Turns over but won't crank. No fire to spark plugs
Oct 28, 2026 at 3:21 PM
No fire to spark plugs?
KEITH CRADDOCK
- MEMBER
- 1 POST
"Turn over" and "crank" are the same thing. Since you're checking for spark, I assume the starter is working.
The first thing we have to do is determine if the automatic shutdown, (ASD) relay is turning on. If it is not, you won't have spark, fuel injector pulses, and on most models, the fuel pump won't run. This is the most common failure, with all those circuits being dead. It's much less common to have a loss of only spark, only injector pulses, or only the fuel pump, but if it turns out to be one of those, we'll diagnose each system as necessary.
Testing ASD relay operation is rather easy, but you may need a helper to run the ignition switch. An old-fashioned test light with an incandescent bulb is the tool of choice. You can use a digital voltmeter, but most of them don't respond fast enough for the first part of the test. Go to the wire that is the same color at every injector connector, ignition coil, or you can use either of the two smaller bolted-on terminals on the back of the alternator. Back-probing an injector connector is usually the easiest. I didn't look up the diagram yet, but I think the wire you want will be dark green / orange. Whichever one is the same at every injector. Prop the test light so you can see it from inside the truck, otherwise, have a helper turn the ignition switch for you. You should see the test light turn on full brightness for just one second when the ignition switch is turned to "run", then it will turn back off. That proves that circuit is working and the Engine Computer has control of it.
Next, what's important is that voltage should come back during engine rotation, (cranking or running). If it does, we have a spark, injector, or fuel pump problem, but not all three. It's more likely the light will not turn back on during cranking. That would be because the computer needs signal pulses from the crankshaft position sensor and the camshaft position sensor to know when the engine is rotating. With a loss of either signal, the ASD relay doesn't get turned on.
You didn't provide any history leading up to this. If the engine stalled while you were driving, there's a good chance there's a diagnostic fault code to indicate the circuit that needs further diagnosis. The problem with these two sensors is the computer needs some time to detect a missing signal, as in when a stalled engine is coasting to a stop. Codes often don't set just from cranking the engine, so if there are no fault codes, we can't take that to mean the sensors are working.
Chrysler made reading fault codes yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then after roughly five seconds, the code numbers will flash as two-digit codes through the Check Engine light. Most of the time it starts with one flash, a short pause, then two flashes, then a longer pause before the next code flashes. Code 12 basically means the ignition switch was turned off earlier. That one can be ignored. It's the next two-digit code(s) we're interested in. Write those down. The last code will be "55". Two groups of five flashes. That one just means it's done flashing codes. On most models, if you think you may have miscounted, just turn the ignition switch to "off" then back to "run" one time, and the sequence will start over. When you turn the ignition switch all the way to "crank", even for an instant, is what takes it out of that test mode.
Once you know the code number(s), you can go here:
https://2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks/
to see the definitions, or I can interpret them for you. The procedure is on that page too.
It's important too to understand that these codes never say to replace a part or that one is bad. If a part is referenced in a code, it is actually the cause of that code only about half of the time. We normally test the wiring and other parts of the system before spending our customers' money on a part. In the case of the cam and crank sensors, a code related to them is more likely to be caused by the sensor itself, but if you get codes for both of them at the same time, it's more like there's a problem with something they have in common, which is their wiring and connector terminals.
I'll be back tomorrow to see how far you got.
The first thing we have to do is determine if the automatic shutdown, (ASD) relay is turning on. If it is not, you won't have spark, fuel injector pulses, and on most models, the fuel pump won't run. This is the most common failure, with all those circuits being dead. It's much less common to have a loss of only spark, only injector pulses, or only the fuel pump, but if it turns out to be one of those, we'll diagnose each system as necessary.
Testing ASD relay operation is rather easy, but you may need a helper to run the ignition switch. An old-fashioned test light with an incandescent bulb is the tool of choice. You can use a digital voltmeter, but most of them don't respond fast enough for the first part of the test. Go to the wire that is the same color at every injector connector, ignition coil, or you can use either of the two smaller bolted-on terminals on the back of the alternator. Back-probing an injector connector is usually the easiest. I didn't look up the diagram yet, but I think the wire you want will be dark green / orange. Whichever one is the same at every injector. Prop the test light so you can see it from inside the truck, otherwise, have a helper turn the ignition switch for you. You should see the test light turn on full brightness for just one second when the ignition switch is turned to "run", then it will turn back off. That proves that circuit is working and the Engine Computer has control of it.
Next, what's important is that voltage should come back during engine rotation, (cranking or running). If it does, we have a spark, injector, or fuel pump problem, but not all three. It's more likely the light will not turn back on during cranking. That would be because the computer needs signal pulses from the crankshaft position sensor and the camshaft position sensor to know when the engine is rotating. With a loss of either signal, the ASD relay doesn't get turned on.
You didn't provide any history leading up to this. If the engine stalled while you were driving, there's a good chance there's a diagnostic fault code to indicate the circuit that needs further diagnosis. The problem with these two sensors is the computer needs some time to detect a missing signal, as in when a stalled engine is coasting to a stop. Codes often don't set just from cranking the engine, so if there are no fault codes, we can't take that to mean the sensors are working.
Chrysler made reading fault codes yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then after roughly five seconds, the code numbers will flash as two-digit codes through the Check Engine light. Most of the time it starts with one flash, a short pause, then two flashes, then a longer pause before the next code flashes. Code 12 basically means the ignition switch was turned off earlier. That one can be ignored. It's the next two-digit code(s) we're interested in. Write those down. The last code will be "55". Two groups of five flashes. That one just means it's done flashing codes. On most models, if you think you may have miscounted, just turn the ignition switch to "off" then back to "run" one time, and the sequence will start over. When you turn the ignition switch all the way to "crank", even for an instant, is what takes it out of that test mode.
Once you know the code number(s), you can go here:
https://2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks/
to see the definitions, or I can interpret them for you. The procedure is on that page too.
It's important too to understand that these codes never say to replace a part or that one is bad. If a part is referenced in a code, it is actually the cause of that code only about half of the time. We normally test the wiring and other parts of the system before spending our customers' money on a part. In the case of the cam and crank sensors, a code related to them is more likely to be caused by the sensor itself, but if you get codes for both of them at the same time, it's more like there's a problem with something they have in common, which is their wiring and connector terminals.
I'll be back tomorrow to see how far you got.
Oct 28, 2026 at 6:45 PM
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