Hi, I'm bringing my Mothers old '98 Daimler LWB back to life and am looking for a bit of help.
Generally lovely car but has been in storage for 8 years so some issues.

Dead fuel pump, changed through the parcel shelf which got it running.

Dead alternator - changed and charging well, battery good.

Split hose behind intake - new one fitted, no discernible vacuum leaks I can find by spraying damp start around.

High pulsing idle and cutting injectors which after reading many posts I realise is due to the limp mode bypassing the TPS and pulsing the injectors. I have an 0121 pedal position sensor range error. I have a gearbox fault, limp mode, no traction control and no stability control, last two and maybe gearbox I believe what happens or related to the limp mode?

Live data shows 0.7v on both tracks at idle rising to 3.5v or so pedal on the floor. Both tracks seem very consistent and even thoughout the range. TPS stuck on 55% and 4.3v, I'm guessing the default limp setting?

It appears from what I've read that this is most likely to be a throttle body swap due to a mismatch between the TPS and the PPS. I have the early vacuum type TB with gold contacts on the PPS don't know if it was ever changed under warranty (that option must have expired now?) as the car was last on the road in 2004. No separate sensors available for this early one I think unless you start taking apart a Toyota Lexus PPS?. No rebuild options in the UK.

I've checked all connectors to the TB, the TP Motor relay (I pull the relay out and the motor moves) Have cleaned the power to the control module fuse box and the earths, currently looking for the other earths on the engine.
Due to the storage there has been plenty of heating and cooling so lots of corrosion on the exterior of the engine but connections do seem pretty good so far. I haven't cleaned the earths behind the TB on the firewall yet due to access.

My car seems to have a different fuse box layout to the manual in that the control module box isn't up my the firewall under the tool tray it's on the nearside inner wing towards the radiator (UK spec maybe?)

My main question is are the readings from the PPS correct 0.7 to 3.5? and how can you test the TPS if it's in limp mode?
Feels like a Ebay TB is the next logical option, sellers say they come from cars with no codes so it seems the best next step for around £100?

Thank you in advance.

 

Hi,

You'll probably find what you need on JagRepair.com - Jaguar Repair Information Resource all the factory electrical manuals etc etc are on there. From the sounds of TB you've got the AJ26 engine so it is a bit different as you're aware.

I'd agree with you that the trac/stability are probably related to the limp mode as they modulate the throttle so would need to know where it is. They are also the first ones to come up with low voltage too (but you'd normally see an ABS fault too then), so as you say it's not run for years & wasn't charging are you sure the battery is good & you're not chasing ghosts? I'd ignore them till you've solved the TPS though, they might well go when that's fixed.

 

ABS fault for a while but now cleared just amber/red lights on the dash. Traction/Stability/Gearbox. Changed both rear abs sensors as well just in case.
Battery is good and strong, 12.75 static 14.3 charging, also tried a new battery to take that out of the equation.

Thanks for the technical link, found the spec in the training guide, doesn't list the two sensors individually except to say voltages range from 0.5v closed to 4.75v fully open so pretty much a standard 0-5 sensor spec.

Removing the inlet again I noticed the dealer eyeball sticker on the TB from 1999 so guess it was changed way back when.
I've just taken the TPS off to test and I have a good ground and solid 5v supply, the sensor does give a signal as much as I can test by rotating it around the motor magnet that varies from 0.7v to 4v but they are always about 1v apart. Think I remember reading that Jaguar designed them that way?

So it seems both sensors are working but maybe not correctly and the difference in signals may be the root cause. As it's stuck in limp mode I can't monitor live data for both in a running situation plus even if I could I can't find any detail on how close the signals need to be for the ECU to be happy.

Think I have to try a SH TB as I can't think of anything else to try.

 

Probably has nothing to do, but have you checked if the gas pedal is doing its full travel? Maybe cable was incorrectly positioned and not creating enough tension?

Also related, maybe you can see what the bore plate is doing when you manually activate the accelerator cable reel, at least to discard any mechanical failure?

I read somewhere once that the motor activating the bore plate could get stuck/an issue with its bearing...maybe somebody can chime in.

 

Yes tried that, checked the live data by moving the pedal and the quadrant on the TB itself, they match. The butterfly is free to move but because it's stuck in limp mode the TP motor and TPS are disabled. The reading is near enough 50% at all times with volts being 1v out of sync. I'm guessing it's default state. It does rev up in neutral but the data readings from the TPS don't change.

The output from the PPS are in sync, and the TPS (Hall effect) are offset by 1v is that correct or should they both be either offset or in sync?

I can't find any data on how the outputs should correlate so hard of know where the issue may be. I do have a S/H TB on the way so I can fit that and compare data. Do hate not knowing for sure though but Jaguar see to keep the precise info top secret!

I'm tempted to drill a hole in the top of the induction tube above the throttle body and stick an endoscope in with a bit of silicone to see what it's doing but not sure that would achieve much.
In my short week of research It seems that this is such a common problem that once you've eliminated everything else it can only be the TB or at least one of the sensors being out of whack so you change it. I wonder if dealers test the sensors in detail and have precise data to compare to or do they just do the same thing?

