Hi I am having issues with a 1998 XJ8 , runs great then when all of a sudden orange then red lights come on and car goes into limp mode and says transmission malfunction , here is what I did so far
1. checked all fuses good
2,did full smart scan , found 3 codes 1 is p1632 , other is p1795 and lat one evap purge ,
3 I real each ecm data and only thing that looks wiers is it says power to signal voltage , scanner says should be 0-6 volts , it reads 16 volts , I put a volt meter on batter and it reads 14.2 volts so seems normal , I then ran new grounds from battery to engine and engine to chassis ect , did not make no difference ,
4 , I then decided to disconnect ECM and also TCM and clean connectors and spray electrical cleaner into all on both sides and dry with air , this also did not cure anything ,
5, I decided for heck of it to disconnect battery and remove alternator output wire , I drove car with output wire missing , and on this road test seemed to run and shift fine , not sure if coincidence or it alternator can cause issue , what i do notice is this
on scanner with engine running and alternator disconnected the battery pos voltage which has value of 1-6v is now reading 12,2v which is battery voltage ,

Please tell me what you think??? p1632 and p1795 my next thing to check is throttle body cable and or maybe a different ecm??? why is it reading higher voltage when alternator is charging then it is also when it goes into fail safe is it normal for engine to feel like it has misfires but no codes showing up for misfires , but when it cleared runs perfect , I never had a alternator cause fail safe mode and it is reading normal on battery with meter but threw caanner on ecm it shows 16v when its messing up thanks LOU , ps 1998 xj8

 

The 1632 has to do with the alternator, check the tensioner pulley and belt. It possibly could be an intermittent issue with the alternator. Get it tested, could be a bad voltage regulator, you may need to get it rebuilt or replaced. I recall Bob saying he gets his rebuilt by a person who does Toyota work, but they new precisely which ones for his Jags. Hopefully he sticks his head in on this for some direction.

The 1795 is a throttle body code, but may be because of the 1632.

When you get down to the 12v range, your getting to where ghost codes appear for lack of proper voltage.

Check the ground lanyard off the battery as well, check this.... https://www.jaguarforums.com/forum/x...-strap-286910/

 

P1795
CAN token messages –inconsistent level

When fault is detected:
– TCM adopts transmission
mechanical limp home mode
(4th gear)
– ECM limits engine power

Possible Causes
CAN control module(s) software error(s):
ABS/TCCM, ECM, INST – check for
additional DTC(s) to locate control module
source
Incorrect control module(s) installed – ABS/
TCCM, TCM, ECM, INST

P1632 CAN loss of throttle data

When fault is detected:
– TCM adopts transmission
mechanical limp home mode
(4th gear)
– ECM limits engine power

Possible Causes
Other CAN control module CAN related
fault(s)
CAN open circuit fault – ECM to TCM
CAN short circuit fault
Throttle failure
ECM failure
TCM failure

 

If the ECU is experiencing an improper voltage condition (be it overcharging, poor ground, etc.) it may go into a protected state (not sure of the failsafe parameters). ECU dropping out or behaving erratically would explain CAN disruption, resulting in failsafe and the codes. Wish these had codes for "improper shutdown" like the new stuff does.

 

HI still having issues , I happened to look at data from ecm and pcm , I am reading 13.8v at tcm data and I see the ecm one of the voltages reads 15,6 volts , i did put my meter on battery with engine running i see only 14 volts , I cleaned connectors and also pins no better , and still getting p1632 and p1795 ,, I disconnected the alternator output , car ran good 2 road tests third one both codes came back and safe mode , can ecm be bad? here is data from scm and tcm thanks LOU

 

I just the other day here on my 2003 XK had a similar issue with the generator. The car had been sitting for a day or two after my other ordeals with the CAN bus, but at the moment everything was supposed to be taken care of.
I started the car in order to move it a little in my garage and after starting the engine i got a red warning "Generator Fault" ten seconds later folowed by "Gearbox Fault"..
With the engine stil running i tok a measurement across my new battery and could read 14.5 volts witch is quite perfect.

After some pondering over the electrical drawing i could notice that the generator has a dedicated lead that goes to the ECM. Futher investigation revealed that the ECM, while monitoring the voltage also moderates the generator output. As i understand it, the ECM increases the voltage of the generator a short while after start to boost back power to the battery and after a minute or so lover the voltage back down.
Shure enough, starting the car from cold today the generator code came back, but this time i waited for few minutes or so and to my surprice the warning disappeared.

So, im thinking that the voltage reading from the generator is erratic to the ECM due to either an internal fault or that the lead from generator to the ECM is done for.. I intend to clean the contact first and se if anything improves..
/E

 

Whisker, I just noticed there’s a discrepancy between the title of this thread states your car being a 98’ but, your profile lists your car to be a 99’. There is a big difference between these 2 years.

