1988 Mercury Tracer 2carpros-HOME-ENGINE CRANKS BUT WON'T START

Tiny
RICHARDCARSON1
  • MEMBER
  • 1988 MERCURY TRACER
  • 62,000 MILES
I was reading engine cranks but does not start. On page two, Step 7. It instructs me to unplug an injector, and with a grounded test light probe both terminal of plug connector. It also says only one side should illuminate. It does not say what to do if both sides illuminate. I stopped my testing at this pint, because both sides illuminated. What do I do next?
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Thursday, July 25th, 2013 AT 6:31 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
Sounds like you read a mistake or there is some confusion. You will find voltage on both injector terminals as long as it's still plugged in and you're back-probing the terminals through the rubber seals around the wires. You WILL find 12 volts on just one wire if you have the connector off the injector.

Assuming it's plugged in, one wire supplies the 12 volts to the injector, then the Engine Computer supplies the ground on the other wire to turn the injector on for a precise amount of time. During that very brief time you would find 0 volts on that "control" wire since it is being grounded by the computer. When the computer wants to turn the injector off, it removes the ground path. The 12 volts you see on that wire comes from the 12 volt supply wire and reads through the injector. I know that sounds complicated but the computer is just an on / off switch. The difference is unlike the light switch in your house that removes the voltage source to turn the light off, the computer's switch is in the ground side and turns the ground path off. Regardless of where that switch is in the circuit the result is the same. Current flow stops and the injector turns off.

If it helps, another way to look at it is you need 12 volts ACROSS, (or BETWEEN) the two injector terminals to turn it on. If you switch off the 12 volts you'll find 0.0 volts on both terminals. If you switch off the ground, you'll find 12.0 volts on both terminals.

What you should find is the test light will flicker on that ground wire when you crank the engine. That indicates the computer is pulsing the injector to spray fuel into the engine.
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Thursday, July 25th, 2013 AT 8:06 PM
Tiny
RICHARDCARSON1
  • MEMBER
I have the connector disconnected from the injector. I have a test light grounded. The ignition is in the on position. When probing both sides of the disconnected injector connector, both sides illuminate the test light.
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Friday, July 26th, 2013 AT 4:13 AM
Tiny
CARADIODOC
  • EXPERT
I don't know what happened but I'm sorry to not have replied back sooner. Have you solved the problem? It occurs to me that your engine may have multiple injectors fired together, and the test light is lighting up on both terminals at one injector because it's getting voltage through the other one. If I'm right, unplug the other injectors, then you should find 12 volts on just one wire on the injector you're working on.
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Sunday, January 19th, 2014 AT 6:56 AM

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