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Re: Clarifications and Additional Info... Posted by Ari [Email] ![]() ![]() In Reply to: Clarifications and Additional Info..., C. Lazzara, Tue, 8 Oct 2002 11:14:09 Members do not see ads below this line. - Help Keep This Site Online - Signup |
An electrical schematic will help a lot here.
I'm sticking by a low bus voltage problem. Yes, current comes from the battery. But there are voltage 'busses'. One bus is powered all the time. Another is on in the Accessory position and On. Another bus is powered only when On. Headlights, panel lights, and stuff like the ECU and fuel pump are powered only in the ON and Start positions. If the voltage on the bus is low for some reason, you won't get lights, and you won't get fuel and spark. That's a recipe for no start.
Let's say that there is a bad connection in the wiring from the battery to the power bus in question. Any point along this string could be a problem. Since the starter cranks, the problem isn't in the battery cable. But it could be in the ignition switch or relay contacts along the way.
A poor connection is a resistance. The more current pulled through that resistance, the bigger the voltage drop. That's less voltage available for the ECU and fuel pump. The headlights take a decent amount of current (about 10 amps total). If the headlights are on, that will magnify the problem of a bad connection.
I would get a voltmeter, and open up the fuse box. Set the meter to Volts, connect the low side to chassis ground, and stick the high side (positive) lead into one of the sides (doesn't matter which) of the fuel pump fuse. Crank the engine and see what the voltage is. Try it again with the headlights off. What you're looking for is a much lower voltage on the fuel pump line, ECU line, or the like correlated with no-start. Compare that with sticking the voltage measured with the meter right on the battery and cranking the engine.
If you are seeing much lower voltages at the ECU and/or fuel pump (2-3 volts lower), then you're going to have to trace back to find out where the voltage drop is.
Since electronics can be funny, don't concentrate just on the + side. If the ground is bad, it will lift with current. Make sure the ground point for the ECU is good chassis ground. Again, I don't have the schematics in front of me, but if the headlights and ECU share a ground, it could be as simple as a bad ground point. Turning the headlights on puts even more current through a poor ground.
Good luck!
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