I have a 97 Dakota 3.9 V6 2WD that has a sporadic crank/no start problem. When it happens, I can only get it started by disconnecting the battery and resetting it. Also when the problem appears it will misfire on the first couple of start attempts, then it cranks without the sound of any ignition at all. I can only conclude that the cause is a bad PCM, since it's fixed (albeit temporarily) by resetting the battery. Otherwise the truck runs fine, without any misfiring--leading me to believe that the ignition coil, crankshaft sensor, cam sensor and fuel injectors should be working OK. Because the problem is too inconsistent and rare, I haven't had a good opportunity to troubleshoot it and I'm reluctant to change an expensive part like the PCM if I'm not sure it's bad. I appreciate any input. Thanks!

 

Start with the basics. How's the fuel pressure? Low fuel pressure could do just what you describe. Do you have a good spark?

You might want to invest in a noid light too, just to see if the injectors are being triggered.

 

Do you really think that an intermittent fuel pump or injectors would be fixed by resetting the battery? If those parts were bad, I would expect the problem to show up when the truck is running, not just while cranking. Besides, I can hear the fuel pump running before I crank it. The one opportunity I had to do much troubleshooting when the problem appeared, I found that the spark was present but possibly weak from the coil to the distributor and not really reaching a spark plug from the distributor. Replacing the distributor cap and rotor did nothing and a resistance check of the coil primary and secondary came close enough to specs. This has led me to suspect that the coil driver in the PCM might bad. This whole problem first occured after 21 years of owning this truck and following replacement of the rear main seal. After putting it back together and filling it with oil, I disconnected the coil to distributor wire to prevent the engine from starting until I saw oil pressure so the oil pump would be primed. I later found out it's not a good idea to crank the engine with the coil secondary disconnected because the high voltage spark needs a gap to jump. I wonder if this might have damaged the coil driver---I cranked the engine 3 times like that before I got oil pressure. I can't confirm this suspicion without taking a $300 plunge for anew PCM---and even then can't be sure because the problem is so sporadic.

 

You're popping what is called a 'poly fuse'. It opens when over-current occurs, and stays open anytime there is an input voltage applied (obv no current flows). When you disconnect the battery and remove voltage, the fuse resets.

I don't know which circuit(s) in the PCM use a polyfuse. The 5 volt rail could very well have one.

Next time it wont start, check for 5 volts at the sensors with the key on.

 

OK, I'll check for 5 volts next time. It seems that the 5 volt reference would only be necessary at the crank and cam sensors for starting, unless I'm missing something. Assuming the poly fuse is tripping, does it indicate a fault in the PCM or elsewhere in the electrical system?

 

The 5 volt line is requires at all times for the engine to run. The crank/cam sensors are what fire the fuel injectors and ignition coil.

IF 5 volts is absent when the key is on/ignition off [might have another person cycle the key just to make damn sure], then either the poly fuse is tripping, or the 5 volt regulator is failing. It's hard to tell which may be the case, other than the 5VR may return to functionality after being cooled down. If an instant power reset cures the issue, it's unlikely to be the 5VR.

IF 5 volts is present when the key is on/ignition off [might have another person cycle the key just to make damn sure], then the issue is downstream of the PCM.

Since it spins over, it's nothing in the NSS or transmission, and being a 97 there shouldn't be much there in the way of electrical complexity (compared to a 99+ truck).

The other thing that raises an eyebrow is that it sputters and stalls when it will not re-fire. That tells me that it's an uncommanded shutdown. That would pass me along to a fueling issue, possibly a spark issue.

When it won't start, pull a spark plug, put the wire back on, and hold the plug to a solid grounded surface on the engine. Have somebody spin the engine over, and see if you get spark out of that plug.

You can also do a fuel pressure test. Put a gauge on the rail and see if you get 45-50 PSI. (Edit- I don't think 97's have rail test ports, do they?)