Re: Isolating the Computer, ESC, and Distrib Module

Seems tempting to apply 12 V to the orange wire - but not tonight. It's raining here.

PS: Got a new and better Job >

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.

Um, don't. Well, what I really mean to say is, DON'T!

There may be a short in this wire, and applying unfused power to it would not be a good thing.

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:

Don't forget to bring your helmet :)

"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

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.

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.

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?

snip

Finally someone brings some sanity to this thread :)

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

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

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