PCM trouble?
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5th Wheeling
Joined: Mar 2019
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PCM trouble?
Hello alll. I have an issue with my 94. I recently removed the engine for new gaskets and heads. After reinstall it started right up for a quick test of function,~3 seconds. Then it sat over weekend while I was out of town. Now it shows low voltage codes for tps, maf, and charcoal canister. It will occasionally start and then die. I have 40 pounds fuel pressure, good spark.
One problem I have found is the tps signal return shows no voltage at any time. The tps signal return shows continuity to ground. Is this normal?
I have gone ahead and removed the pcm and opened it up. It does have some corrosion on the inside wall.
Any ideas?
1994 f150 2wd 5.0
One problem I have found is the tps signal return shows no voltage at any time. The tps signal return shows continuity to ground. Is this normal?
I have gone ahead and removed the pcm and opened it up. It does have some corrosion on the inside wall.
Any ideas?
1994 f150 2wd 5.0
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
What are the codes by number and when were they displayed?
Completely normal. Signal Return is a dedicated path to ground for several key sensors. Did you measure VREF with reference to ground?
Completely normal. Signal Return is a dedicated path to ground for several key sensors. Did you measure VREF with reference to ground?
Fleet Owner
Joined: Oct 2005
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From: Kentucky
There is never a mix of two AND three it's one or the other. Your truck displays three digit codes only. I assume you are counting flashes to run the Key One Engine Off (KOEO) tests and Stored Code display?
Fleet Owner
Joined: Oct 2005
Posts: 20,789
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From: Kentucky
Try running the KOEO tests and Stored Code display again. As I previously stated you will never get a mix of two AND three digit codes. It just does not happen. What code reader are you using?
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5th Wheeling
Joined: Mar 2019
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Will do tonight. FORD OBD 1 code reader 1981-1995, it doesn't have a brand. Thank you for your help.
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Fleet Owner
Joined: Oct 2005
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From: Kentucky
If that no name brand reader is still displaying Code 35 I suggest you change it's battery then retry again. If you still get Code 35 chuck it in the trash can then run the tests/code display using a jumper and count flashes.
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5th Wheeling
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I took pictures of the codes as they came up. Friday at 6;40pm I got, 35 then 565 then122 then 157. I understand when you say it can't, but it did. I am assuming since I removed the pcm all previous codes will be erased. The 35 I believe relates to the smog pump. Smog pump has been removed.
Last edited by llama; Jun 13, 2022 at 12:00 PM. Reason: I live in Mississippi.
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5th Wheeling
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I checked the codes last night. I got 565, 122,157,111.I may have miss stated yesterday. When i stated the signal return had ground continuity. It is the TP wire that has a beginning reference of ~1 volt and goes up to ~4.9 volts with throttle wide open. This is the black wire on the TPS. It has no voltage and does show continuity to ground. Sorry if I used the wrong terminology yesterday..
Fleet Owner
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From: Kentucky
The black wire is Signal return.
1988-1991 TPS Wire Colors
VREF - Orange/White
TP SIG - Dark Green/Light Green
SIG RTN - Black/White (My 1990 is actually Red/Gray)
1992-1996 TPS Wire Colors
VREF - Brown/White
TP SIG - Gray/White
SIG RTN -Gray/Red
Motorcraft TPS Wire Colors
VREF - Orange
TP SIG - Green
SIG RTN - Black
Code 122: (KOEO) TPS circuit below minimum 0.6 volts.
Code 157: (CM) MAF sensor went below 0.4 volts during the last 80 warm-up cycles.
Code 565: Canister purge circuit failure
Verify the CANP solenoid is plugged in
Code 111: System Pass
1988-1991 TPS Wire Colors
VREF - Orange/White
TP SIG - Dark Green/Light Green
SIG RTN - Black/White (My 1990 is actually Red/Gray)
1992-1996 TPS Wire Colors
VREF - Brown/White
TP SIG - Gray/White
SIG RTN -Gray/Red
Motorcraft TPS Wire Colors
VREF - Orange
TP SIG - Green
SIG RTN - Black
Code 122: (KOEO) TPS circuit below minimum 0.6 volts.
Code 157: (CM) MAF sensor went below 0.4 volts during the last 80 warm-up cycles.
Code 565: Canister purge circuit failure
Verify the CANP solenoid is plugged in
Code 111: System Pass
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