 | Home > Puma (Tdci) > Should the engine be holding revs between gears? |
 
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| andy63 Member Since: 30 Jun 2023 Location: north east Posts: 514  
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My understanding of the two switches.. Is one is normally open... One normally closed..
When pressed the normally open closes and provides an ignition fed supply to the brakes.. The normally closed opens and removes an earth connection to the ecu.. They are as mentioned used by the ecu as a plausibility function of the accelerator pedal position.. That's what I remember reading.. But don't really know the fine details.. Ie what earth path is removed and does that ignition feed play any part in the abs.. Dsc set up🤔 |
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6th Jun 2024 7:13am |
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| blackwolf Member Since: 03 Nov 2009 Location: South West England Posts: 17378 
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The wiring diagram makes it clear that both switches take a 12V ignition-controlled supply from CJB Fuse 13 (as does the clutch switch) and switch it to the ECM. Both switches are identified as S215 in the diagrams, but there are two separate connectors identified, C0029 and C0075. C0029-1 and C0075-1 are the supply pins.
C0075-2 is the switch connector pin which, when the pedal is depressed, energises the brake lights and trailer socket connector. This switch is therefore evidently an NO switch which closes when the pedal is pressed. The switched output of this switch is connected to the ECM on pin C872-E3 on an ECM signal identified as "BOO1", clearly active high. C0029-2 is connected only to the ECM on pin C872-F3, to an ECM signal identified as "B002", and nowhere else. This strongly suggests that it is an NC switch which opens when the pedal is pressed. Signal B002 is therefore undoubtedly "pulled down" internally in the ECM so opening the switch by pressing the pedal removes the 12V signal on B002 and effectively earths B002. This type of arrangement is extremely common in the design of such systems and serves to protect against the failure of either switch. I have no doubt that the purpose of BOO1 and B002 is, as I stated earlier, to protect against a failed throttle position sensor (TPS) which is giving an unwanted wide open throttle (WOT) signal and to ensure that pressing the brake pedal will override the WOT signal and stop the vehicle. This will firstly ensure a safe failure mode in the event of WOT TPS failure, but will also ensure that a failure of a single NO brake light switch in the closed position won't disable the throttle, which could be an equally dangerous failure in the even that the vehicle becomes immobilised. It is however completely unclear why a failure of fuse F13 results in the "rev hang" phenomenon, and there doesn't seem to be any logic in this. It should be readily detectable in the ECM SW that there is no power to the NC switch, and a more intelligent response would be to log a fault (and possibly an IPAC warning light). The ECM would not as it stands be able to differentiate between a failure of F13 and a failure of the NC switch. Incidentally an open circuit failure of the NC switch would probably cause the same "rev hang" phenomenon, and I can only assume that the "rev hang" phenomenon is the result of a programming oversight, a condition which nobody thought to consider. Although earlier I suggested that the OP check the brake light bulbs, upon reflection it is easy to understand how a failure of F13 could have this unintended consequence, but I don't see how failure of either or both bulbs could do it, since there should be 12V on the brake light circuit irrespective of the bulb status. It would be easy to test (by removing the bulbs or disconnecting the lights), but I suspect that actually it has to be a fuse or supply failure to cause the rev hang. The best advice in the future therefore would be to check fuse F13 rather than the bulbs. One interesting result of this investigation is that it should be both possible and in fact relatively easy to disable this "safety" feature to allow one to drive with a foot on the brakes to dry them out! |
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6th Jun 2024 8:19am |
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