[xj40] Transmission Limp Mode - can't find the cause

Good day Gents!

About 2-3 months ago, my car started going in limp mode. It
would sometimes go away with stopping the car and simply
restarting. Other times it would not go away at all.

The transmission appears to have enough fluid. It looks to
be towards the max level when warm. I do not know when the
last fluid & filter change was.

I cleaned the throttle body in December of 2009. The
problems didn’t start until 2010.

About a month ago, I drilled a hole in the Throttle Position
Sensor and sprayed electrical contact cleaner inside several
times. It behaved a little better, but not majorly.
The voltage for the TPS was about .6 at both contacts before
I did this. I have not checked it since.

I opened up the plug for the connection to the TPS. I took a
small round file and cleaned up the inside of every female
plug. I removed each male pin, one at a time, and ran a
small file over them as well to make sure I had a good clean
surface. I sprayed them both with electrical contact
cleaner, then WD40 and put it back together. It has worked a
LOT better since then. I thought it was solved. It worked
for 2 days and then I had limp mode again.

A week later, I jacked the car up and opened the rotary
switch on the transmission. I sprayed that with cleaner. I
didn’t notice any corrosion or damaged areas. The round
connector was checked and looked fine. It worked for a day
or two and I’m back to random limp modes again.

This weekend, I attacked electronics under the dash. I
pulled the purple relay mounted on the right blower motor,
that looked nice and shiny. I removed the transmission
decoder module on the back side of the relay rack panel (far
right side). I opened that box and did not notice any
corrosion or damage. I sprayed the components and the plugs.

Next stop, check the TCM. The car was dead cold and sat
overnight.
Pin #19-># 5 was 37
Pin #19->#24 was 37.4
Pin #19->#42 was 37.6
This is not the 34 ohms mentioned in the XJ40 online book,
but seemed close enough to not be an issue.
Pin #19-># 6 was 6.2 (within the 5-7 range)
Pin # 2->#38 was 337.4 (a touch higher than the 300 +/- 10%,
but seems reasonable)
Pin # 7, # 9, & #26 all showed .000 to ground/earth.
Pin # 1 & #39 had 12v
Sprayed the connections and put it all back together.

Removed the ‘Sport’ switch by the gear shifter. Opened it up
completely - right down to the circuit board. Sprayed that
with cleaner and reassembled.

Connected the negative battery cable (was off for 2-3 hours
for all this) and started it up. Drove around for about 45
minutes with no problems at all whatsoever. I did local
street all the way to highway speed and back. No limp mode.

I drove to work this morning (less than 15 minutes away) and
I drop into limp mode as I make a right turn into the
parking lot.

Why am I still getting random limp modes?

My next guess would be to replace the TPS. I’d rather not
throw $250 at a guess. Can anyone see anything I missed, if
my numbers were bad, or what to look for now?–
1991 XJ6 (XJ40) Sovereign - 137,000 miles
Oyster Bay, NY, United States
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In reply to a message from Rick sent Mon 12 Apr 2010:

Unfortunately the only way I know to read codes for the TCM is with
PDU and most Jag dealers probably discarded it years ago. They
promised WDS would be backwards compatible as soon as the software
was developed but it never happened. They even provided the cables
to connect to the 1990 to 1994 cars. The most likely causes are the
TPS and the TCM relay (the blue ones were always failing
intermittantly)

Without PDU you have to guess at the TCM faults.

bob gauff–
The original message included these comments:

Why am I still getting random limp modes?
My next guess would be to replace the TPS. I’d rather not
throw $250 at a guess. Can anyone see anything I missed, if
my numbers were bad, or what to look for now?


Bob Gauff Owning and Repairing Jaguars Since the Early 70’s
Decatur, TX., United States
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In reply to a message from Rick sent Mon 12 Apr 2010:

To be honest I don’t think anyone has come up with a
definitive answer to a very common problem , I have had this
sporadically on my high mileage car and I’ve noticed that it
definitely occurs more when the battery is a bit run down in
the winter months .

