Engine/Transmission (1994 - 1998) Computer won't Scan

[McCRam]

The Tach, speedo, air conditioning, alternator and other things quit working yesterday. Was able to drive home on the batteries, they lasted the whole four hour drive and still have 11 volts in them. Hooked up the autoxray scan tool, but the truck isn't talking to the scan tool. I'm going to get serious and crawl under the hood in the morning, but I'm kind of foggy on what I should look for now. I was hoping it would be the engine speed sensor, but since the computer isn't scanning, I'm afraid that the computer is the problem. I did give a quick looksie at all the conections I could find. So far everything appears normal. Any tips, hints, suggestions,or guidance would be appreciated.

 

[tp0d]

well, I havent had a problem like the one you are describing... but mebe i can suggest a few things. . have you 1- checked your fuses, mainly under the hood? 2- unplug and replug all the connections going into the PCM, and make sure there are no damaged wires (melted/cut/etc). Sounds like your battery connections are OK, so it probably isnt a terminal problem. hmm. . i would check the fuse thing first tho



good luck



-jerry

 

[McCRam]

I tested every fuse I could find and they are all OK. I visually inspected the harness and I have not seen anything wrong yet. I unpluged the computer like you said and cleaned the terminals while I was there but it didn't change anything.



Other things that do not work are the grid heater and fuel heater, or the cruise control. It seems as if the computer is not working at all either.



Would the computer scan if the speed sensor was inop? Would the temp and compass work? (it does by the way)



I did a few searches and kept reading to check the grounds, but my manual is awol, so I do not know which wires those are or where they go. Thanks for the help.

 

[I-6DZL]

Bad News!!

Too me it does sound like the computer!! But. depending on how you checked the fuses. how i check fuses are : get a 12 volt test light and turn the key to on position and connect the ground for the test light to the neg battery terminal and touch each side of each fuse, if light lights up, good fuse. check all fuses, inside and out. Normally when a computer wont communicate with our scan tools, it's either a fuse or a bad computer.



Good Luck

Curtis

 

[McCRam]

I was afraid of that. I checked the fuses with a fuse testor. I did check that the computer chassis is grounded with an ohm meter. None of the labels come out and say which one is for the computer, so which one is it?

If I new, I would replace it just to make sure.



Any body know of any good deals on computers?

 

[I-6DZL]

computer

The computer should be on the passenger side of vehicle, bolted to the firewall. kind of tucked down inbetween the inner fender and the firewall. I beleive it's best to pull the air box out.



Good Luck

Curtis

 

[Bill Lins]

I'm not a mechanic, but I know that low voltage will make a computer go nuts. Have you charged your batteries back up? If not, you might do that & then retry the scan tool. You might also check the big (140 amp) alternator fuse. Good luck.

 

[John - K5AWO]

I was just going to post that very thing. Before you make any tests, get the batteries back up to full charge.



Computers do not like low voltage.



Once that has been done..... or while it is being done would be better. Disconnect the batteries and let the ECM go to sleep for a couple hours. Then put it all back together.



Should get results then.

 

[DSiemens]

I have had that problem on my 95 Ram. The pcm needs a signal from the engine speed sensor to "turn on". The sensor also sends a tach signal. My connection just above the crankshaft/harmonic balancer went bad. Fixed connection, pcm got the needed signal... . all good.



Dan

 

[McCRam]

I think the computer would let me scan it if it was only the speed sensor, that is what I was hoping the problem was. I did not even think of the batteries though. I will start them on a charger when I get home in the morning. I took out the computer to get #'s and to give it a rest like John said. If that is why it wont scan, it could still be the crank sensor. I hope so cuz it will be a lot cheaper. Dealer quoted me $560 for a new pcm, and Bumper to Bumper can do a reman for 260, although it will take two weeks.

 

[McCRam]

Well I charged up the batteries and tried again to scan the computer after I pluged it back in. Still no luck. I started and ran the truck without pcm and nothing changed versus when it was pluged in. I'm positive now that the problem is the pcm. Can anyone tell me what wire or wires power the pcm. I would like to confirm that there is power before I replace it. Anyone ever take a pcm apart? If its broke I guess it wont hurt. I might even find a bad connection in there.

