Help with CPS circuit
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Help with CPS circuit
I have a no start 1997 F350 Powerstroke. I've changed out the CPS since the no start condition. Code reader gave code for intermittant CPS signal. Need to know what to do to check CPS circuit for short, etc..
Post Fiend
Joined: Mar 2007
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From: Texas
Where did you buy the CPS? We've seen too many cases of the ones from the parts stores being bad right out of the box. Make sure you're running a Ford or IH CPS, then check the connector to make sure the little rubber gasket on the plug is there, then start tracing the wiring looking for any chaffed areas.
If you want to check with a meter, unplug the CPS connector and check the top wire against a good ground with the meter set to ohms. Look for a reading of less than 5 ohms for a good wire.
Switch the meter to read volts, turn the key on, and check the middle wire against ground looking for close to 5 volts (Vref)
Turn the key off and check the bottom wire (still reading volts) against chasis ground looking for a reading around 12 volts (or whatever your battery voltage is with the key off).
Welcome to FTE.
If you want to check with a meter, unplug the CPS connector and check the top wire against a good ground with the meter set to ohms. Look for a reading of less than 5 ohms for a good wire.
Switch the meter to read volts, turn the key on, and check the middle wire against ground looking for close to 5 volts (Vref)
Turn the key off and check the bottom wire (still reading volts) against chasis ground looking for a reading around 12 volts (or whatever your battery voltage is with the key off).
Welcome to FTE.
Thread Starter
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New User
Joined: Sep 2010
Posts: 3
Likes: 0
Where did you buy the CPS? We've seen too many cases of the ones from the parts stores being bad right out of the box. Make sure you're running a Ford or IH CPS, then check the connector to make sure the little rubber gasket on the plug is there, then start tracing the wiring looking for any chaffed areas.
If you want to check with a meter, unplug the CPS connector and check the top wire against a good ground with the meter set to ohms. Look for a reading of less than 5 ohms for a good wire.
Switch the meter to read volts, turn the key on, and check the middle wire against ground looking for close to 5 volts (Vref)
Turn the key off and check the bottom wire (still reading volts) against chasis ground looking for a reading around 12 volts (or whatever your battery voltage is with the key off).
Welcome to FTE.
If you want to check with a meter, unplug the CPS connector and check the top wire against a good ground with the meter set to ohms. Look for a reading of less than 5 ohms for a good wire.
Switch the meter to read volts, turn the key on, and check the middle wire against ground looking for close to 5 volts (Vref)
Turn the key off and check the bottom wire (still reading volts) against chasis ground looking for a reading around 12 volts (or whatever your battery voltage is with the key off).
Welcome to FTE.
Rick
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I checked the circuit for the CPS. The top wire I got zero ohms. After switching on the ignition I got 5 volts on the middle wire and 12.8 volts on the bottom wire. When I checked the bottom wire after turning the ignition off I did not get any voltage off it. Can you help me out?
Rick
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Joined: Mar 2007
Posts: 26,966
Likes: 50
From: Texas
I'll add some in put Rick, but we'll need others to chime in too. The info I have is based on the newer 7.3's, so I'm not 100% sure there wasn't a computer change somewhere that would affect when that circuit get's energized.
You've got 12.8 volts with the key onm, which would be correct, but 0 with the key off. The newer trucks are supposed to always read the battery voltage. I'd try wiggiling the wiring harness to the CPS with the meter on there (key on) and look for any voltage drops. Also check the pin with the key off against a battery positive, or check the pin against the frame looking for something that is grounding out the signal. Then disconnect the PCM and do the same check to see if any ground signal is being caused by the computer.
With the PCM disconnected, a reading of 5 ohms or less against a good ground indicates a short to ground in that wire. You can also ohm the wire itself from each end. On the newer trucks I show that to be test pin 21 where it plugs into the PCM. I don't have a pin out chart that old, so someone else will need to cough it up or verify it's the same as the newer ones so you know where pin 21 (or whatever number for your model year) is.
You've got 12.8 volts with the key onm, which would be correct, but 0 with the key off. The newer trucks are supposed to always read the battery voltage. I'd try wiggiling the wiring harness to the CPS with the meter on there (key on) and look for any voltage drops. Also check the pin with the key off against a battery positive, or check the pin against the frame looking for something that is grounding out the signal. Then disconnect the PCM and do the same check to see if any ground signal is being caused by the computer.
With the PCM disconnected, a reading of 5 ohms or less against a good ground indicates a short to ground in that wire. You can also ohm the wire itself from each end. On the newer trucks I show that to be test pin 21 where it plugs into the PCM. I don't have a pin out chart that old, so someone else will need to cough it up or verify it's the same as the newer ones so you know where pin 21 (or whatever number for your model year) is.
