I need help! My car went into Limp home mode about 2 miles from home. I got it home and the diagnostics started.
Here is what I have learned so far:
1) No fuel pressure in the fuel rail ( Motor will start and run, seemingly normal, with fuel sprayed into the throttle body ).
2) Replaced fuel pump. NOTHING CHANGED. I checked the old pump and it seems fine.
3) I was able to secure a fuel pump driver module at a reasonable price and replaced it. NOTHING CHANGED.
4) I was able to obtain a copy of the Jaguar Service Manual
5) Studying the fuel pump and driver module, I learned that is was controlled by a signal, received from the ECM, a 150 Hz PWM signal which the driver uses to control the speed of the pump and thus the fuel rail pressure. ( the signal is a 5 v. 150 Hz pulse with a duty cycle between 4% and 51% ). With a heavy background in electronics from decades ago in the USAF, I was able to duplicate this signal with a PWM square-wave signal generator and fed it to the input line of the driver module. In this condition, the motor will start and run, seemingly normal. So it would appear that the ECM is failing to produce this essential signal. WHY?.
4) By disconnecting first the crankshaft position sensor and subsequently the camshaft position sensors while still bypassing the fuel pump driver module, I concluded that those devices were functional. Disconnected = no start while connected = start !
5) In putzing around checking those sensors, I determined that the motor could not be revved above 3000 rpm. At this rpm, the motor was acting fuel starved and misfiring. Since I was in control of the fuel pump through its driver module with my signal generator and fuel pressure was being maintained at the rail, something else was responsible.
6) Recalling the initial symptom of the car alerting me of a 'Limp home mode' status, I suspect that I am chasing the characteristics of the 'Limp home mode'.

Some moron at Jaguar engineering decided that I was incapable of understanding and properly responding to an ALERT of a malfunction and decided that disabling my machine was a preferred alternative. Reminds me of a once owned Dodge Daytona that shut off the fuel pump if a monitored electric radiator fan did not get started quickly enough ! My wife will throttle that engineer if she ever gets hands on him. But I digress.

Has any of those illustrious Jaguar engineers ever listed the events that will trigger the 'Limp home mode' ? Is anyone there at the Jaguar Forums intimately involved with this ill-conceived concept? I have convinced myself that the car is still in the 'Limp home mode' and that something silly ( like a late responding fan ) is responsible. Can you help me with a recommended process for discovering the cause of this nightmare?

I need help !

Thanks
Paul E. Ouellette
PS I did get a very informative response from Motorcarman.

 

Model & model year? No OBD error codes? I had looked though the codes before and found OBD errors will go to limp mode but not disable fuel/ignition. I hope the 03 error code PDF file helps.
TPS/adjustment- frequently the culprit behind limp modes, but not to the point of incapacitating fuel and maybe spark, too.

 

It can be difficult to troubleshoot this as you can't 'see' exactly what's happening.

You've almost certainly eliminated the pump & its associated control module with your sig gen test. It would be worth pulling any DTCs as there are a few that relate to issues in this area.

Do you have access to an oscilloscope? If so, then I'd look at what (if anything) is being fed to the pump module from the ECM:- normal duty cycle is 4-51% as noted, but the ECM can push that to 75% to turn the pump off. See this snippet from the 2003 MODEL YEAR XK SERVICE TRAINING TECHNICAL GUIDE:

"
To support the installation of the 4.2-liter engines and continuing the drive for fuel
consumption benefits, a new fuel system including fuel tank, fuel pump, pipes, breather
pipes has been installed.

The XK now benefits from the use of the return-less fuel delivery system very similar to the
X-Type and S-TYPE models.

One major change on supercharged (S/C) variants is the use of only one fuel pump instead
of the usual two as used on all previous S/C models (XJR, XKR and S-TYPE R).

The fuel pump uses the same principle of operation as the on the X-TYPE where the ECM
monitors differential pressure across the fuel injectors and the injector pulse width and
duration to accurately calculate the fuel quantity being delivered to the cylinders. It uses
this to demand a specific fuel flow rate, which it communicates to a fuel pump driver
module located in the right hand side rear wheel arch.

The ECM use a frequency of 150 Hz (PWM) signal during average conditions, varying its
duty cycle between 4 and 50% to control fuel delivery rate or turning it to 75% to turn off
the pump.
"

Note that there is an electronic rpm limiter. The ECM limits rpm to ~3000 if the car is in neutral/park, so what you're seeing is normal behaviour.

