Coil runs on 12 volts. It will indeed be higher than 5v. No avoiding that.

 

jlowmiller - "Now I could see that being a coolant temperature sensor malfunction, the PCM adjusting fuel trim and screwing it up with bad readings.. "

Just checked the CTS and it crumbled into pieces when I went to pull the connector off. Cheeky little bastard has been hiding under the alternator all these years, like Saddam in his spider hole. Like all evil-doers, it could run, but it couldn't hide forever. New one comes in at noon. I have to believe it hasn't been working for years - we'll soon see if that has any bearing at all on my misfiring. I tend do doubt it, but I also have to believe that having a functioning CTS will improve matters in general.

heyyou - "Have you tested the spark at the coil, and verified that you get intermittent weak spark?"

As I had said, I tried, but they come so fast that it's hard to observe skipping. That's where a scope comes in, either the home-made one, or a store-bought. I'm going to work on making/procuring one.

As for the voltage issue - there are two wires on the connector to the bottom of the coil. Is one of them 12 volts from the battery, and the other a 5-volt reference signal from the PCM? I tried to take voltage readings and got inconclusive results. It seems that one of them has to come from the PCM, so I'm hoping to make a nice graph of that signal. Should tell a lot.

Just this morning, I again got no spark at all on #4, even after trying different wires. Then I got a weak and erratic spark on #4, all the while getting strong and steady sparks on the cylinders that have been consistently strong and steady.

I called this thread "Interesting spark problem" with reluctance, because it will be either just that, or something that makes me look like a complete idiot. Stay tuned, because we're going to find out which one it is.

 

At the coil, one is 12 Volt supply from the ASD relay. It will have 12 volts for about 3 seconds at key on, and then nothing till the PCM seen the engine turning. (crank sensor signal)

Other wires goes to the PCM to ground the coil. PCM charges coil by grounding that wire, fires coil by removing ground. All it is, is a switch. (just a very fast one.) On a scope, should see a nice, consistent square wave off that one.

 

Cam signal actually triggers the ASD relay.

If you look at the scope I posted, the third line down is the coil negative wire from the PCM, nice and square like you say.

 

John,

Make sure when you follow the circuit to make the probe you set it up for voltage division of the 12 volts down to about 1.5 to be safe so you don't toast your audio card if you end up making a probe.

Edit.. if you need help making a voltage divider for this PM me.

 

So there is a relay as well between the PCM and coil on this model?

 

ASD relay (auto shutdown relay) provides power to the coil, and injectors. It is controlled by the PCM. Will be enabled for three seconds at Key On, or, if the PCM sees the engine turning. (cam or crank sensor is my understanding.)

 

No progress on making/borrowing a scope, and a Fluke 99B goes for $1200 new, $600 used.

Replaced both temp senders (PCM, gauge) and the only difference is that the engine runs worse now. Maybe an already confused PCM is even more confused with new input (assuming the old sender/sensor wasn't working at all). It just might not have been; both were very crusty, corroded and fragile.

There's some confusion at NAPA and with me about which sender is which. They say the gauge sender is on the top front center of the intake manifold, and the PCM temp sensor is tucked behind the alternator on the passenger's side/front of the manifold. I say the opposite. When I disconnect the sender behind the alternator, the gauge stops reading.

Checked ASD relay for ground and cleanliness, and looks fine. One other relay had some "greenies"; cleaned it up, no change.

Want to know how frustrated I am? I even yanked out the pre-cat O2 sensor. (No change, just a little noisier).

I'm going to bump up my other thread from long ago about my long-running stalling/stumbling problem that might be the early onset of this problem. maybe the symptoms will ring a bell with someone.

Anyone had a misfire problem, as opposed to a no-start, due to a bad PCM?

I'm about to bite the bullet and slam a different one in; prices range from $150 used to $230-495 new.

Some are listed as "RFI" (circuit board shielded to reduce Radio Frequency Interference), and these are the ones that NAPA lists. I'm wondering why they made non-RFI models? Do they work better? Hell, the radio doesn't even work anyway - maybe I should get one.

Anyway, this is a great link for looking up PCM numbers by year and engine:

http://www.dodgetrucks.org/docs/PCM%...%20Updates.pdf

Gonna go sniff some glue now, that always cheers me up.

 

My brother in law had a similar problem on his Jeep Grand Cherokee. parts tores tried to sell him all the sensors on the engine - one at a time.

I did some research on another forum and it appears that grand cherokees ofter suffer this problem, and most people would change out the PCM, only to have the problem come back. He finally brought it over this weekend so I tried a fix that I learned about:



The steel hose clamps hold the two outer connectors in firmly, but the tie wraps are needed to secure the center connector.

Don't know if this will help, but if the connector on the PCM is loose and not making good connection, there is no telling the problems it may present.

(I also wrapped a mesh shield around cables to the PCM to eliminate any potentiao stray emf interference.)

 

Single wire sender is for the gauge, two wire sensor is for the PCM, doesn't matter where they go.