jetjock
06-12-2007, 03:35 PM
Since the code 51 confusion continues by those just learning here's an easily found (by searching) post and one people can point others too. First some background. I'll keep it simple:
The ECU is stupid. All it knows is what programming tells it to do. It's programmed to flash code 51 whenever the key is on *and* diagnostic mode (T1 and E jumped) is on *and* one or more of the following conditions exist:
1) The idle circuit is open. Part of that circuit is the switch in the TPS.
2) The air conditioning signal is active. In other words the AC button is illuminated.
3) The vehicle is not in Park or Neutral (if an automatic transmission) with the starter installed on the engine and connected.
Since condition 1 is almost always the cause of code 51 conditions 2 and 3 will not be discussed. That said be sure your AC is off and the vehicle is in P or N when troubleshooting a code 51.
The ECU needs to know when the engine is idling ie; when the throttle is fully closed. It does this by monitoring an input called IDL. When IDL is connected to ground the ECU judges the throttle to be closed.
The TPS is what connects IDL to ground. Inside the TPS there is a switch. One side of the switch is connected to the IDL wire going to the ECU. The other side is connected to ground. The switch closes (gets "turned on") when the throttle is closed. Therefore a code 51 should *not* be present at closed throttle if everything is working.
So you have a code 51. Assuming conditions 2 and 3 aren't involved the reason has to be either an open IDL contact in the TPS or open wiring in the circuit. Since an open IDL contact can be caused by the throttle being mechanically held open the first thing to do is verify it's fully closed. First, take your damn foot off the accelerator pedal (thanks Nick ;)). Next disconnect the throttle linkage and either look in through the 3000 pipe or back out the throttle stop screw until you see space between it and the throttle lever. Then bring the screw in until it touches and tighten the check nut. See the TSRM for details. If the throttle body is off the car all this is much easier to do. Course, if you find the throttle fully closed there's no need to mess with any of this, but the bottom line is the throttle plate must be fully closed as a starting point for setting the TPS.
Checking the throttle was done first because it effects other things besides the TPS. The second thing to do is check the wiring. You're going to do this by connecting IDL to ground, same as the switch in the TPS does. Unplug the connector from the TPS and insert a jumper wire into the bottom two pins on the harness connector with it oriented the same way it was when plugged into the TPS. In the TSRM these pins are designated IDL and E2. With the pins jumped ignore any new codes that come up but verify code 51 is no longer present. If it remains either the wiring is bad or your jumper isn't making a good connection.
With the wiring verified you can turn your attention to the TPS. With the throttle verified closed measure the resistance across the pins that mate with the ones you jumped on the harness connector. They will be the bottom two facing the TPS. They should be less than 2K ohms (2000 ohms), much less in fact. If not loosen the TPS and adjust it until they are. If you can't get it to less either the TPS is bad or the throttle is still cracked open. If the throttle is closed and you still can't get the pins to read less than 2000 ohms it's time to check the TPS off the TB.
Remove the TPS and twist the lever fully clockwise. Check for smoothness and that the lever returns all the way to a vertical position with the TPS connector facing left. If that's OK place the TPS face up on a table with the connector facing towards the left and again measure the resistance of the bottom two pins. Again, they should be 2k ohms or less. If not it's bad. If it is 2k ohms or less twist the lever slightly. The resistance should quickly switch to the same reading as when the meter leads are open ie; not connected to anything. If it does this the IDL switch in the TPS is good. Put it back on the TB and try adjusting it again.
As an aside once you get all this done try checking for code 51 by turning the AC on or placing the transmission in other than P or N. The engine doesn't have to be running. If code 51 pops up it proves the wiring between those items and the ECU is good ie; the ECU is seeing those two inputs.
It should be obvious you can test the idle switch at any time by entering diag mode and pressing the accelerator pedal slightly. Code 51 should come up and then disappear when the pedal is released. Another way is to raise rpm to around 2000 and carefully drop it 100 revs at a time while watching the tach and listening to the engine. Fuel cut should be indicated by an abrupt drop in rpm commencing at around 1500. Using the MIL is easier though.
Code 51 is not stored in ECU memory. It's a real time code that clears itself when the condition that caused it is corrected. Therefore no need to pull the EFI fuse the way you'd clear other codes.
One more note: Initial ignition timing (setting the CPS or distributor position) *cannot* be set unless the IDL contact is closed, even if you have jumped T1 and E in the diagnostic block. Do not attempt to set initial timing with a code 51 present. Do so and you'll be wasting time (pun intended).
Ok, so all this concerns what happens when IDL stays open. What happens when it stays closed? If it remains connected to ground when the throttle is opened? The TCCS will see increasing engine speed and airflow yet still judge the throttle to be closed. It's a condition that logically should never exist. In response the ECU will halt fuel injection at around 2000 rpm. It'll be restored once rpm falls to around 1200. It'll do this even if the pedal is held to the floor. Deliberately shorting IDL and E2 then opening the throttle is the method used to verify this fuel cut function is working, as it's used to lower emissions during deceleration. See the TSRM for details about this procedure.
