Re: Isolating the Computer, ESC, and Distrib Module

And he is an electrical engineer!

"John Alt" wrote

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 any difference.

Ian

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 grasshopper too.

NH rules. And they just abolished the need for a permit to carry concealed... yay.

~KJ~

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 pressure switch.

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.

"John Alt" wrote

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.

Ian

running/cranking,

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 'high'

I once had a celeca with a bad Fuel Pump Relay (or some related circuit)... To start it I had to crank it for about 30 seconds - long enough to build 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 some>

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 diagram.

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.

"John Alt" wrote

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.

Ian

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.

The Chevy Monzas and Vegas were wired that way. As far as I know none of the fuel injected cars were though. Bob

Progress but not there yet... Hotted the ornge wire (via a piece of that old ornge extension cord). That gets the fuel pumping and it starts - for a moment. Gets up to about 1500 RPM and shuts down. Doesn't seem to be timing related (timing bolt is still loose - tried everything that would make it start). Best I can figure is that the computer is sutting it down for some reason... Would the oil-pressure switch be a possible reason? Should I just short the leads that plug into it? Thanks for the help so far. Elliott