No Fan....PCM?

[jacuzzo]

Hey all,

I have a need for some moral support and advice. My 95 3.5 overheated the otherday, the fans did not come on. I have checked all fuses and relays, they are fine. The temp sensor is also fine as the dash gauge works fine, and it is new. I think I have narrowed it down to the computer. This is my reasoning. The fan comes on for no good reason just after the car starts. So, it works, but the only thing that turns it on is the PCM. Also, the car was acting goofy the other day. when I turned off the heat the car would drop idle and sputter and the fan would come on. When I turned heat on, fan would shut off and idle returned to normal. This leads me to believe that the voltage regulator (in pcm) is goofy. And, before this, the car was idleing irradically (it was cool), I swapped out the coil and it seemed better. But that was just before the heater incident.

The question is: have I missed something in the fan circuit? Should I take the $300 pcm gamble? Is there another test I can run.

Oh and, the check engine light (which has been on for awhile with innocent codes) no longer comes on.

Any suggestions would be great.

Thanks
Jacuzzo

[bigj-dog]

I'm not positive on this model, but some have 2 temp. sensors, one for the gauge/dash and one for the computer. The other symptoms you're describing could very well be attributed to the computer getting bunk readings from its temp sensor. Check it out.

[jacuzzo]

This one has only one temp sensor. I have checked the wiring diagrams and the vehicle to ensure that this is correct. The gauge on the dash is in working order. In fact it corresponds exactly to the state of the engine. Light comes on and H is achieved at the same time steam shoots out of the overflow. The sensor goes through the PCM to the interior computer. So it seems that the connection between these is appropriate too.

Thanks for responding.

any other thoughts would be great.

Jacuzzo

[bigj-dog]

I looked it up on Chilton's Online, and you are correct, there is only one sensor. The only time I had symptoms similar to yours on my '97 3.5 was when a BCM ground was loose. Inside the console just below the dash on the drivers side. I think you can get a used PCM from a yard for about $50. That's the going rate in Minnesota anyway. They give a pretty detailed method of testing the sensor/wiring on their site, I can send you the steps if you want.

[jacuzzo]

Thanks, those steps would be nice to have. I have the factory service manual, but no real testing steps there. The problem with a junk yard computer is that these things are specific to the features of your car. The dealer found the part number with my VIN. He even asked what size tires I had. So, I wonder if I'll get the correct one.
I will check that ground.

jacuzzo

[bigj-dog]

Here's the steps..
Make certain that the engine is cool (i.e. has not been run for a couple of hours).
Unplug the engine wiring harness electrical connector from the coolant temperature sensor.
Connect an ohmmeter between the terminals 1 and 2 of the sensor.
At room temperature of about 70° F (21° C) the ohmmeter should read approximately 7000-13,000 ohms.
If the resistance readings are not within specifications, replace the sensor with a new one.
Attach the engine wiring harness electrical connector to the coolant temperature sensor, then turn the engine ON (without starting the engine).
Backprobe the sensor's tan/black tracer wire to ground with a voltmeter. Refer to the accompanying wiring illustration. Then start the engine.
Watch the voltage of the sensor until the engine coolant reaches normal operating temperature.
If the voltage does not change, or changes in erratic jumps the sensor is defective and will need replacing.
If the voltage changes gradually and in a smooth manner, continue to the next step.
Once the coolant temperature has reached normal operating temperature, turn the engine OFF but keep the ignition ON.
Unplug the engine wiring harness electrical connector from the coolant temperature sensor again.
Connect the ohmmeter between the terminals of the sensor. The sensor resistance should be between 700-1000 ohms at 200 ° F.
If the sensor has not functioned as described in any of the various steps, replace the sensor with a new one.
Make sure to retest once the new sensor has been installed to rule out the possibility of numerous problem spots.

If you look at VikingAutosalvage.com (the yard I usually go to), they have a very good part search engine. I think for the 3.5, it matters if you have autostick and VR rated tires, instead of normal 225/60s. I'd bet a good yard in your area would have the same, otherwise those guys would shipt to you. Still a little risky though.

[jacuzzo]

Thanks again. One problem is that it isn't going to be 70 degrees till next june.
I will check out that junk yard, but I am leaning towards the additional certainty one gets with a new one.

[bigj-dog]

Either way let us know what happens. It seems to be that the Chrysler dealers blame everything on bad PCMs and that it ususally doesn't work out that way. Good luck.

