Mechanics

300M. CAN A FAULTY CRANK SENSOR CAUSE THE PCM TO NOT SEND VOLTAGE TO INJECTORS AND COILS?

1999 Chrysler 300 • 13,000 miles

1999 chrysler 300m. I have a crank, but no start condition that happened after the car sat overnight. Trying to isolate the problem.
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Texdrag1!
October 6, 2012.




Scan for codes and check fuel pressure first. If you think it's the crank sensor check connections.
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Hmac300
Oct 6, 2012.
My friend replaced the fuel pump thinking that was the problem and still crank but no start. Unhooked the cam sensor and no start. I Would really like to know about testing the crank sensor. I've heard about the tach bouncing when cranking if the crank sensor is ok. True/false?
Going to get back to the car for more testing later today.
No codes were found, but will scan again.

Tiny
Texdrag1!
Oct 6, 2012.
Check to see if lcranksensor or cam sensor are getting power and then check resistance of crank andcam sensor, we don't have any specs for there, if the reisitance is high then it may be bad. You'll have to check against a new one unfortunately.

Hmac300
Oct 6, 2012.
Hi guys. You have to check for voltage to the coil(s) and injectors while cranking the engine. If you do have 12 volts, the cam and crank sensors are working. The bouncing tach is not valid. It is run by the instrument cluster which is a computer module, when it gets information from the Body Computer.

Unplugging either sensor will guarantee a no-start condition. Both signals are needed for the Engine Computer to know the engine is rotating, (cranking or running), then it turns on the automatic shutdown (ASD) relay which sends current to the coils, injectors, alternator field, and fuel pump or pump relay. That's a safety feature to prevent a fire in the event of a crash that ruptures a fuel line. With fuel spilling onto the ground there can be no pressure so the engine will stall, the signals stop, the computer turns off the ASD relay, and the fuel pump turns off. Most people jump on the dead fuel pump but you rightly noticed the spark is missing too.

Hmac300 suggested checking for codes. That is always the place to start but if you disconnected the battery any codes will be erased. Cam and crank codes often will not set during cranking. One of the sensors has to fail while driving.

The best way to tell if the cam and crank sensors are working is to use a scanner to view live data during cranking. They will be listed with "no" or "present". The crankshaft position sensor has the higher failure rate, and both of them often fail by becoming heat-sensitive. The most common complaint is a no-start after the car is parked for 20 minutes with a hot engine. The heat migrates up to the sensor, then it works again after cooling down for an hour or so.

I don't know which style of crank sensor is used in your engine because you didn't list the size, but some of them are adjustable. They use a thick paper spacer stuck to the end to set the critical air gap. If you pull it out, you need to use a new spacer from the dealer when putting it back in. Some aftermarket replacements have a thin plastic rib molded onto the end to set the gap. When reinstalling one of those you are to cut the remaining part of the rib off and use a paper spacer.

Caradiodoc
Oct 6, 2012.
If you haven't actually jumped time, then.

Apparently a lot of Chrysler 300M's at one point or another go through a problem similar to this. The symptoms include (often in this order): Acting like the rev limiter is turning on at greater than approximately 2,500 RPM

Rough idle followed by engine shutdown and inability to start

The Fuel Shutdown and/or Automatic Shutdown Relay clicking on and off multiple times per second or every couple of seconds

Intermittent spark when cranking the engine

Trouble codes for crankshaft position sensor fault, camshaft position sensor fault, or both

People will try replacing the crankshaft position sensor, the camshaft position sensor, or both, followed by the ECM (Engine Control Module), and the car may start up, then stop working again. This will make you pull your hair out.

I just went through this problem myself, and I was finally successful in correcting the fault. If you follow this procedure, you will be too.

Pull the battery and clean and tighten the terminal clamps. Clean the negative cable jump terminal on the passenger side fender under the hood. Clean the terminals connected to the positive jump terminal in front of the air cleaner assembly. Clean the positive cable terminal feeding power into the power distribution center (PDC). Test your battery and make sure it is charged. Lack of power due to a discharged or bad battery or corroded terminals will prevent adequate spark and will stop cranking. After cleaning these terminals, try starting the car. If the car still doesn't start, proceed to step 2.

Test fuel pressure at the fuel rail while having an assistant crank the engine. Upon startup, the fuel pump will pressurize for approximately 3 seconds, so this test is simply to eliminate a bad fuel pump as the cause of your non-start. You will probably read in the neighborhood of 50-60 PSI if the fuel pump is working.

Test for spark at one of the coil packs. If you have no spark, your ASD relay is probably not getting a ground.

Following the procedure outlined in the Haynes or Chilton's repair manual for your car, test your camshaft position sensor and your crankshaft position sensor. One wire feeds approximately 8 VDC to the sensors, one wire grounds the sensors, and one wire sends a square wave (approximately 5 VDC) to the ECM. This test involves “backprobing” the connectors, but you can simply use a voltmeter and push GENTLY through the wire insulation (if you push too hard, you'll break the copper wires inside and create a high resistance wire which will be more of a pain to fix) and test for power and/or ground as specified by the manual.

