WHO CAN SOLVE THIS DIAGNOSTIC PUZZLER? 2000 Grand Caravan: PCM won't delegate authority

Maintenance/Repairs
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1

Short & Sweet Version:
2000 Grand Caravan (3.3L) cranks but doesn’t run. If I bypass the automatic shut-down relay it’ll run for about 3 seconds, but starts surging and conks out. Both the cam & crank position sensors test okay. What other sensor(s) might be telling the PCM (PowertrainControlModule) that it shouldn’t let the engine run (not energizing the automatic shutdown relay)?

The rest of story (with extra clues):
After several problem-free years, the engine mysteriously died a few times while driving, but started back up immediately, and then twice it took several tries to start from cold. The last day, I drove it for 2 very short errands without incident, and then it refused to start, permanently.

Usually, when I find fried wires I can trace them and figure out the cause; not so with this trickster. Two of the three leads to the camshaft position sensor had gotten hot enough to make the insulation shrivel up leaving about 3" of each lead completely bare. The odd thing is, the lead that supplied the power in the circuit is the one that didn’t get fried. Well, I replaced that sensor with a known good one but the engine still just cranked without firing.

Traced the cause to the automatic shutdown relay; it wasn’t sending juice to the ignition system and the fuel injectors. But the relay tested out perfectly.
For some reason, the PCM is not sending a signal to the automatic shutdown relay authorizing it to shoot some juice on over there.

So I hot-wired 12V directly to the ignition coil pack and fuel injectors…and got spark and fuel. This arrangement gets the engine to run nicely for about 3 secs before it starts surging, and then within about another 3 secs it dies.

I’m guessing that the root of this problem is probably another sensor that’s telling the PCM that something is whacko enough that the PCM should not authorize the auto-shutdown relay to let the engine run.

Both the cam & crank position sensors check out okay. Whatever sensor is the cause had a few intermittent episodes over several weeks. The wires to the cam pos’n sensor were fried, but may not have been involved with the underlying problem.

By the way, the 3 wires on the cam pos’n sensor are:
Org 8v supply from PCM
Blk/LtBlu Sensor ground back to PCM
Tan/Yel Sensor signal back to PCM {5.0volts and 0.5volts, alternately}
And, somehow, enough power passed thru the last two wires to cause their insulation to shrivel up leaving nearly 3 inches of the wires bare!

Who has an idea of what could be going on?

2

I had a similar problem on a 2002 Caravan. Apparently, to start and run properly, the BCM will communicate to ALL the modules through the CAN, a twisted pair communications network under the dashboard. The BCM is the network CAN hub. ALL of the modules have to report ‘ready’ to allow the van to start and run, including the PCM, the security module, and even the gauge cluster. Yes, a bad gauge cluster can cause a ‘no-start’.

In our case, the BCM went bad and caused an intermittent ‘no-start’ condition. The gauge cluster had already been worked on. A replacement BCM fixed it, but finding a match was tough. And, since the odometer reading is stored in the BCM, the replacement module changed the odo from 136,000 to 223,000.

3

Good job on your troubleshooting.

From what you stated about the burned wires it sounds like the trouble is within the PCM module. Too much current caused not only the wires to burn but it most likely damaged what ever circuit those wires are tied to inside the PCM. If you are savvy enough you might be able to get inside the PCM and trace out the wiring, find where the wires tie to, and repair the unit by replacing the device that carried the current. You might even find signs of damage due to the high current. I have repaired ECUs in the past with success. Even easier, you should be able to find a used PCM at a salvage yard for pretty low cost and replace your unit. Hopefully that will cure the problem.

4

This seems like a situation where some experienced help might be the best route. Do you know of a local shop who has the manufacturer’s scan tool for this vehicle? That’s probably where I’d start. Let them do the diagnosis, then you may be able to do the fixing yourself depending on what they say. But if you don’t know what the cause is, well, you can spend a lot of time and money trying this and that, replacing this and that, even run out of money and/or patience before ever finding the actual cause. Which might be something simple.