[rhandwor]

Go to http://www.alldata.com/ and purchase a years subscription for your vehicle. It used to be $15.00 then print off a wiring diagram. Buy a logic tester on ebay for around$25.00 with shipping. You have to start at the fuse and go to the fan. A lot of time you will have 12volt at the fan but no groung.
If you hook up a ground the fan will run all the time. Definately check the ground in the previous post. Check all computer grounds and the wire to the relay for a ground.

[jacuzzo]

Thanks,
The chrysler manual that I have includes very informative wiring diagrams. But it does not include technical service bulletins, so it might be nice to have the subscription. I have studied the diagrams and found no problems.

The ground in the previous was described as being under the console. I took a look, I could not see anything amiss, but I could not decide what the console was exactly, nor what 'under' meant. Does it mean "below" or "Behind"? I also did not know if I was to look under a console on the driver's side or look at the driver's side of the space under the console. So I am at a loss in that regard.

Are you suggesting that I run a wire from a fan motor to the frame? I guess I could splice into the ground that is in the connector. But wouldn't I run the risk involved in having an under heated engine? Also, if it is the computer, as I suspect, doesn't it seem likely that I would run the risk of a variety of malfunctions that might leave me stranded on the side of the road?

What is a logic tester? I have a standard electrical testing kit and have been shopping around for an OBD1 program for my laptop. Is there an interface that is only $25? Do you know of any good programs?

This is my reasoning.

The fuse is always powered when the engine is on. Since that fuse is fine, the relay is next, that is fine. Then there is the fan, which runs for no good reason, but runs, so fine. The relay is run by the PCM, which is informed by the sensor. Since the sensor is linked to the dash through the computer and the dash works, the computer is getting info from the sensor. But, if the sensor works and the computer works, the fan should work. But the fan does not work, therefore it must be the computer.

Is there a flaw in this reasoning?

Thanks for all of your help.

Jacuzzo


Any info would be great!

Jacuzzo

[rhandwor]

In my manual you have a ground rear of engine,on asdm bracket,left kick panel,right front frame rail, left front frame rail. These are all for the computer system.` Collant temp sensor top front of engine. Power train control module right front frame rail. A logic probe either has a red,green and amber light.
The cheap ones are computer safe and they tell you if you have a hot or ground. The better ones pick up a pulsating signal like a cam position sensor or crank position sensor. The temperature sensor tells the computer the temp. is to high then the computer normally closes the ground for the relay control circuit which turns on the fan. You can feel the relay click when it turns on.
Pull the relay up slightly and with a logic probe check for both hot and ground on the control side. If you have this check the other side of the relay. If it is four pronged pull it out and hook up a hot and ground to the control side. You should have continuity to the other side of the relay using your ohm meter.
If you don't have control voltage I always use an ohm meter and check the wire before buying computer. You should be able to buy a used one for around $50.00. Check with a parts store for a price on a rebuilt unit.

[bigj-dog]

The ground bolt I was referring to is directly to the right of your right knee if you're sitting in the driver seat. It's under the plastic of the center console and it grounds the computer to the metal structure support of the dash itself. You'll have to partially remove the console (ie shifter and disassemble some of the lower dash componets to gain access to it. I'm not 100% sure about your car, but mine is a '97 3.5 with autostick and that ground bolt got loose and played hell with a bunch of things. Lost parking lights, power windows, blower fan and the idle was a mess. I'm pretty sure that ground is for the bcm, but it did affect my idle, so I figured I'd mention it.

[jacuzzo]

Thanks for all of the helpful information. I I have discovered that the ground ribbon on the left frame rail is broken. But I cannot find the other end. I supect that it is supposed to be under the bolt that has a big black wire, most likely the negative Battery cable, under it too. Any help?

Jauzzo

[rhandwor]

You can clean up the end and crimp on a new terminal. Sand a clean spot on the frame and use a new self tapping bolt and attach it. Very hard to do without power tools. Or use a splice terminal and add a piece of wire and a new terminal end. Remove the corroded bolt and find the broken terminal remove it. Clean all connections and reinstall.
Use silicon to seal the joints on your connections. Coat the ground connection with greese.

[jacuzzo]

I ordered a new pcm and the dealer got me the wrong one. It was then that I realized that I do not have the appropriate pcm in the car. The dealer called chrysler to verify. I should have it tomorrow. Update to follow.