Test the ASD relay.

Resistance test terminals 85 and 86. You should read about 75 ohms of resistance.

Resistance test terminals 87 and 30. They should read open (infinite resistance).

Jumper terminal 86 of the relay to 12 VDC, and jumper 85 to ground. Resistance test terminals 87 and 30. You should read continuity (roughly 0 ohms of resistance).

If the relay meets these specifications, the relay is good. If not, replace the relay.

Test the Fuel Pump Relay using the same procedure as with the ASD relay (the terminal numbers are the same and the relays operate the same, even thought the Fuel Pump Relay is narrower).

At this point, you'll have established that you have a good battery, good power distribution, a good fuel pump, a good crankshaft position sensor, a good camshaft position sensor, and good ASD and Fuel Pump relays. If your car still won't start, and you're still reading bad crankshaft and camshaft position sensor codes, and you're still getting “chattering” ASD and Fuel Pump relays, it's because the relays are getting an intermittent ground through the ECM. Most likely your problem is that there is a fault in the engine wiring harness that has shorted and ruined your PCM. The harness must be repaired and then the ECM must be replaced. If you just replace the ECM without repairing the harness, odds are you'll just fry the new one.

To remove the harness, first remove the upper intake manifold. Place clean rags in the intake holes on the lower intake manifold to prevent debris from entering the manifold. Disconnect the C1 connector from the ECM (this goes to the engine, the C2 connects to the PDC). Follow the harness and disconnect it from the PDC connectors, the alternator, fuel injectors, coil packs, upstream O2 sensors, throttle body, etc. Note the portion of the engine harness that passes under the upper radiator hose connection at the lower intake manifold. On my 300M, the insulation on about half of the wires at this point had been melted due to the heat from the engine coolant passing through the hose.

Repair the harness. Separate the individual wires from each other. Wire by wire, cut out any parts that have melted or brittle insulation. Solder and heat shrink replacement wires into place.

After repairing the damaged portions of the wiring in this section of the harness, inspect the rest of the harness for cracked, brittle insulation, melted insulation, chafed insulation, etc. Repair the wires as necessary (this part took me a couple of evenings in my shop). Inspect all of the connector plugs for damage, missing lock tabs, or any other damage. You can still get most of the connectors at a dealer, so replace them as necessary (I found about half a dozen wires that were chafed at the connector plug and were probably grounding out on my harness; any of these could have fried my ECM), although if you need a C1 connector, you're going to have to go to a junkyard and splice it in (I didn't need one). Use solder and heat shrink, or you'll just end up redoing the job when your crimped splices corrode out.

After repairing all the wires in your engine harness and replacing any connectors as necessary, chafe wrap your rebuilt harness. Between Auto Zone, O'Reilly's and Harbor Freight, I got plenty of 1/4”, 3/8”, 1/2” and 3/4” plastic anti-chafe corrugated tubing (or whatever it's called). Chafe wrap every sensor lead right down to the sensor and secure the chafe wrap with zip ties. After chafe wrapping the entire harness, use plenty of electrical tape where sections of chafe wrap meet to secure the sections together.

At this point, you have a harness that's probably better than factory. Reinstall the harness, rerouting it ABOVE the upper radiator hose and pushed forward more so you don't get melted wiring again anytime soon from radiated manifold heat. Reconnect the harness to your ECM (you might get lucky and NOT have a fried ECM). Try to start the car. If you're still getting the chattering relays, your PCM is shot. I recommend going to Auto Zone for a new one. Dodge wanted $500 for the part, plus another $100 to flash program the ECM; they wanted $900 to install and program the ECM themselves. O'Reilly's wanted $130 for the ECM, $20 to ship it in from out of state (plus a three-day wait), and then I'd have to take it to Dodge to flash program it. AutoZone had the part I needed for $130, and when I went in, I brought in my VIN and my mileage, they got me the part in three days already flash programmed from the remanufacturer. It works like a charm, plug-n-play. Just install the part and you're ready to go.

Tiny
Combat Missionary
Oct 18, 2013.
Lots of good points, Combat Missionary. Thanks for helping texdrag1! There's a couple of things I'd like to add. First, I never allowed my students to poke through wire insulation to take readings. When I found holes on my prepared cars, the offender got to splice in a new piece of wire right up to the terminal. Moisture will get in there and cause corrosion. (If you do that on an older GM vehicle with aluminum wires, you will have a fully-corroded wire within a few weeks). It was also not acceptable to seal the hole with silicone gasket sealer because that would leave a "footprint" behind showing the next student where you were working. You'll never get your car back from a shop with holes in the wires. Back-probe through the rubber weatherpack seals alongside the wire. Stick pins make dandy probes. Stretched-out paper clips can work too if there's room to get them in.