I'll be editing this post as needed...
The ECU is stupid. All it knows is what programming tells it to do. It's programmed to flash code 51 whenever the key is on *and* diagnostic mode (T1 and E jumped) is on *and* one or more of the following conditions exist:
1) The idle circuit is open. Part of that circuit is the switch in the TPS.
2) The air conditioning signal is active. In other words the AC button is illuminated.
3) The vehicle is not in Park or Neutral (if an automatic transmission) with the starter installed on the engine and connected.
Since condition 1 is almost always the cause of code 51 conditions 2 and 3 will not be discussed. That said be sure your AC is off and the vehicle is in P or N when troubleshooting a code 51.
The ECU needs to know when the engine is idling ie; when the throttle is fully closed. It does this by monitoring an input called IDL. When IDL is connected to ground the ECU judges the throttle to be closed.
The TPS is what connects IDL to ground. Inside the TPS there is a switch. One side of the switch is connected to the IDL wire going to the ECU. The other side is connected to ground. The switch closes (gets "turned on") when the throttle is closed. Therefore a code 51 should *not* be present at closed throttle if everything is working.
So you have a code 51. Assuming conditions 2 and 3 aren't involved the reason has to be either an open IDL contact in the TPS or open wiring in the circuit. Since an open IDL contact can be caused by the throttle being mechanically held open the first thing to do is verify it's fully closed. First, take your damn foot off the accelerator pedal (thanks Nick ;)). Next disconnect the throttle linkage and either look in through the 3000 pipe or back out the throttle stop screw until you see space between it and the throttle lever. Then bring the screw in until it touches and tighten the check nut. See the TSRM for details. If the throttle body is off the car all this is much easier to do. Course, if you find the throttle fully closed there's no need to mess with any of this, but the bottom line is the throttle plate must be fully closed as a starting point for setting the TPS.
Checking the throttle was done first because it effects other things besides the TPS. The second thing to do is check the wiring. You're going to do this by connecting IDL to ground, same as the switch in the TPS does. Unplug the connector from the TPS and insert a jumper wire into the bottom two pins on the harness connector with it oriented the same way it was when plugged into the TPS. In the TSRM these pins are designated IDL and E2. With the pins jumped ignore any new codes that come up but verify code 51 is no longer present. If it remains either the wiring is bad or your jumper isn't making a good connection.
With the wiring verified you can turn your attention to the TPS. With the throttle verified closed measure the resistance across the pins that mate with the ones you jumped on the harness connector. They will be the bottom two facing the TPS. They should be less than 2K ohms (2000 ohms), much less in fact. If not loosen the TPS and adjust it until they are. If you can't get it to less either the TPS is bad or the throttle is still cracked open. If the throttle is closed and you still can't get the pins to read less than 2000 ohms it's time to check the TPS off the TB.
Remove the TPS and twist the lever fully clockwise. Check for smoothness and that the lever returns all the way to a vertical position with the TPS connector facing left. If that's OK place the TPS face up on a table with the connector facing towards the left and again measure the resistance of the bottom two pins. Again, they should be 2k ohms or less. If not it's bad. If it is 2k ohms or less twist the lever slightly. The resistance should quickly switch to the same reading as when the meter leads are open ie; not connected to anything. If it does this the IDL switch in the TPS is good. Put it back on the TB and try adjusting it again.
As an aside once you get all this done try checking for code 51 by turning the AC on or placing the transmission in other than P or N. The engine doesn't have to be running. If code 51 pops up it proves the wiring between those items and the ECU is good ie; the ECU is seeing those two inputs.
It should be obvious you can test the idle switch at any time by entering diag mode and pressing the accelerator pedal slightly. Code 51 should come up and then disappear when the pedal is released. Another way is to raise rpm to around 2000 and carefully drop it 100 revs at a time while watching the tach and listening to the engine. Fuel cut should be indicated by an abrupt drop in rpm commencing at around 1500. Using the MIL is easier though.
Code 51 is not stored in ECU memory. It's a real time code that clears itself when the condition that caused it is corrected. Therefore no need to pull the EFI fuse the way you'd clear other codes.
One more note: Initial ignition timing (setting the CPS or distributor position) *cannot* be set unless the IDL contact is closed, even if you have jumped T1 and E in the diagnostic block. Do not attempt to set initial timing with a code 51 present. Do so and you'll be wasting time (pun intended).
Ok, so all this concerns what happens when IDL stays open. What happens when it stays closed? If it remains connected to ground when the throttle is opened? The TCCS will see increasing engine speed and airflow yet still judge the throttle to be closed. It's a condition that logically should never exist. In response the ECU will halt fuel injection at around 2000 rpm. It'll be restored once rpm falls to around 1200. It'll do this even if the pedal is held to the floor. Deliberately shorting IDL and E2 then opening the throttle is the method used to verify this fuel cut function is working, as it's used to lower emissions during deceleration. See the TSRM for details about this procedure.
I'll be editing this post as needed...