 

[tp0d]

Dsiemens has a good point about the crank sensor mounted just above the harmonic balancer. I would think this sensor would go bad before a complete PCM. The PCM is usually a pretty tough unit, I would look at sensors before buying something that costly.



I think you can check the crank sensor using a multimeter. It should just be a coil that responds to the rotation of the harmonic balancer. If you find an open circuit at the plug to the sensor, I`d put my money there. . I`ll see if theres a test procedure in my service manual...



I`d give ya a wiring pinout for the power to the pcm, but I`m at work away from my service manual. If someone else doesnt get to it first, I`ll post the pinouts to check on the pcm when i get offa work. .



-jerry

 

[tp0d]

Ok... found some test points you can hit with yer meter...



circuit A22 (red/white) - dedicated 12v+ power from PDC fuse #8

A2 (lightgreen/black) - ignition 'on' 12v+ fused thru block #9

a31 and a32 (black/tan) - ground (test continuity to ground)

b10 (darkgreen) - generator driver (alternator turn on)

a8 (grey/black) to a4 (black/lightblue) - engine speed sensor (crank sense) - should have an ohm reading, or continuity. i think its a coil.

a17 (violet/white) - +5v feed to crank sense (should have +5v to ground when pcm on)





well... those should give ya some reference. might want to use paperclips or something to press into the connector because meter probes are usually too big. you could test the first three and the crank continuity with the connector off though.



good luck



-j

edit---- ohya, theres a pcm for $50 on ebay. . its from a 98 gasser, but i`m pretty sure you can take it to the dealer and have em flash the right code onto it to make it work in yer rig. might check with the dealer just to be sure. .

http://cgi.ebay.com/ebaymotors/ws/e...tem=7920873258&category=33596&sspagename=WDVW

 

[DSiemens]

I dion't remember which pin on the PCM it is, but if you have a frequency meter you can test that pin for the correct hertz. No hertz, crank sensor is bad and PCM doesn't turn on.



Dan

 

[McCRam]

Well I tested as much as I could. I got reading on the crank sensor and at the wires tp0D listed. (thanks for that info by the way, it was a big help) So The new pcm arrives tomorrow. I hope I checked everything. I still have a question though. From the above posts I gather that if the crank sensor is inop, the computer wont scan? That doesn't sound right. I understand that the crank sensor tells the computer that the engine is running. But it recieves power from the battery and the ignition. I would think it would still be on, it just wouldn't tell anything else to turn on.

 

[DSiemens]

I follow your logic and I agree. Please post the result when you have the new PCM installed. I want to know. Thanks,



Dan

 

[McCRam]

It wasn't the pcm dangit! Everybody has been right about the crank sensor allowing the computer to turn on, unless you unplug the sensor. After I unpluged the sensor to check continutity of the wires, suddenly the grid heaters started working. I hooked up the scan tool and the computer was working, although there are no trouble codes. I also made a mistake when I was checking the sensor. I had continutity between the 5v feed and the center gray(black wire), not the black(light blue) wire like it was supposed to be. So I hope the dealer will take the pcm back, they said they would, I hope they are good on their word. Thanks for all the help, I get the new crank sensor on wednesday, I let you know what happens.

 

[tp0d]

yea... kinda thought it was the crank sensor. . the pcm is usually a tuff bugger. . cept for the charging circuit. . ive heard of a few failures. sounds like your crank sensor failed and shorted the gray/black to the 5v feed... just for reference, how much did you pay for the new sensor? hope you didnt buy it at the ********... prolly could have got one thru ebay for 1/4 the cost. also. . do you have instructions on how to recalibrate the new sensor? I think you will need a barring tool to rotate the engine so the sensor flat on the dampner will be in line so you can set the air gap. . its pretty damn small so you`ll need a set of feeler gauges. .



good luck



-j

 

[McCRam]

I didn't know anything about calibrating the sensor. I know the gap between the sensor and crank needs to be adjusted( haven't found what that is supposed to be yet). anything else?

 

[DSiemens]

Sensor gap is 0. 050 inch. A brass feeler gauge is nice as the sensor is magnetic. A steel feeler gauge will stick to the crank sensor.



Dan



Edit: I've had my crank sensor on & off a few times. Calibration must refer to the gap. It is straight R&R.