Chiltons and Haynes manuals go through a lot of tests that professionals don't have time for when their customers are paying by the hour. We use some tests like you listed for the relays here because we can't see the cars and have to get the owners to provide the information we need. It's far simpler to measure the voltage at an ignition coil or injector to see if the ASD relay is working. You will see 12 volts for one second after turning on the ignition switch. Many digital voltmeters don't respond fast enough to catch that so a test light is a better choice for this test. If you have 12 volts, you're done. The relay is working and there's no need to check its coil and contacts. If it's not working, just swap it with one of the others like it. The starter relay is a good choice because you know it's working.

If you hear the fuel pump hum for one second, that relay is working too and the Engine Computer has control of it.

I don't like using electrical tape anywhere on a car. It will unravel into a gooey mess on a hot day. I've seen aftermarket radios fail because do-it-yourselfers twisted wires together and covered the splices with tape. Eventually two splices will touch, and if a speaker wire touches a ground or 12 volt feed wire, it usually destroys the output IC. If you had a bunch of melted wires, tape will fare much worse. I'm happy to read you used heat-shrink tubing on the splices. You can get it with hot-melt glue inside to really seal the joints. The Chrysler dealer's parts department has it in little pieces. You can get it from any auto parts store too.

I think the tubing you're referring to is called "convoluted tubing". I used a lot of that when building my "bugged" cars. Some came from donated cars we were scraping out, but for longer pieces I bought it by the box, and the guys at the auto parts stores always knew what I wanted when I said "convoluted tubing". Instead of taping the joints, consider using a few inches of the next larger diameter to go over the ends. Little zip ties will insure it stays in place but it usually will stay put on its own.

I doubt you're going to run into a problem by rerouting the harness but keep in mind that often the path the engineers used defy common sense for a reason. AC generators can produce voltage spikes, especially on '87 and newer GM vehicles. Those spikes cause corresponding current spikes in the large wire going back to the battery, and current flow sets up a magnetic field around that wire. A changing magnetic field will induce a voltage into any nearby wires that run parallel to it. That's how an ignition coil works. GM has a lot of trouble with engine running problems that defy diagnosis until the generator is disconnected. To reduce that, computer sensor signal wires will not run in the same harness as other wires that have large voltage pulses.

Along with this, you may have found one or two pairs of wires that were twisted together. If you replaced any like that, the new ones should also be twisted. Those are the "data buss" that allows all the many computers to talk back and forth to each other. They do that by taking turns transmitting square wave pulses that are only about 0.2 volts high. You would be almost guaranteed of getting magnetic interference high enough to confuse some computers. By twisting the wires, those interfering pulses hit each wire equally and cancel each other out.

Oxygen sensor signals are another one that is pretty touchy. They generate a voltage that varies from 0.2 to 0.8 volts and switches a couple of times per second. It wouldn't take much to induce a different voltage. You would likely never notice an engine performance issue but the Engine Computer could misinterpret those signals as running rich or lean too long, or there could be excessive emissions during a compliance test. Things like that are rare because the engineers have already looked for that. It's when we modify things that we have to keep the ramifications in mind. Magnetic interference was never an issue on older cars where there was just steady direct current flowing. It's the computers and their signals that are affected.

As for those computers, I can't argue with your experience, and I'm disappointed you had a failure, but in general, most computers are very hard to damage. If a driver circuit for an ignition coil or injector becomes grounded, the computer disables that circuit and sets a diagnostic fault code to that effect. The same is true of the 8 volt supply for the cam and crank sensors and the 5.0 volts supply to the other sensors. If those lines become shorted, the computer will shut them down. You have to turn the ignition switch off, repair the short, then turn it back on to get the voltage supplies back. If accidentally grounding a wire would damage a computer, I would have had to have dozens of them on hand for my bugged cars. If anything can be done wrong in the name of learning, a student will do it. The only computer that was ever damaged was the Engine Computer on a '97 Dakota. By grounding the wrong wire on the alternator to perform a "full-field" test, the 12 volt feed wire from the ASD relay burned a copper trace between two terminals inside that computer. That was repairable with a 1/4" jumper wire but for the average person that would have required a new computer.

GM did have a real lot of computer trouble in the mid '80s to early '90s. That's why so many mechanics still jump on a computer before eliminating all the other possibilities. Chrysler had hardly any failures through 1995. In fact, I still have the original computer in my '88 Grand Caravan daily driver. The '96s were the first to really have some failures on the redesigned Caravans, but Body Computers caused a lot more trouble than the Engine Computers.

Even accidentally connecting a new battery backward rarely takes out a computer. Some GM cars from a few years ago could have as many as 47 computers. They have a trick designed in on purpose that can render all of them useless if the radio / Body Computer fails, but typically when we read that someone connected the battery wrong, all that's needed are a few new fuses. I've never seen or read about a car with all the computers damaged, but I DO read about batteries being connected backward about once a week.

The point of all this is to not assume a computer has been damaged by a grounded or shorted wire until extensive testing has proven it. Thanks again for sharing this repair. It sounds like this is the answer for a lot of people.

Caradiodoc
Oct 19, 2013.

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