 

2003 XK8 (VIN A32xxx) convertible. No OBD codes that made any sense to me at the time. My "Limp home mode" lasted long enough to get to my driveway where my "Crank / No start" odyssey began. I recall a DTC indicating a Fuel Rail Pressure Sensor which was replaced to no avail. Not needing the car, it was pushed into the garage until this week when my odyssey resumed.
Research in these forums and elsewhere leads me to the point where I will need to construct another dedicated computer ( 2001 BMW Z3 was 1st one ) about six or seven years ago.
As previously noted, only fuel seems to be disabled, lacking 5v.150 Hz PWM from ECM pin 27 to Fuel Pump Driver module pin 3. Replacing this signal with a PWM 5v. square wave generator at a 20% duty cycle was sufficient to activate the driver and produce fuel rail pressure and a resulting quick start of the engine. Modifying duty cycle modified fuel pressure therefore I conclude that the driver module and pump were OK.
As previously noted, putzing around checking crankshaft and camshaft sensors, I determined that the motor could not be revved above 3000 rpm. At this rpm, the motor was acting fuel starved and misfiring. Since I was in control of the fuel pump through its driver module with my signal generator and fuel pressure was being maintained at the rail, I concluded something else (Limp mode ? ?) was responsible.
I am open to suggestions as to what (other than an internal ECM failure) would cause the ECM to not produce this essential signal.
I am suspecting the Fuel Rail Pressure Sensor as the only input needed for the ECM to generate it. My efforts today will concentrate on verifying that no vacuum leak is present at the FRPS and that it is actually delivering its 0-5v. signal to pin 73 of the ECM.
The TPS (Throttle Position Sensor) adjustment idea is very much appreciated. Is there a Tech Bulletin on how this is accomplished?
Any and all assistance will be greatly appreciated and seriously considered.

Paul E. Ouellette

 

Thank you michaelh for your response. I learn something from everything I read.
I do have an oscilloscope and I will check the input to the Fuel Pump Driver Module on pin 3 for any signal coming from the ECM. I had ? assumed ? no signal since no pump operation, but your relaying the info about a 75% duty cycle turning off the pump is interesting. How is it reset? Would my injecting my sig gen signal reset it? Also thank you about the RPM limit in Park/Neutral. Very interesting ! Leads me away from my thinking the car was still in Limp Mode. Nice to know that the car is doing something that is normal behavior.

 

The PCM does not allow the engine to be revved beyond 3000 rpm if not under load to reduce the possibility of damage to the connecting rods and bearings.

One other possibility is the CKP, which sends a message to the PCM to keep the fuel pump running. If the CKP is faulty, the PCM believes the crankshaft is not rotating and shuts off the fuel pump.

 

Double check that the inertia switch isn't tripped or has poor continuity. It wouldn't cause your limp mode but a shutdown. Fuel pump grounds through that and then ignition switch. If you only used your PWM signal to drive the pump and grounded through frame, switch should be OK, but if you grounded through the pump wires, do check the switch that is half hidden under dash on left inner fender. ECM gets a ground signal if inertia switch is tripped so you'd have to think there'd be a warning signal displayed when tripped.

I just posted "TPS and PPS adjustment via ODB." ECM calls it quits and goes to the programmed limp mode after it is too confused when pedal position, throttle position, & throttle motor... don't jive or missing a feedback signal. Just adjusting the TPS manually throws it into "reduced performance."

Fig 03.1 shows the BPM gets serial signals from the immobilizer "security acknowledge" and "OK to fuel" which might be the go-ahead for the 2 second pump priming. You'd need the oscilloscope to witness that. ECM still has to tickle the pump into action.

I tried uploading the "jagxk2003 electrics" but it seems like this won't attach PDF files. It is likely I downloaded it and other very informative documents from this BBS.

 

Make sure the gauges and lights on the instrument cluster are working right. Jaguar routed a lot of signals through that- it is the only multiplexing module, translating signals between BPM and ECM.

 

I imagine that it would revert once the control signal from the ECM dropped back into the normal operating duty cycle range. There is also a 1Hz 'heartbeat' output from the fuel pump control module (pin 4) back to the ECM which might give you a little more of the story. Pretty low frequency, but you should be able to capture & measure mark/space with the scope. Apologies for the caPs LoCk - it's copy n' paste from the EG:

FUEL PUMP MODULE MONITOR: PWM, 1 Hz, 50% POSITIVE DUTY CYCLE = NORMAL, 25% POSITIVE DUTY CYCLE = NO CONTROL SIGNAL, 75% POSITIVE DUTY CYCLE = FUEL PUMP INOPERATIVE

 

Thank you NBCat, I appreciate your response. michaelh got ahead of you on the RPM limitation in Park/Neutral
As for the CKP (crankshaft position sensor), I believe that I have eliminated it and the camshaft position sensors from culpability as using my sig gen to run the fuel pump and disconnecting the three sensors one at a time would not allow a start and run while connected, the motor readily started and ran. Please advise if you think that I am making improper assumptions.

 

I thank you for your response. I did check the inertia switch, pressed it to no avail.
I am very interested in your post re: "TPS and PPS adjustment via ODB". I will be reading that as soon as I get caught up with this thread.
I have the jagxk2003 electrics already and thanks for that effort.

 

I am not as conversant with the Jaguar instrument cluster as I would like to be, however, it does seem to be working correctly.
I also wish that I was more acquainted with the acronyms used throughout these threads. A lot of hunting and disrupted reading could be avoided if you senior members would give us jr members a break and identify the acronyms you are using such as PCM, BPM, TPS, etc. Thanks in advance.

