Mechanics

NO SPARK

2001 Dodge Ram • 160,000 miles

Initially my truck died while driving. I though it had just run out of gas. Well I bled system and have gas going to injectors but have no spark at the coil. I've replaced coil and checked relays and fuses. Even replaced camshaft sensor and distributor is in good working condition as well. So I'm at a loss as to why I'm not getting spam and engine not turning over
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Biggibbs
April 30, 2013.




How are you checking for spark if the engine won't crank?

The fuel pressure at the injectors is misleading because the fuel pump will run for one second after turning on the ignition switch. That will get the pressure up in case it bled down overnight but it doesn't verify the pump is running during engine cranking.

Which engine do you have? Are we working on a failure to crank or a failure to start?

Caradiodoc
Apr 30, 2013.
It cranks all day long. I have the 5.2 liter engine. Old school spark plug and screwdriver method for checking for spark. Have a voltmeter I used to make sure power is getting to coil. We are working on failure to start

Tiny
Biggibbs
Apr 30, 2013.
First check for any diagnostic fault codes. Cycle the ignition switch three times from "off" to "run" within five seconds without cranking the engine, leave it in "run", then watch the numbers appear in the odometer display.

Next, measure the voltage at the ignition coil positive wire or any injector wire. Those are typically dark green / orange. You will see 12 volts there for one second after turning on the ignition switch. What's important is if that voltage comes back during cranking. If it does not, the most common suspect is the crankshaft position sensor. You may have better luck checking that with a test light. Most digital voltmeters don't respond fast enough to catch that one-second blip.

Caradiodoc
Apr 30, 2013.
I've replaced the crankshaft sensor. When I cycled the ignition the odometer reads pdone. And the light flutters upon cranking

Tiny
Biggibbs
Apr 30, 2013.
If the light is flashing you're on the switched side of the coil or injector but that still tells us the automatic shutdown relay is turning on. That means the camshaft position sensor and crankshaft position sensor are working, and that leaves an ignition system or fuel supply system problem. Listen by the gas tank too hear if the pump runs for one second when a helper turns on the ignition switch.

If you had the test light at the ignition coil and it was flashing, you should be getting spark. Try a squirt of starting fluid to see if you have a fuel pressure problem.

Caradiodoc
Apr 30, 2013.
The light did not flash during cranking it stayed lit the entire time engine cranked

Tiny
Biggibbs
Apr 30, 2013.
What did you mean by "and the light flutters upon cranking"? Where were you taking that measurement?

Did you listen for the hum of the fuel pump?

Caradiodoc
Apr 30, 2013.
Yes it makes a hum sound. Checked coil wire plug. The green orange wire but light never turned off or even dimmed during cranking
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Tiny
Biggibbs
Apr 30, 2013.
Just so we're clear, on the dark green / orange wire, the test light should light up for one second after turning on the ignition switch, turn off, then turn on only during cranking. It must go off within about one second when you stop cranking. If that's what's happening, the Engine Computer is turning on the automatic shutdown (ASD) relay as it should. On the '90s models that would prove the crankshaft position sensor and camshaft position sensor are working. Both have to send signals to the computer to tell it the engine is rotating before it will turn on the ASD relay. That relay is what switches on the 12 volts for the injectors, ignition coil, alternator field, oxygen sensor heaters, and fuel pump or pump relay. If either sensor failed while driving, the engine would stall.

Beginning on some 2000 models one of those two sensors could fail while driving and the ASD relay would stay on and the engine would run on a backup strategy, but once the engine was stopped it would not restart. To verify that you need a scanner to view live data. Under the "Sensors" menu you would watch the cam and crank sensors during engine cranking. They will be listed as "present" or "no". If both are present you have an ignition system problem or a fuel supply problem. If either sensor is listed as "no", both the ignition and fuel supply systems will be dead.

Until we learn differently, we can assume both sensors are working since the ASD relay is turning on. With no spark that leaves the ignition coil, wires to it, and the Engine Computer. You already replaced the coil, and you measured 12 volts to it. Put your test light on the other small coil wire and see what happens during engine cranking. If it's on steady the computer is not switching it or there's a break in that wire.

No mechanic likes to throw a computer at a problem because it introduces too many new variables, and it rarely solves a problem like this, and before you do, you can use the scanner in "Actuator Test Mode", (ATM) to command the Engine Computer to fire the ignition coil. That will instantly tell you whether the circuit is working or if there's a problem in the computer.

Was that a new or used crank sensor you installed? Did it have a thick paper spacer stuck on the end, or a thin plastic rib molded in?

Caradiodoc
Apr 30, 2013.

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