99 mustang gt 4.6l v8 no communication with pcm only

Bursey16

New Member
Oct 15, 2018
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Hey guys i got a 99 mustang 4.6 l automatic i have no communication with pcm miles displays ------- theft light flashes and the fans are on with key on and it only cranks! All my fuses are good im getting power to pcm all wiring is fine can a faulty pcm be the culprit?
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,818
501
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Houston Texas
Just wondering. Have you confirmed the PCM ground? As a reminder there is a separate ground wire that goes back to battery negative. Recommend using a test light that will "load" the circuit. This will confirm the circuit's ability to carry real current.

Also, has key on power in and out of fuses F2.34, F2,2, and F2.8 been confirmed? Again using a test light that will load the circuit.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

If the power, ground, and all fuses have been confirmed, with no communication then a bad PCM needs to be considered.

Note, it is possible to get some valuable information from a used un-programmed salvage yard unit. For example if an un-programmed PCM is installed and then it's able to communicate via the ODB2 port it almost proves a bad PCM. Try car-part.com for some sample used units.

Here's some more information that my help.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
 
Last edited:

Bursey16

New Member
Oct 15, 2018
3
0
1
26
New york
Just wondering. Have you confirmed the PCM ground? As a reminder there is a separate ground wire that goes back to battery negative. Recommend using a test light that will "load" the circuit. This will confirm the circuit's ability to carry real current.

Also, has key on power in and out of fuses F2.34, F2,2, and F2.8 been confirmed? Again using a test light that will load the circuit.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

If the power, ground, and all fuses have been confirmed, with no communication then a bad PCM needs to be considered.

Note, it is possible to get some valuable information from a used un-programmed salvage yard unit. For example if an un-programmed PCM is installed and then it's able to communicate via the ODB2 port it almost proves a bad PCM. Try car-part.com for some sample used units.

Here's some more information that my help.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
Just wondering. Have you confirmed the PCM ground? As a reminder there is a separate ground wire that goes back to battery negative. Recommend using a test light that will "load" the circuit. This will confirm the circuit's ability to carry real current.

Also, has key on power in and out of fuses F2.34, F2,2, and F2.8 been confirmed? Again using a test light that will load the circuit.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

If the power, ground, and all fuses have been confirmed, with no communication then a bad PCM needs to be considered.

Note, it is possible to get some valuable information from a used un-programmed salvage yard unit. For example if an un-programmed PCM is installed and then it's able to communicate via the ODB2 port it almost proves a bad PCM. Try car-part.com for some sample used units.

Here's some more information that my help.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
Just wondering. Have you confirmed the PCM ground? As a reminder there is a separate ground wire that goes back to battery negative. Recommend using a test light that will "load" the circuit. This will confirm the circuit's ability to carry real current.

Also, has key on power in and out of fuses F2.34, F2,2, and F2.8 been confirmed? Again using a test light that will load the circuit.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

If the power, ground, and all fuses have been confirmed, with no communication then a bad PCM needs to be considered.

Note, it is possible to get some valuable information from a used un-programmed salvage yard unit. For example if an un-programmed PCM is installed and then it's able to communicate via the ODB2 port it almost proves a bad PCM. Try car-part.com for some sample used units.

Here's some more information that my help.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
hey thanks for the reply . so what I have done is completely disconnected the pcm and looked at a pinout diagram and checked all the power wires and also checked all the ground wires to the ppm by just going from body ground to the pin . Is that the way you are talking about to check ground? also when I turn the key to the on position the fans turn on and stay on when the key in in that position
 

wmburns

SN Certified Technician
Aug 14, 2009
5,818
501
194
Houston Texas
Is that the way you are talking about to check ground?
Just wondering. Did you review the information provided? Testing in the way mentioned does not confirm the ability of the wire to actually carry a high current.
also when I turn the key to the on position the fans turn on and stay on when the key in in that position
The CCRM controlled cooling fans operate in a "fail safe" fashion. This is common symptom IF the PCM is bad or not getting power.
 

Bursey16

New Member
Oct 15, 2018
3
0
1
26
New york
ok that makes sense about the cooling fans! thanks guess I still don't quit understand how to actually load the ground wire?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,818
501
194
Houston Texas
So you don't see HOW running a real current of a known value through the actual wire path actually "tests" the circuit?

A volt-ohm meter (VOM) uses a current that is sooooooo small that it's possible to have a high resistance connection that would read OK with a VOM but would fall on it's face under higher current flow. With a small current comes a small voltage drop. With a larger current comes a larger voltage drop.

Under ideal conditions the test light have a current draw close to the expected draw. Few people would go to that trouble unless a voltage drop problem is suspected. The other reason is that it's often easier to test a really large current flow using the "voltage drop" method. No army of different test lights required.

If you still don't understand, then please watch the video links provided. Eric O goes into great detail the WHY's and HOW's.
 
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