 

For all the sensors and the Powertrain Control Module (PCM) to correctly function, there needs to be sufficient voltage available. It's a good practice to always begin any diagnosis with determining the available voltage.

What is the voltage across the battery terminals with the ignition in the OFF position when measured with a voltmeter?

 

For all the sensors and the Powertrain Control Module (PCM) to correctly function, there needs to be sufficient voltage available. It's a good practice to always begin any diagnosis with determining the available voltage.

What is the voltage across the battery terminals with the ignition in the OFF position when measured with a voltmeter?

Does Powertrain Control Module (PCM) = Engine Control Module (ECM) ?

The voltage across the battery terminals with the ignition key in the OFF position =12.65 volts. I keep a trickle charger on it at all times to keep it up to snuff.

 

Yes, ECM is the same as ECU and PCM. The techs frequently use the latter, and it caught me out in the beginning.
Fortunately, there's only a few others in common use:

IC Instrument Cluster
BPM Body Processor Module
KTM Key Transponder Module
SL(C)M Security and Locking (Control) Module
TPS & PPS are the Throttle and Pedal Position sensors

Hope this doesn't come across as a lecture.

 

Some have pointed out the connections to the inertia switch can be corrupted from moisture under the fender so an open will prevent grounding through the ignition switch in run, but not alert the Electronic Control Unit. With battery disconnected, checking any one of the Red/Green wires, from Body Processor Module, trunk fuse box or engine fuse box (Figures 01.1 & 02.1 2003 electrics) to ground will verify grounding with ignition in run position for the Inertia Switch AND THROUGH that ignition switch circuit. Note the two fuse boxes ground via relays, other spots to swap relays around. I'm just stressing this as it is one problem area that shuts the engine down without throwing a code.

Just for your information, other valuable documents in my library for our '06 XKR...
XK8 '03-'06 Workshop Manual
Third Edition Released in 2003
The_2003_2005XK8_RevL
Service Manual 2003
Jaguar-XK-2003-service-training -guide
Jag training manual B
2003_2005_XK8 tech
2003 Update Training Manual
I'm always on the hunt for more information- no such thing as too many technical info sources. There are many questions that none of them answer.

 

Not at all, at 82 I am still willing to learn.And so, I appreciate getting good information.

 

Send me a PM (personal message) if you'd like to borrow my Toughbook with Jaguars SDD loaded on it. It, coupled with my mongoose clone OBD cable works with both my cars. You won't be able to reflash any modules, but it has a ton of monitoring capabilities. Takes awhile to get used to the archaic interface as well.

 

I used a sparkplug tester that you can get in any auto parts store and plugged it in line with the coilpack and it worked see video.

 

I really appreciate the offer and I may accept it. But first let me update anyone following this thread.

1.- In the wire harness that travels along the tank edge and goes to the top of the tank: I cut both wires that go to the pump !
2.- I installed a second fuel pump driver module and connected pins 1 & 2 to the fuel pump in the tank. Pins 5 & 6 were connected to keyed power and ground.
3.- I connected pin 3 to the output of a 5v. key-powered signal generator at 150 hz 23% duty cycle (ZK-PP1K 1 Channel 1Hz-150KHz Dual Mode Pulse Frequency PWM Signal Generator available on Ebay for $10.29 .
4.- CAR NOW SEEMS TO START NORMALLY ! This set-up produces a fairly constant 35 lbs of fuel pressure at the rail. CLEARED ALL CODES ! Ran the CAT for about a half hour down to the gas station, put in some fuel, and drove it home.
5.- Expected ALL MANNER OF NEW CODES TO POP UP, but I was ? disappointed ? when only a few popped up, including the check engine icon and the "dreaded" LIMITED PERFORMANCE message in the speedometer display !

HERE IS A LIST OR WHAT POPPED UP:
ECM abnormal 3 Engine Control Module
P1236 - Fuel pump not working when requested ( My note: The signal to the car FPDM and the monitor line from it are intact)
P1582 - Flight Data Recorder data is stored. (My note: Based on code reader provided info, this code results from "limp home" or "limited performance" action.
P1000 - System check not completed since last memory clear.
IP abnormal 1 Instrument Pack
0X00000 - Watchdog Reset ( My note: What is this ? )
ATC abnormal 2 Climate control (My note: The AC fan was blowing but not cooling ! )
B1266 - Left air intake servo motor - circuit failure
B1263 - Vent servo motor - circuit failure
BPM abnormal 1 Body Processor Module
U1041 - Vehicle speed not obtained.
DDM abnormal 1 Driver Door Module
B1236 - Door window glass feedback, loss of signal.
PDM abnormal 1 Passenger Door Module
B1236 - Door window glass feedback, loss of signal.
SLM abnormal 1 Security Locking Module
U1041 - Vehicle speed not obtained.

ALL OTHER SYSTEMS CHECKED AS NORMAL
SPEEDOMETER WAS READING NORMALLY