No pulse from injectors when cranking

TJb

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I have a bad case of "crank no start" on my 1998 jeep wrangler tj 2.5L and have managed to trace it back to my fuel injectors. When I turn the key to the ON position, they receive a short impulse as they should, but they don't receive any impulse when cranking. What could be the cause of this? Is there a communication error between the PCM and the injectors or is it a problem with the wiring? It seems weird that they manage to receive the impulse when turning the ignition on but fail to receive anything when cranking.
 
A bad crankshaft position sensor can cause that, it provides the engine's master timing signal and the PCM requires its signal to generate the pulsed ground connections that fire the injectors.

If you decide to replace it try to find a Mopar replacement, avoid store brands and cheap online replacements.
 
A bad crankshaft position sensor can cause that, it provides the engine's master timing signal and the PCM requires its signal to generate the pulsed ground connections that fire the injectors.

If you decide to replace it try to find a Mopar replacement, avoid store brands and cheap online replacements.

How do I check if the crankshaft position sensor is working? I assume it has to receive 5v from the PCM and send out a pulsating signal when cranking?
 
The crankshaft position sensor generates short fast pulses when the engine is being cranked, you might be able to see something with a meter set on its lowest DC voltage scale by probing the gray with black tracer wire exiting the CPS. But even if you see something with a volt meter that doesn't mean the pulses are correct or strong enough.
 
I checked the 3 relays (horn, fuel pump and ASD I think) that my TJ has by placing them in the horn slot and the horn worked with all three so they should work normally.

Ok, but you need to verify it is working (remaining closed) while you’re trying to start the engine, that will verify you have a valid crank signal among other things.
 
The crankshaft position sensor generates short fast pulses when the engine is being cranked, you might be able to see something with a meter set on its lowest DC voltage scale by probing the gray with black tracer wire exiting the CPS. But even if you see something with a volt meter that doesn't mean the pulses are correct or strong enough.

Ok thanks for the info, might want to look into an oscilloscope then. How do I connact an oscilloscope and what values/measurings should I be looking for when reading the signal from the CPS?

Ok, but you need to verify it is working (remaining closed) while you’re trying to start the engine, that will verify you have a valid crank signal among other things.

Is there an easy way to test this?
 
Ok thanks for the info, might want to look into an oscilloscope then. How do I connact an oscilloscope and what values/measurings should I be looking for when reading the signal from the CPS?
It'd be a LOT cheaper and faster to just replace the CPS than it would be to buy an oscilloscope and learn how to use it to test it. Oscilloscopes are expensive to buy, I have owned a couple over the years and used them since the 60's.
 
Ok thanks for the info, might want to look into an oscilloscope then. How do I connact an oscilloscope and what values/measurings should I be looking for when reading the signal from the CPS?



Is there an easy way to test this?

If you have a meter you can check the “ASD Sense” output coming from the the ASD to the PCM, or simply have someone monitor (put their finger on it) the relay while you’re cranking the engine.
 
It'd be a LOT cheaper and faster to just replace the CPS than it would be to buy an oscilloscope and learn how to use it to test it. Oscilloscopes are expensive to buy, I have owned a couple over the years and used them since the 60's.

I forgot to mention this here because I have another post about my crank no start issue but I already have replaced my crankshaft sensor as I came to the conclusion that this would have a high chance of solving my problem. But it didn't change anything...
If you have a meter you can check the “ASD Sense” output coming from the the ASD to the PCM, or simply have someone monitor (put their finger on it) the relay while you’re cranking the engine.

Thanks! Will give that a shot and I'll let you know how it went
 
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If you have a meter you can check the “ASD Sense” output coming from the the ASD to the PCM, or simply have someone monitor (put their finger on it) the relay while you’re cranking the engine.

Just tried the finger method, and when I turn the key to the "ON" position it clicks once, and 1-2sec later clicks again. When I crank the engine it appears to click aswell, and it gives a second click when I stop cranking.
 
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It'd be a LOT cheaper and faster to just replace the CPS than it would be to buy an oscilloscope and learn how to use it to test it. Oscilloscopes are expensive to buy, I have owned a couple over the years and used them since the 60's.

Jerry is 100% right that it's much easier to just replace the sensor. And that (most good) oscilloscopes are expensive, and lastly that learning how to use them isn't necessarily the easiest thing in the world.

BUT SINCE YOU ASKED..... I found a cheap ($30) oscilloscope on amazon and it works great for this sort of thing. I posted an article in our how-to section for testing the camshaft position sensor (which I am too lazy to find & link), and I have a video of the process as well...aside from the connector itself and the specific pinouts for your year, testing the cam sensor is identical to testing the crank sensor. And also obligatory comment about how the cam & crank work together to determine timing & ignition advance, and you can't tell if they're working together with a single channel scope. But knowing how to tell if they are "in phase" enough further underscores Jerry's comment above.

 
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That was crazy great I have to get that $30 oscilloscope! Great video thank you! 😍

lol it's like the 4th time I've posted it for various reasons, you must have me blocked. ;) I'm working on a series for testing all/most of our sensors. New job is getting in the way though.

There's another scope that is multi channel for about $150 on amazon. It's tempting, but I'm not done being impressed with this little guy just yet.
 
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Having established the fact that the crank sensor is working the need for a scope is moot, unless you’re set on getting one regardless, in which case a dual channel scope is a far better option.