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Old 04-28-2019, 08:34 PM   #1
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Need help with CHT issues Please

I recently got a 2002 focus with zetec engine. Previous owner replaced alternator and after replacing, the temp. gauge quit working and gauge and cooling fans run constantly with engine running. I took the alternator back off and found that the person who replaced the alternator pinched the CHT sensor wires and it had been stripped bare when it got pinched. So i bought new CHT sensor and sensor pigtail, replaced both. Unfortunately the car still has the aame problem. The cooling fans run constantly and the temp gauge drops below cold after 5-10 seconds after starting the engine then the engine idles down. So my question is, is it possible for the ecu to have shorted out since the last person crushed the CHT wires?


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Old 04-28-2019, 10:38 PM   #2
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Moved to "General Technical Chat".
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Old 04-29-2019, 11:26 PM   #3
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Quote:
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...So my question is, is it possible for the ecu to have shorted out since the last person crushed the CHT wires?
The short answer is Yes, the PCM (aka ECU) could have been damaged. The better or longer answer requires an explanation.

That circuit is not protected by an external fuse. So, an electrical short of those 2 CHT wires should equal something gets fried right? No, not so fast. That circuit, like many or most PCM sensor circuits is designed to protect the PCM from dead short situations like you encountered.

It works like a voltage divider (if you know about that stuff) where 2 resistors are used. One resistor is inside the PCM (CHT detector circuit) and another resistor is outside of the PCM (the CHT Sensor). On paper, this circuit is designed to protect the PCM via the resistor inside the PCM during an external wiring short. Off paper, in the real world, this protection can fail, and damage to the PCM can result.

Use a multi-meter and check for DC voltage at both pin-1 and pin-2 of the CHT sensor connector. You can do this while that connector is disconnected (easy) or while that connector is connected (tricky). Use the car battery negative for the meter black lead, and use the meter red lead at pin-1 and then again at pin-2. Record your voltage test results.

Also, you do NOT have to use the car battery as ground reference. You can measure this voltage across CHT pin-1 verses pin-2.

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Old 05-01-2019, 07:34 AM   #4
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There is no voltage at pin 2 and there is .06 volts at pin 1. So im guessing the pcm took the hit. Sound about right marde?
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Old 05-01-2019, 11:23 AM   #5
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Yep. Highly likely the PCM is toast. This assumes your prior wire damage is properly repaired.
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Old 05-01-2019, 12:35 PM   #6
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Yes properly repaired, got new pigtail and sensor from ford and replaced both. Matching correct wires to new pigtail
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Old 05-14-2019, 07:27 AM   #7
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So just had new pcm programmed at dealer and it did not fix my problem. Any ideax?
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Old 05-14-2019, 06:05 PM   #8
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Quote:
Originally Posted by Tankf100 View Post
So just had new pcm programmed at dealer and it did not fix my problem. Any ideax?
Are you using any kind of diagnostic tools/code scanner as a failed PCM would have fault codes.
Did the dealer do any diagnostics on the old PCM or just swap the parts?

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Old 05-14-2019, 06:10 PM   #9
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I have checked voltage at cht harness and was only getting minimal voltage. Which led me to ask if someone else thought it could be pcm failure. I got another pcm and had the dealer swap and reprogram it. They did no diagnostics all they said was that there must be a short or bad cht since it was telling the pcm engine was running hot causing fans to stay on. But they said nothing as to why the temp gauge burys past cold after start up.
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