Keep in mind that these cars are old and to see a 15+ volt readout within your scanner should be ignored. You’ll get used to identifying what you should believe and what not to believe.

 

Hi, I am Klaus from Germany and have the Same Problem with an XJ8 1998. And I am already searching for a long time to fix this randomly hapening issue. Sometimes the car Runs 1000mls without trouble, sometimes it Happens several times a day. If it Happens, the car goes into fail Safe Mode, only Rolling up to 30 Miles is possible. 3 days ago I got an additional error P0790, which comes from the sport switch. But the switch is ok, I changed it against the switch out of a well Running xj8. I made a lot of Measurement alread, checking ground and voltage and so on. Couldn t find any unregular values. What I found out is, that all wires related to the faults, are going over EM53 connector. Can anybody Tell me, which connector is EM53, where is it located, how does it Looks like? EM53 Connects ECU and TCU, and I also had once the error of Bad Can Communication. So I would like to Check this important connector EM53. Even a picture of EM53 is welcome. Thanks Klaus

 

One additional info. I found a very Strange Voltage behaviour in the instrument panel, see photo. But I am Not shure, if there is a Relation between the Error Codes and this Strange voltage. Because this strange voltage is also shown in a well Running xj8. Any idea if this is normal?

Strange voltage from instrumental panel

 

Hello,

I think what you're looking for will be in the appropriate electrical guide here JagRepair.com - Jaguar Repair Information Resource

 

HI still having issues here is what I did since last post , I xhecked throttle body with lab scope at pedal sensor and tps both sweep normal no glitches at all no opens or drop offs

I got a 2003 jag here did data stream and compared both line by line with ecm and tcm everything was similar except
1 voltage is way off and it is in red meaning it is bad ,

Battery Positive, Central Processor Unit 1 current value 15.94v normal value 0-6v
that is with car at idle and battery voltage is normal at `14.2
I checked on another car it read 6v
I so car not overcharging Im stumped but same 2 codes p1632 and p1795

so I am thinking this hi voltage is causing the p1632 and putting the transmission in limp mode
I did order a used ecm plugged it in key on is
Battery Positive, Central Processor Unit 1 12v
car wont start with this ecm maybe needs program IDK but I was more concerend to see if volatge would be in normal range ,

when I google Battery Positive, Central Processor Unit 1 it seems to be the body module in back of glove box which controls accys it looks like I will see if I can access it and unplug it to see if that volatge does to zero ,

any more ideas I am stumped i do know when i do smart scan when codes come up only same 2 codes 1632 and 1795 but the trans module is red alsso but no trans codes
I am running out of ideas can a bad body module cause a hi voltage to Battery Positive, Central Processor Unit 1 ????????????? does it regulate can voltages?????????

 

A replacement ECM will require an IMMOBILIZATION SETUP to introduce it to the CAN bus.

The setup is required every time a different ECM is installed.

 

Hi whiskermouse, on which Controller did you measure the 15,8V? I remember, that I also measured such a value, I think it was on the Instrumental Panel Module. I will Counter Check in the two XJ8, I have here. My one is Running well, my friend’s XJ8 Shows randomly the same error Codes Like yours. As it is randomly, it is very difficult to find the cause. I cleaned all mass connections and last week the Regulator wires at the alternativ. That were very corroded. But anyway, I beleave we have any bus Communication Problem, and that Drives down tcu and ecu into limb Home mode. Sometimes I get the error Message of missing Can communication between those controller. My feeling is, the base Problem is around the transmission electronics. But all wires I checked are ok. Once I got. your errors, when I Stifted from D to N. Head up, going on searching🙂

 

Today I checked the Voltage on Both xj8. Both ECU Controllers Show 15.8V, and that is a correct value, it Must be more than 12 V. The shown setpoint of Max 6 V is totally nonsense. My diagnosis Scanner read the Same value from ECU. Don‘t worry about this. Do you have the error codes permanent? I have it only randomly. Today I mojnted a new Sport switch , it had a pending error P0790. This can also bring the tcu into limp home mode. If you have your errors permanently, then you may have a voltage Problem. It could be caused by defect ignotion contact plate​​​​​. Many guys gave me this hint.

 

I wasted countless hours on my first 2000 VDP with symptoms similar to yours. Finally replaced the pedal position connector on the throttle body and all the trouble went away. (Or at least that problem went away. Many problem have happened since then!)

Also, I never had an ECU go bad on the five 308's I have owned but I have had the instrument cluster cause a CAN fault on three of them. Replacing the speedo with a junk yard unit cleared the problems.
I understand your frustration; hope you get it resolved. I am about worn out fooling with my two cars. Even though they both run very well at the moment, they both have various issues that need addressing.

 

Worth a watch of this video, might give you inspiration!

 

I think I would take it to an alternator/starter rebuilder -- these guys rebuild Denso stuff all the time . . . they can sort out the issue faster than you can and solve your frustration.

 

David, what is the Junk yard? Ah I know..🙈