Question: Field service mode
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Question: Field service mode
On a 87 350 TPI Z28, what does it mean if the engine runs good in field service mode
(ie with A and B connected on the ALDL while the engine running) and runs very roughly in regular mode? Is my chip bad or could it be something else? As I understand the computer does not run off the chip while in FSM, right?
(ie with A and B connected on the ALDL while the engine running) and runs very roughly in regular mode? Is my chip bad or could it be something else? As I understand the computer does not run off the chip while in FSM, right?
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iTrader: (2)
Joined: Nov 2000
Posts: 1,188
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From: Ft Worth, TX USA
Car: 2016 Ram 1500
Engine: 3.0L Diesel
Transmission: 8sp
you have some thing there .... oh man this sucks I cant remember what that means but I think when you run the field service mode it bypasses a sensor or does something like that.
I guess i am just verifying that you can narrow down the problem this way but I will have to appeal to a more knowlegeable person than myself to recall exactly what field service mode bypasses.
Im thinking vader should definately know this but i think I actually read something about it in my chiltons of all places.
vader.....
I guess i am just verifying that you can narrow down the problem this way but I will have to appeal to a more knowlegeable person than myself to recall exactly what field service mode bypasses.
Im thinking vader should definately know this but i think I actually read something about it in my chiltons of all places.
vader.....
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The field service mode is designed to be used to check the car while its running. What you are not supposed to do is start it with the terminal grounded... although I have accidentally done that and had no problems. I dont recommend trying to see if you have my luck starting the car with the connector grounded tho 
Anyway, what it does is it ignores most everything and supplies fuel from the base fuel map and base timing settings. Have you gotten a code from the car? Try disconnecting the TPS and starting the car and driving it, but dont ground the diagnostic connector. Thats my guess anyway.
Anyway, what it does is it ignores most everything and supplies fuel from the base fuel map and base timing settings. Have you gotten a code from the car? Try disconnecting the TPS and starting the car and driving it, but dont ground the diagnostic connector. Thats my guess anyway.
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Kris,
Field Service Mode really shouldn't change any operational parameters, only monitor the fuel offsets and ECM feedback mode, and relay that information through the MIL lamp.
Field Service Mode
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
Backup Fuel Mode
A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.
If your engine seems to run better in this mode, there is likely a sensor input that is reporting incorrect data to the ECM. Likely suspects are the oxygen sensor, CTS, and MAT sensor, but a MAP or MAF can skew the mixture as well.
------------------
Later,
Vader
------------------
"No matter how hard you try you can't stop us now..."
Adobe Acrobat Reader 4.0
Field Service Mode really shouldn't change any operational parameters, only monitor the fuel offsets and ECM feedback mode, and relay that information through the MIL lamp.
Field Service Mode
On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.
WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.
If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.
Backup Fuel Mode
A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation. Backup fuel is usually around 12:1 or richer from the factory.
If your engine seems to run better in this mode, there is likely a sensor input that is reporting incorrect data to the ECM. Likely suspects are the oxygen sensor, CTS, and MAT sensor, but a MAP or MAF can skew the mixture as well.
------------------
Later,
Vader
------------------
"No matter how hard you try you can't stop us now..."
Adobe Acrobat Reader 4.0
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