1998 Super V8. Intermittently cruise becomes unavailable - the message centre displays “Engine Failsafe Mode” and code P1797 is stored. My code reader gives “Neutral switch circuit malfunction” for P1797. Code U1261 is also stored “SCP circuit failure” but I can’t immediately see the relevance of this to the cruise fault. This is the Mercedes box so would I be right in thinking that the neutral switch is incorporated into the dual linear switch and that this switch or the connections to it are the most likely cause of the problem? Or could it simply be a matter of cable adjustment?
I’d bet my right arm this is a failing brake switch. I had the exact same issue on the first XJ8 I had and my original Super V8. I’m fully expecting it to crop up on the replacement Super V8 I now have!
Enjoy being upside-down in the driver’s footwell! Great exercise for your lower back!
JagFab and Robin - the brake switch does seem the likely culprit. I fiddled about under there today to see what was what - pulled and re-plugged the connector with contact cleaner and cruise engaged perfectly afterwards. If the fault returns I’ll break open the piggy bank and get a new pedal switch.
Might need more than the piggy bank they are reasonably expensive, if it does return I would try electrical switch cleaner into the micro switches first and after that maybe try to source replacement min,micro switches.
As other have already mentioned, your most likely culprit is the brake switch. Inside the switch body is a rotating cam. Somehow that cam gap closes up over time. I removed my switch, opened up the gap about 1/4" and reinstalled. That solved the problem and I didn’t have to purchase a new switch. Maybe MotorCarMan can explain better the operation of that switch and WHY Jaguar designed it with the adjustable cam.
Forgot to include some info on the microswitches. OEM micro’s were made by Cherry and are fully sealed. Spraying them with contact cleaner, etc. is fruitless as they are a sealed unit. They are also soldered to a circuit board making replacement unlikely. The switches are pretty bullet proof. Haven’t found a bad switch yet, just the cam gear gets out of adjustment!!!
Hi Frankie, Removing the switch is a real PITA. As I recall it takes a combination of metric wrenches and small sockets (7mm-9mm) PLUS some good tape (Duct, etc.) to re-install the switch. You will figure it out!!! Ha Ha Also look for a small wear area. A little JB Weld, smoothed out will fix that too! Hope this solves your problem. Haven’t had a problem with mine since I readjusted the cam striker. Saved a bit of cash too.
It failed again so I tried the adjustment suggested by Dick Wells. I have had limited opportunity to road test but so far its now working perfectly. I didn’t find removal/refitting too much of a problem and the following tips may help anyone else doing this.
After removing the single securing bolt it is necessary to depress the brake pedal to relieve the spring pressure on the switch so it can be wiggled free from its upper locating tab. On refitting, the brake pedal must be depressed to allow the top of the switch casing to engage in the top locating tab and the peg on the brake pedal arm to engage in the slot in the switch casing.
With the switch refitted and bolt tightened the brake pedal will only come up as far as the opened up ratchet adjustment mechanism of the switch will allow; to complete the adjustment the pedal should be pulled up against the resistance of the switch (clicks will be heard as the ratchet mechanism takes up the adjustment).
I did not find it necessary to make the JB weld repair suggested by Dick - even though wear was visible and the switch is original with 100,000 miles of service. It seems that adjustment alone adequately compensates for wear.
Thanks everyone for the advice - the piggy bank is still intact!!
they are reasonably expensive, if it does return I would try electrical switch cleaner into the micro switches first and after that maybe try to source replacement min,micro switches.