I have as a result made sure this last winter to keep it
fully charged , I do a lot of short journeys so it never
gets fully charged by driving , as a result I’ve hardly had
it happen this year ,

Derek…–
The original message included these comments:

would sometimes go away with stopping the car and simply
restarting. Other times it would not go away at all.
The transmission appears to have enough fluid. It looks to
be towards the max level when warm. I do not know when the
I cleaned the throttle body in December of 2009. The
Why am I still getting random limp modes?
My next guess would be to replace the TPS. I’d rather not
1991 XJ6 (XJ40) Sovereign - 137,000 miles
Oyster Bay, NY, United States


Derek, kent '93 Daimler 4.0L solent ,'93 4.0L,westminster .
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I must admit that this has plagued me for a long time with the current car.
It is most embarasing to give the car a bootful in kick down mode to have
the trans go into limp mode.
I have changed out the TPS, the fluid is fine, the only other thought that I
have had is the wiring going across the trans tunnel may be a bit tired and
a possible short when the engine kicks at the point of gearchange.
Having said that I have had it occur randomly at start up when selecting
reveres to leave the garage.

Robin
'92 XJ40 4.0 lt----- Original Message -----
From: “happyshopper” delandcarol@hotmail.co.uk
To: xj40@jag-lovers.org
Sent: Wednesday, April 14, 2010 9:58 AM
Subject: Re: [xj40] Transmission Limp Mode - can’t find the cause

In reply to a message from Rick sent Mon 12 Apr 2010:

To be honest I don’t think anyone has come up with a
definitive answer to a very common problem , I have had this
sporadically on my high mileage car and I’ve noticed that it
definitely occurs more when the battery is a bit run down in
the winter months .

I have as a result made sure this last winter to keep it
fully charged , I do a lot of short journeys so it never
gets fully charged by driving , as a result I’ve hardly had
it happen this year ,

Derek…

The original message included these comments:

would sometimes go away with stopping the car and simply
restarting. Other times it would not go away at all.
The transmission appears to have enough fluid. It looks to
be towards the max level when warm. I do not know when the
I cleaned the throttle body in December of 2009. The
Why am I still getting random limp modes?
My next guess would be to replace the TPS. I’d rather not
1991 XJ6 (XJ40) Sovereign - 137,000 miles
Oyster Bay, NY, United States


Derek, kent '93 Daimler 4.0L solent ,'93 4.0L,westminster .
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services and resources including Photo Albums, Event Diary / Calendar, On
Line Books and more !

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In reply to a message from Robin Maureen sent Tue 13 Apr 2010:

I don’t know what the relationship is, but I found the plug
to the engine temp sensor had gone flaky and brittle from heat.

A good clean of the contacts seemed to cure my cold start
limp mode.

Any thoughts?–
The original message included these comments:

Having said that I have had it occur randomly at start up when selecting
reveres to leave the garage.


Crownwayne
Victoria, Canada
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I have just been through the CTS and cleaned the contacts. But I don’t think
there is any relationship with the CTS to the trans?
I know some vehicles have top/overdrive locked out until the temperature in
the ATF is up to operating temp but I don’t think the XJ40 has, Bryan??
I went on an autocross with our local MG club this past Sunday and the only
way that I could stop the limp mode kicking in was to ensure that I didn’t
give it the full welly on the throttle.

Still I was only about 3 seconds slower than the fastest MG (a midget with a
very hot MGF motor VVC, goes like stink!) :)))

Robin.
'92 XJ40 4.0 lt----- Original Message -----
From: “Crownwayne” vipg75@hotmail.com
To: xj40@jag-lovers.org
Sent: Wednesday, April 14, 2010 12:38 PM
Subject: Re: [xj40] Transmission Limp Mode - can’t find the cause

In reply to a message from Robin Maureen sent Tue 13 Apr 2010:

I don’t know what the relationship is, but I found the plug
to the engine temp sensor had gone flaky and brittle from heat.

A good clean of the contacts seemed to cure my cold start
limp mode.

Any thoughts?


The original message included these comments:

Having said that I have had it occur randomly at start up when selecting
reverse to leave the garage.


Crownwayne
Victoria, Canada
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services and resources including Photo Albums, Event Diary / Calendar, On
Line Books and more !

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In reply to a message from Robin Maureen sent Tue 13 Apr 2010:

Robin,

No temperature sensor in the ATF AFAIK! :slight_smile:

Just to show my ignorance on the subject further, I have
always been a little confused by the reference in the
Electronic Technical Guides to the ‘Limp Home Default’ in
the section dealing with EMS fault codes.

I understand what the term means but I don’t know if, when
an EMS sensor fault is detected and the EMS ECM substitutes
a nominal value for the missing input to ‘get you home’, the
limp home light appears on the dash and if so, the tranny
also remains in third gear as it does when there is a fault
in the tranny electrics.

Sounds unlikely to me - but what do I know? :-)–
The original message included these comments:

I have just been through the CTS and cleaned the contacts. But I don’t think
there is any relationship with the CTS to the trans?
I know some vehicles have top/overdrive locked out until the temperature in
the ATF is up to operating temp but I don’t think the XJ40 has, Bryan??