 

We’ve had a rash of 97’ - 98’ XK8/XJ8 tow ins due to dead ECM’s. The next thing you should do before going any further is to pull the ECM out, remove the top panel of the ECM and inspect for any burn marks on the underside of the panel you just removed and anything that resembles any black goo on the circuit boards especially by the big capacitor.

 

I did see mention of that on a diagnostic site but it threw different codes but guess everything is worth checking.
Am I right in thinking it's behind the glovebox under the Pollen filter housing?

 

I love your bio interests!!! 😂😂😂

You remove the ECM from your passenger side in the engine bay. Remove the single screw and black passenger side plastic flat panel and remove. The other large black panel underneath houses the ECM, TCM and one or two other little modules I believe. There’s 4 T30 Torx (don’t lose any of those) screws that you need to remove and remove the panel. The ECM is right on top. Carefully remove the 6 electrical connectors and lift out the ECM. It’s just 4 Phillips screws for the ECM panel and cut the security tape.

 

Everything I've ever owned is made of rust and paint when I first get hold of it, I make it my goal to adjust the balance back towards the shiny side!

OK apart from the awful one way security screws on the cover and the ECM (I would have been perfectly happy with Torx) the circuit boards seem good as gold, no sign of anything burnt or overheated. I have a few more earths to check in the engine bay and a SH TB on it's way.

Really appreciate you guys advice and help, it's sent me off in the right directions as far as being able to nail down how everything works. The technical guides have made sense of what I'm seeing. My TB is the early one with the vacuum canister and the mechanical guard that drops into place in limp mode so I was seeing the butterfly move directly with the pedal (and the PPS data changing) but nothing for the TPS or the motor moving with ignition on. I now realise that that is all disabled when in limp mode. The motor does work if I pull the relay and put it back in so it feels like it is a sensor issue subject to cleaning a few more earths. I'd love to find a broken wire somewhere but so far everything is checking out ok.

My Range Rover is quite picky with voltages and the whole brake light switch traction/abs/gearbox thing too, similar vintage so I suspect similar setup but the Daimler seems far more delicate!

 

All looks good in there. It was important to look just for peace of mind.

 

Agreed, over the years I've learnt not to try and short cut stuff. The one tiny thing you take for granted and don't check is always the fly in the ointment. Just the engine strap left to clean up...

 

is this an aj26 or 27 ecm? the way these throttles work are totally different, 1998 isn’t specific enough because uk likes to assign registrations to build year not model year.

aj26 is what happens when japanese engineers do too many drugs the aj27 is simplified full electronic control same as all the early throttle by wire toyotas.

26 TPS is a hall effect sensor, pretty reliable i’ve never seen one fail usually it’s the mechanical guard sensor that goes before anything else. early aj27s have poor contact on the TPS and they’re pots so the tracks wear out eventually

 

It's a 26 with hall effect, the error error code is 0121, Pedal position sensor range so although the outputs from the pps are pretty consistent from 0.7 up to 3.7 for both tracks I don't know if that is what the ECM expects to see so I'm assuming that is still where the error lies. The mechanical guard is in place as limp mode is on and the butterfly moves directly with the pedal so I'm not getting any live data from the TPS which is therefore disabled, but I can get it to read by just rotating it around the magnet, again it reads consistently but the readings are about 1v out of sync (is that correct?)

Replacement TB should be here today so I will have something to compare with later. Only thing I have left to check is continuity of sensor wires back to the ECM but as I get live data from all of those I feel they are probably ok but will double check.

I was looking at the technical docs last night and the cutaways of the 26 TB and it does seem to be a ridiculously overly complicated mechanism , great when it works but I can see why it was redesigned!

 

Isn't it possible that your error code is because obviously, since the TPS is at a fixed position, it will always mismatch the PPS?

That will mean that the only solution to the equation is the wrong range in the PPS (3.5 vs 4.5).

Anyway, you'll know more with the new TB.

 

Yes entirely possible, don't know exactly how the diagnostic flow works but it does seem that it's a long interrelated chain so anything along that can effect all the others be it gearbox, abs, traction etc and send you off in the wrong direction. Thank goodness for forums like this though that are full of people who have experienced the same issues. I find after a few weeks of reading you start to join all the pieces of info together and it begins to make some form of sense.

Usually just before the next thing goes wrong.

 

Which means you're an expert on an issue your car never suffers again

 

Indeed. Makes for great dinner party chat and the Missus loves to hear about such things.

 

p0121 is for the TPS. 1123, 1221-3 are for the PPS. just throw another tb at it

 

That's what I read in the Jag documentation but my code reader (iCarsoft) says p0121 for the pps, pretty sure it's one or the other!

 

I think that's one of the times Jag used a non-standard meaning for the code. There's at least one other which generic readers often say is an O2 sensor but it's something totally different that I've forgot.