with a definitive answer...or perhaps some more knowledgable
"driveability" tech can chime in.
the pump is switched on by the ECM during the prime mode, and then again
when the ECM 'sees' ignition pulses
this is to insure that a lack of ignition does not cause the catalytic
converter to fill up with unburned fuel
watch the voltage with a meter, as soon as you start cranking, it goes back
I once had a celeca with a bad Fuel Pump Relay (or some related
To start it I had to crank it for about 30 seconds - long enough to
up oil pressure. Then it ran fine. Since I only paid $1 for the car,
and had 2 other trucks plus a car, I ran it that way as a winter-beater
(abusing the %^&* out of the starter/flywheel). So I sold it for $2 to
a friend who sold it for $4 who sold it for $8, and they they got greedy
- I later met someone who had paid $200 for it and they couln't figure
out how to get it running - I told him how but the battery was dead.
Anyway... the parallel circuit seems the common, diagram-reinforced,
realistic interpretation of the information. John and Ian's
of the PCM's control of it seem logical too.
Per your comment:
"the pump is switched on by the ECM during the prime mode, and then again
when the ECM 'sees' ignition pulses";
How does the ECM 'see' the ignition pulses?
Seems my system is shutting down within about 2 seconds of startup - could
be related - ideas?
The diagram looks equivlant... Arranged quite differently, but
as I look it over it seems to connect similarly. The one thing
I didn't realize that it illuminates is that the Fuel Pump Prime
goes to the OBD1 connector. Anybody know which OBD1 pin is a
hot-wire, or better a pin-out list/diagram for the OBD1???
Seems a small jumper in the OBD1 would be a reasonable way to
prime the pump - though I may still need to hot the ORG wire
for the ESC module (My number 1 speculation at this pint is
the ignition switch).
Anybody want to buy this truck? It's in MD and I'd make a good
deal. It's $200/mo for a parking space in LI, and my wife
doesn't want to drive my other truck - seems we'll be buying
something else. I think with an ignition SW and some check-out
this truck will be very deascent for what it is - though I haven't
seen it move under it's own power yet. Comes with another engine
(carb'd) and manual tranny, etc. $1K asking price.
John Alt wrote:
That diagram shows it pretty well. The connector is shown from a front
view, with the attachment ears facing up. Look at your connector, and
see which side is up, as it changed on various models. The fuel pump
test point is the red wire, terminal G. Again, refer to the connector
I could swear that I read the relay drops out after the prime, but since
Ian and Gary both say no, I'd believe them. I'll have to look it up
again. Damn, I feel stupid. But putting a system in parallel like that
is stupid in the first place. It might be something GM thought better
about and changed the computer's EPROM, rather than change thousands of
pre-built wiring harnesses.
You mentioned the celica needing oil pump pressure. The ford system is
the same, also. That one I know from working on a Taurus a friend
changed the oil in and forgot to add oil after draining. Damn, I'll
never let him live that one down.
I talked with some of the more knowledgeable guys in our
shop about the oil switch/fuel pump issue. Apparently, when
GM first came out with the electric fuel pumps, there were some
vehicles that did use the oil pressure switch as part of the
pump circuit...ie: these vehicles would quit if the oil switch
went open, or you lost oil pressure. They haven't done
this for a while now.
I'd bet the Astro Van was one of them. My first conversion used an Astro
wiring harness, motor, and transmission. Most of my research was in the
Astro manual. I only have the wiring diagrams I photo-copied from the
factory manual left. Well, I only feel half stupid now. But it's Friday,
so there's still hope to achieve total stupidity by 2 am.
Now it's more fustrating... Maybe I broke something...
Hotting the ornge wire brings the fuel pump to life. And
earlier today it would start for about 2 seconds and then
shut itself off. Now nothing. The pump is on (can hear it),
but no gas at the injectors like there was earlier - and
it is getting a spark but it doesn't even seem to be trying
to start now. Maybe just that I'm out of starter fluid. :(
And the fuel like I didn't replace (the return line) has
now sprung a leak. One of the tranny lines is still leaking,
the ignition switch seems blown (hotting the ornge), and now
something else (it seems).
This project has turned really sour. I hate to take it
somewhere arround here - that would be hundreds. And I don't
see a use to having it in LI. Maybe it's time to sell it for
It's getting frustrating because you are throwing jumpers in and doing
other half-ass things to solve the problem, rather than looking for the
You have no power at the ECM B fuse. Find out why. Note the names of
those 2 fuses: ECM B and ECM I. Those stand for Battery and Ignition.
The ECM B fuse is ALWAYS hot. The Orange wire, therefore, is always
hot. The orange wire has nothing, repeat nothing in common with the
ignition switch. Well, about as much as the dome light, but still....
That same orange wire powers the computer. The computer isn't firing the
injectors. There is a clue located somewhere in those last 2 sentences.
Stop jumpering and use the old thinking cap. If you have a tone
generator, break it out and chase down that wire break. If not, hook up
the VOM at the fuse holder and work out from there until you lose
As a footnote, the injectors are powered from (In order) ECM I fuse
(pink/black) to white wire injector 2, red wire injector 1, and then out
green and blue to the computer, where the computer grounds them in
pulses to fire the injectors. If your pink/black didn't have power,
there would be no spark, since the computer would be off. So fix the
orange wire. Oh, you might want to trace out the orange wire too, and
When you have 12 volts on all the orange wire, try and start it. Don't
bother until you do. If it still wont start after that, then post back.
So to get it going I have:
1. Hotted the ornge wire to the fuel pump
2. Shorted across the oil-pressure-switch connector
3. Shorted OBD1 pins A&B (puts it into diagnostic mode).
Surprisingly it's not setting any codes.
I'm gonna let it run for quite a while now.
Thanks for the suggestions guys!!!!!!!!!!!!!!!!!
PS: OK maybe I'll keep it now. May even decide to part
with the 96 Sonoma instead of this one.
I'm not sure it was clear... the Celeca described previously had a
problem (in the Fuel Pump Relay circuit). The oil pump switch did
over-ride the circuit - by cranking the starter long enough to get oil
pressure. It was truly a $1 car.
It's been running for a couple of hours now ---
Thank GOD! And thanks NG!
Is the computer fried??? If I pull the AB jumper
it dies instantly. I have the oil pressure switch
bypassed (and it doesn't make a difference - the
connection isn't so good, but no diff (though that
may be a test-mode thing)).
The idle is too low- rigged a hose clamp to increase
it (there's no idle screw on this one).
So to pass inspection and drive it...
1. Headlights (install 2nd)
2. Turnsignals/B-Lights (non operating - one bulb comes on)
3. Gas leak (replace return line)
4. Tranny line (either slowing or burning off on the Y-pipe)
5. Ignition Switch (hotting the ONG sucks).
6. Computer ??? (test mode only)
7. Tighten up the timing adjust bolt.
AAAAAAAAAAHHHHHHHHHH - so much more encouraging with it
finally running - the list doesn't seem so terrible now.
This was the one with the home-sanded heads (posted
3/18) - it took too long this time).
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