5

I think as GeorgeSanJose said that a pro might be a worthwhile expenditure. I think that someone with a professional or factory-level scan tool needs to look at this thing and see what’s going on with the ECU and other modules. The ECU is normally an extremely reliable component on a car, but they do fail, often due to other problems with the electrical system.

As a side note, I’d start looking at the crank sensor circuit. If the crank sensor is bad or not generating a signal, it will not necessarily set a code and turn on the MIL, but will instead look like the engine isn’t spinning to the ECU–like you’re not trying to start it, so why provide spark and fuel? I would get a schematic for your car and backprobe the wires to the crank sensor and make sure you have a clean path to the ECU electrical connector that is not shorted to power or ground anywhere. Unless there is hot exhaust blowing on the sensor wires, the insulation should not ever melt–these wires do not carry any kind of massive current. If the path to the ECU is good, then I’d suspect the ECU. If it is corrupted with a short to power or ground, you need to correct that and also suspect that the ECU and crank sensor may have been damaged by the circuit problems.

6

Well said @oblivion … good info!

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I don’t know how to go about testing here unless you have a scan tool, not just a generic code reader but an actual scan tool. You say the ECM is not actuating the ASD relay. Is it able to? Does it want to? Does the relay have proper voltage at both the control leg and the load leg? First thing In would do is tell the ECM to turn on the ASD and see what happens and go from there. Other than that, I would test for battery voltage at ignition coil and injectors while cranking. I would test for reference voltage at TPS and MAP sensors KOEO and cranking. I would monitor fuel pressure. The results of these tests would determine what I did next.

How have you tested cam and crank sensors?

8

Thanks for all the great suggestions! Sorry for the delay responding. Here’s an update on what I’ve done, and answers to your questions.

I was able to test the outputs from the cam & crank pos’n sensors with an oscilloscope during the fleeting moments that the engine would run (had to video the scope’s screen and play it back to catch it), and they both look appropriate-- pulses alternate between about 0.5V and 5.0V.
{In the attached pic, the voltage scales for the two traces were not set the same. That’s why the Cam Sensor’s output signal looks smaller.}

The ASD relay never receives a “signal” (grounding of the DkBlu/Yel lead) from the PCM to energize the ASD, causing it to send power to the injectors and ignition coil pack. I fed power to them with a jumper lead to get them to operate-- and eliminate them as possible faults.
Fuel? Check.
Spark? Check.
Engine cranks? Check.
Engine starts? Check.
…then, some parameter (apparently controlled by the PCM) throws a monkey wrench in the works and the engine dies.

I swapped the PCM with a known good one with no difference.

I’m going to try testing the TPS (throttle pos’n sensor) next, once I find a place to tap into the wire. As I understand it, the output should vary between 0 to 5 volts as the throttle is opened.

Can anyone tell me how I can test the MAP sensor (manifold absolute pressure)?
Also, there doesn’t seem to be a mass airflow sensor (MAS) on this engine. Is that true, or am I missing seeing it someplace?

How the cam pos’n sensor’s wiring got toasted is still a mystery. The replacement is doing fine so far. There’s no heat source, ie: exhaust, nearby. I think I’ve managed to link some pics. After taking the sensor off the engine, I broke away the black plastic, exposing the tin can within that you see in the second pic.

I should be able to get a BCM to swap-in for a trial…if I have to…it does look like a pain to get to based on the pics I’ve found.

By the way, do you know of a good source online for schematics and/or test procedures?

Thinking about the ASD relay again,…
Right from the get-go, before the engine even begins to rotate, the PCM has decided to not energize the engine. What does that tell us? Apparently there is something on its “pre-flight checklist” that doesn’t make the grade. And it’s something that can flip intermittently and cause the PCM to cut the engine as you’re driving down the road – and be restarted a second later.
I’m wondering if it could be something like a roll-over switch? It must have one; I’ll google it (google knows all–almost).
What other conditions are there that could be bad enough that the engine would be shut down?

