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Re: Isolating the Computer, ESC, and Distrib Module

John, Thanks for the inputs. I do think I've covered these steps. The Gray wire on the schematic is "Gray or Tan/Wht" (mine is Tan/Wht). I did unplug the fuse - and had 0V on both pins. If the ORG wire were shorting to ground I would expect the ECMB fuse to be blown... no it's not that - but upstream of the fuse. Probably the ignition switch or wiring to the ignition switch. Seems more likely the switch because other functions do turn on from the switch (so there is power to it). Applying power to the orange wire???... Well, via a 2A/6A battery charger onto a 10A circuit it should be ok - even if there is a short somewhere. I concur that via jumper cables would be a bad idea - but it won't happen tonight. Elliott

John Alt wrote:

> > > fixing a typo/acronym: > > A, B, and C (Tan/Wht, Blk/Wht, and Grn/Wht) are all 0v. > > The ornge wire that (the other side w/o a label) has 2.3V. > > I do have a Haynes schematic. Thus this seems to represent the output > > of the ECMB fuse - that I have checked is good. Also, I have a charger > > on it and the battery voltge is 14V. Ignition does beep, but obviously > > no noise comming from the fuel pump. Per the schematic this same ORG > > wire gots to the PCM on 2 pins. > > The ECMB fuse has no voltage on it, but the ACCY pin in the fuse block > > does. Is the ---PCM--- supposed to be a power source or recipient for > > this > > signal that obviously should be 12+V to get the pump going? Is this > > looking like the ignition switch or a wire to it? It's obviously > > switching something, with the beep inside the truck. > > Elliott > > > You missed the grey wire. Fuel pump prime. The Orange wire is connected > at pins b1 and c16 on the computer, and are the battery inputs to it. If > you are getting 2.3 volts, you have found your problem (well, at least a > place to fix). The power comes FROM the ECM B fuse to the relay, then to > the computer. The relay has no effect on the computer receiving voltage, > as the relay is merely serving as a terminal block as far as this wire > is concerned. > > First, unplug the relay. Check the voltage on the orange wire. It should > be battery voltage. If not, fix the problem at the ECM B fuse. It's > probably corroded, or there is a bad wire there somewhere. > > Now, if that didn't get it, pull the ECM B fuse and measure voltage at > the terminals. One should be battery, one should be zero (I know, duh). > If this is the case, then your orange wire is shorting to ground > somewhere. To find where, pull the connector off the oil pressure > switch. It has an orange wire and a grey wire on it. The orange comes > from the relay, the gray goes to the fuel pump/relay. Check again for > voltage at the relay connector, with the connector unplugged and the ecm > fuse back in. If low still, I'd start looking between the fuse box and > the relay. If it goes high, your oil switch is probably the culprit, > shorting the wire to ground. Haven't seen it, but with the motor off, > the orange and grey have no continuity to each other.
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reitze
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Don't forget to bring your helmet :)

Reply to
Battleax

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reitze

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reitze

"John Alt" wrote

Are you getting this information from an actual schematic? The reason I ask, is that many people are under the illusion that the oil switch is in series with the relay and can shut off the fuel pump if it fails, or if the engine doesn't develop oil pressure. In fact, as far as I can see by looking at an older schematic (and I know for certain on newer vehicles), the oil switch is nothing more then a parallel circuit that provides an alternative power source to the fuel pump in the event that the fuel pump relay goes belly up while the engine is running.

Ian

Reply to
shiden_Kai

Reply to
reitze

No, if you lose oil pressure, you lose the pump. Pulling the oil pressure switch connector will prove that. AFAIK, all TBI units were like that. I have no idea about the newer units, but I have no doubt you are right about them. I haven't had the good fortune to have to do any real serious troubleshooting of the wiring on anything newer than 94. I'd bet people getting left on the side of the road for a stupid oil pressure switch or a corroded connector woke up someone at GM.

Computer terminal b2 is fuel pump input. It does not drive the fuel pump, nor are there any other circuits to the pump except off the relay. They grey wire powers the pump in prime mode, while the orange goes to the oil switch, then connects to the grey wire to provide power during normal operation. The grey wire is in parallel, but it is hot only when the pump is priming. I had to look all this up when I converted a couple trucks to TBI. The factory manual I have has the diagram I am referring to on page 1a-57. This one is for the 89 Astro, which is the closest I have to his truck with the same 1227747 computer. I know for sure there are many, many errors in the Haynes diagrams. I can't find the post where I talked about it, but I'd given some specifics about an S-10 blazer I was working on a few months ago.

Reply to
John Alt

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reitze

Pay attention, young grasshopper. The orange wire is the battery input for the computer. You don't have the battery regulating that line, since there is no voltage on it. Now you are going to put a poorly regulated, roughly 16V source with horrible ripple voltage onto your computer's power supply. These computers are tough, but there is no good in doing this.

If you must supply power to this wire, do so with a wire slightly smaller than the factory wire. Hopefully, if the smoke comes out, it will be through your smaller wire and not the factory harness.

Reply to
John Alt

Wouldn't it be a lot easier and safer to disconnect the battery, unplug the ECM and connect a ohm meter between the orange wire and ground to see if it's shorted?

Reply to
Neil Nelson

snip

Finally someone brings some sanity to this thread :)

Reply to
Battleax

Aww come on Neil, what would be the fun in that? Bob

Reply to
Bob

Well, after reading the ring and pinion thread, I'm not sure I can handle any more fun from Elliot.

Reply to
Neil Nelson

Ooops, sorry.... didn't mean to. ;-)

Reply to
Neil Nelson

He doesn't want to listen to reason. Besides, this is fun.

Reply to
John Alt

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reitze

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reitze

And he is an electrical engineer!

Reply to
hank

"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

Reply to
shiden_Kai

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reitze

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