PCM and CPS Issues

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May 8, 2017 | 01:21 PM

ChrisMelton78

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Mountain Pass

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PCM and CPS Issues

So, I'm chasing a no start issue in my 95 F250 7.3 Powerstroke after reassembly. One thing I've noticed is that when the truck is turning over, the tach is not bouncing at all. Which would initially make me think the CPS is an issue.

My question is, if the CPS is good, should it bounce regardless if the PCM is good or not? Or does the CPS require a good, working CPU before one would see the tach bounce?

Thank you for your assistance!!

May 8, 2017 | 01:27 PM

madpogue

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From: Madison, WI

The CPS connects ONLY to the PCM; the PCM thus feeds the tach. So yes, a bad PCM could cause a tach failure.

Are you getting a WTS light? Is the GP relay closing?

May 8, 2017 | 01:34 PM

knottyrope

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From: Bahstun

weak batteries can make it not bounce too from lack of RPM

May 8, 2017 | 01:41 PM

ChrisMelton78

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Mountain Pass

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Posts: 219

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Quote:

Originally Posted by madpogue

The CPS connects ONLY to the PCM; the PCM thus feeds the tach. So yes, a bad PCM could cause a tach failure.

Are you getting a WTS light? Is the GP relay closing?

Yes. I am getting a WTS light. How would I know if the GP relay is closing?

May 8, 2017 | 01:42 PM

madpogue

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Joined: Oct 2003

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From: Madison, WI

Listen for it. Also check for voltage at the terminal that leads to the GPs.

Do you have a device that can read real-time data, do PCM-driven diagnostic tests and obtain codes?

May 8, 2017 | 01:47 PM

ChrisMelton78

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Mountain Pass

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Quote:

Originally Posted by knottyrope

weak batteries can make it not bounce too from lack of RPM

These two batteries are only about 6 months old and I had a fresh charge on both of them.

Something odd (or maybe not) after turning the engine over for a bit the "Service Engine" light comes on.

May 8, 2017 | 01:54 PM

knottyrope

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Joined: Apr 2016

Posts: 7,213

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From: Bahstun

corroded terminals can make good batteries not perform at 100%

scanner to monitor your engine

Amazon

Torque Pro
https://play.google.com/store/apps/d...g.prowl.torque
Use the communication in the profile
SAE J1850-PWM
make sure you add in Ford PIDs
https://play.google.com/store/apps/d...ch.CarGaugePro
people like it cause it will pull codes, buzz test and CCT
Warning, Some OLD PCM's have to be flashed before you can pull codes

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May 8, 2017 | 02:00 PM

ChrisMelton78

Thread Starter

Mountain Pass

Joined: Sep 2012

Posts: 219

Likes: 0

Quote:

Originally Posted by knottyrope

corroded terminals can make good batteries not perform at 100%

scanner to monitor your engine
https://www.amazon.com/Vgate-Bluetoo...rds=vgate+OBD2
Torque Pro
https://play.google.com/store/apps/d...g.prowl.torque
Use the communication in the profile
SAE J1850-PWM
make sure you add in Ford PIDs
https://play.google.com/store/apps/d...ch.CarGaugePro
people like it cause it will pull codes, buzz test and CCT
Warning, Some OLD PCM's have to be flashed before you can pull codes

My terminals are brand new military spec terminals and the terminals on the battery were cleaned once I placed them back in the truck from charging.

May 8, 2017 | 02:05 PM

#10

knottyrope

Lead Driver

Joined: Apr 2016

Posts: 7,213

Likes: 1,342

From: Bahstun

if you don't have a spare Ford CPS, you should get one and keep it along with a 10 mm wrench in glove box

I would change that before the PCM and check the wiring down there for chaffing

May 8, 2017 | 02:21 PM

#11

ChrisMelton78

Thread Starter

Mountain Pass

Joined: Sep 2012

Posts: 219

Likes: 0

Quote:

Originally Posted by knottyrope

if you don't have a spare Ford CPS, you should get one and keep it along with a 10 mm wrench in glove box

I would change that before the PCM and check the wiring down there for chaffing

Ok. I've got a spare CPS in the glove box. I just bought this one a few months ago, so I didn't suspect it. However, I know they are notorious for going belly up.