Hey, thanks again; I really appreciate all your help with this!

9

Again, very good troubleshooting here. You are going to need it for this problem.

Have you made sure that all the inputs, especially power and ground, connections to the PCM are good? If not that would be my next move. I am surprised the replacement PCM didn’t fix this so now I have to wonder about the connections to it.

As for service info I suggest you purchase a factory service manual for the vehicle. There may be some on Ebay for a good price. They do have factory diagnostic manuals available also to help you find problems like this. They are worth every cent they cost.

10

You may be able to run down every problem if every basic thing is OK. Fried wires on the ground side could be a hint of a bad ground from engine to frame. Since you can do work on cars; make a ground wire to go from the engine to the body and see if anything at all improves.

If a bad ground is a problem, fixing that situation could make it possible to eventually fix all the problems that it caused. The negative battery cable is your friend if it works, so check it well. If there is a small wire on it that leads to ground, look it over too. After you install the home-made ground wire change the ignition switch because it’s old and it doesn’t like ground problems either. Also because I have no faith in electrical stuff on a Chrysler product.

11

I believe the PCM turns off current to the ignition system and fuel pump if no attempt is made to start the vehicle within a few seconds of turning the key on–and no pulses received at the ECU from the crank sensor is the same to the ECU as not trying to start the vehicle. I think the relay should briefly energize as soon as you turn the key to “run”, but may not remain on–but don’t quote me on that.

Have you verified that you have a clean path to the ECU connector from the crankshaft position sensor? Just seeing pulses doesn’t necessarily mean that the ECU is properly receiving them. I don’t think a bad TPS will affect the relay energizing or not–but you can disconnect it and the computer will work around it for testing purposes.

Chrysler electrical systems are just as good as anyone else’s, despite what pleasedodgevan2 says. With a 14 year old vehicle, just about any kind of failure is possible. You really need a schematic to understand what is going on. Since you can obviously use a scope, I’m sure you can read one. Google is your friend–I found a downloadable service manual for my car in a couple of minutes and very cheap. I’m sure you can do the same. Money well spent.

Stupid question, but are you also sure the ASD relay is good?

12

I have seen cam sensor wiring like that, it is a poor grade of insulation and distorts when in contact with oil for a long time.

Check that there is 5 volts to the sensors with the ignition on and when cranking. The orange wire or green with an orange stripe, I don’t remember but I can check tomorrow. I believe the cam and crank sensors are on one 5 volt feed and the MAP, TPS and a few others are on a different feed. If one of these 5 volt feeds is shorted to ground the vehicle won’t run.

If that other PCM came out of a vehicle with SKIM (Sentry key immoblizer) it won’t work in your van. Do your keys have a gray or black handle? Is there a red dot light on in the instrument cluster? With a scan tool you could check the fuel status, “disabled due to VTSS” or “enabled”.

13

When I have a problem I purchase a subscription to www.alldata.com . They have the schematics and diag procedures for most cars. Plus all the TSB’s and location diagrams.

14

The DG/OR wire leads to anything that keeps the car going, coils, fuel injectors, O2 sensors… I would get the alldata and start ohms checking.

15

I have no idea what all the abbreviations mean, and my electrical knowledge is minimal, but I do know that Chrysler minivans of this era had problems with the soldered connections where the connector pins were attached to the printed circuit of the instrument cluster. They can cause all sorts of mysterious issues, in addition to flashing or partial odometer displays, gauges and meters randomly on and off, etc. If you instrument panel is doing anything even slightly wrong, you can remove it, unplug it and take it apart and resolder the pins.

16

@wentwest: It is very unlikely that an instrument cluster problem will cause the issues he’s having. These are not used as a junction point or communications node on these vehicles to my knowledge.

17

Like I said, my knowledge is very limited, and I could easily be all wrong. At 67, I’m used to being wrong. My suggestion was based on an experience I had where a cluster seemed to cause a driveability problem, and fixing the solder seemed to fix the problem.

18

Sounds like the SKIM (sentry key immobilizer module) may be flaky.