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from motorcarman sent Tue 13 Apr 2010:

My 94 had this problem once in a while. I found the solution to
the problem by changing out the relay module on the passenger side
firewall. Have not had this problem in over a year.–
OldFatGuy
La Vernia, Texas, United States
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In reply to a message from OldFatGuy sent Fri 16 Apr 2010:

I drove the car this weekend from New York to Virginia. This
tip is about 8 hours long each way - approximately 410 miles
each way.

I made it all the way to Virginia without a hiccup. It ran
great and the cruise control made it even nicer to drive.

On the way home, it did go into limp mode one time. This was
after I had gone over a bridge in Delaware and had to pay a
toll. I was the second car in line to pay. I let off the
brake for a second, rolled forward a touch and the limp mode
kicked in. I rolled up, paid my toll, pulled out and went
off to the side. I turned the car off for a few seconds and
limp mode was gone with the restart.

This was after about 12 hours of serious driving. If this is
what I have to deal with, it’s not so bad. For now I can
live with it.

I’ll change the TPS down in the near future and hopefully
that will be the end of it.

Thank you all for your help.–
1991 XJ6 (XJ40) Sovereign - 137,000 miles
Oyster Bay, NY, United States
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So got a “funny/not funny” story to revive this topic. Found a 2002 XJR low mileage (relatively) for estate sale. Inquired.

Took my 1994 XJS 6.0 for her weekend drive. She jumped and continues to jump into limp mode. 1st time ever.

Groan.

I think she’s jealous.

Any thoughts on “begging for her forgiveness?” I’m not more than a basic mechanic but love this car like family.

David, my car is also a '94 MY but with a 4.0L 6cyl. engine. In my case there are two main reasons for a limp home mode: 1, relay in a purple base in the engine bay, 2, another relay that’s in the transmission control module. Last time when the transmission was stuck in the 3rd gear, I opened up the TCM and noticed a cracked solder joint.

2 Likes

Thank you Joseph. I am a 4.0 6 cyl. as well. Have to look at my post as may have messed up the ID. I’m going “fishing” for the cure this weekend. So far all fluids check out. No obvious (to the eye) wiring issues. Did find some “unusual-as-they-all-seem-to-have-some” leakage mid-front under the engine. I’m thinking it’s electrical short/code, maybe brake or clutch area. Pure guesses.

I’ll also give your suggestions a try. Much appreciated.

I’ll likely surrender to my go-to “wizard” by Monday. But if I ever get a second shot at life it’s Jag mechanic for me. I envy the folks who fix.

Id service transmission, mine flashed transmission light and did a few weird things, no drive in drive for five seconds so i serviced it, all good.

1 Like

Thank you. I’m likely to end up there. Part of the “fun” of my own Jag experience is learning my way around mechanics.

As one knowledgeable mechanic said: maybe you should have started with an old Chevy.

I loved my Jag. Once got new relay modules installed (and a few for spares) i never had those problems. The ac modules fail sometimes also. My most aggravating problem was when the fuel pump failed. The tank has to be removed through the trunk area. After doing that twice I converted to an external fuel pump. No more problems. Unfortunately, I have gotten old and sold off my stable of cars. A young Dr bought the Jag and was tickled to get it. He told me later it really looked good in his parking lot.

Black Jag at bottom was mine.

1 Like

So, I feel compelled to tell you the abbreviated story of my XJS. Got it through an estate sale. I was thrilled. My dad (Rest In Peace) always wanted one. I loved him, but we butted heads. I only realized some things after he passed.

No regrets but wanted to honor him with “his car.”

I was doing a little interior fix and found some eye glasses. Obviously the former owner’s.

They now hang from my rear view.

I think of both my dad and the original owner every time I drive “their” car now.

Love the story about the doctor and your car. Sounds like it’s in good hands.

Like I hope my XJS is as well.

Continuing this to solution for the next poor Jag soul. Clearly looks to be electrical. Going to follow the “old soul’s” and other advice, have my relays replaced after a “code search” by someone who “bleeds Jag racing green.” Also found a leak under engine right side toward back of front right tire passenger side. Looks like the wiper fluid with a test.

Might be a little leak of something else, but all levels good and hoses/“rubber tubes with stuff running through it” appear intact and connected.

She’s going in next week to the “man.” I’ll post the fix. Thanks for indulging and help.

Never know who will benefit (other than me).