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XJ XJ6 / XJR6 ( X300 ) 1995-1997

tach stopped working

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  #1  
Old 10-09-2013, 12:35 AM
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Red face tach stopped working

Hello everyone

As I mentioned, I finally got my 95 XJR running, after 4 years, due to a sheared woodruff key, ( go figure ) anyway the tach stopped working within a few minutes of starting. It still starts and runs fine, just no tach, I know it is not the cps, as it still starts and runs ( and I replaced it, with same results ) I also swapped out the ecu as I have a spare ( also no difference ) so does anyone know where and how the tach gets its signal from. The car drives fine but it trips the transmission light, and throws codes
P1775 (general transmission fault ) and
P0726 ( engine speed input circuit range/performance ) and
P0727 ( engine speed input circuit no signal ) as from what I have read on this forum the trans light and code 1775 for the trans would be expected as the trans says its turning, but it is not matching the engine rpm, as it is not getting the signal that the engine is turning. So the tach must get its signal from somewhere other than the cps. I have been searching the forums for answers. I even have the useless disc from JTIS but I am not able to find anything usefull at all.

Any ideas, if its just a bad tach that would be good, but I find it hard to beleive that the transmission gets it engine speed signal from the tach

Does anyone know

Darryl
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  #2  
Old 10-09-2013, 03:49 PM
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Darryl:
It ain't so simple- The "tach" is an electronic function that is absolutely DERIVED from the CKPS, (not the CPS). Obviously your CKPS is working or the car wouldn't run. I am not at my computer with the diagrams, but I am pretty sure the ECU, which gets the CKPS signal, sends the derived speed to the instrument module and the transmission module over a network connection. As I said, I don't know the exact connection on an X-300 but I will look it up in a day or two for you if someone else doesn't chime in before then. In the mean time, i would look for loose plugs on the TCU and the instrument module.
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  #3  
Old 10-09-2013, 05:29 PM
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I just came across this same issue this morning! As I'm heading to work, I noticed that my fuel gauge was on Empty (which I definitely didn't leave it at), and my gauge for coolant temp was all the way down (assuming the car still needs to warm up). So i'm driving to the gas station when I notice that none of the needles are moving on the Tachometer or other gauges. I was hoping to see that this is a simple fuse type of situation, but i'm not to savvy on cars when it comes to the electrical lingo. If I could get a hand from anyone it would be appreciated!
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  #4  
Old 10-10-2013, 10:18 AM
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Smile thanks for the help

hello sparkenzap

In my post when I said cps, I ment the crank position senser, that is what I replaced, and as you noted it starts and runs fine so it has to be working. I will continue checking conections for trans and such.
It did the car no favors sitting for that long, it had a bunch of issues the first couple of days, ( traction, abs, etc ) but all that seems to be left is the non working tach and trans light.
I really appreciate everybodys help

Darryl
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  #5  
Old 10-17-2013, 12:39 PM
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Default no tach

hello sparkenzap


I was wondering if you found those link connections for the tach, I did check all the transmissions connections, the 3 at the tranny and the TCU. It is still running good, just no tach and I have noticed that the fuel gauge every once in a while wobbles a little bit, I would think that it is just a instrament pack issue except for the fact that it trips the trans light and and throws the code for no engine speed signal, and like I said before I cannot beleive that the trans TCU gets its engine speed signal from the inst pack but I guess it is possible,
Any ideas anyone ?

Darryl
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Old 02-13-2016, 03:52 PM
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I know is an old thread, but was this issue ever resolved? I have a 96 XJR with similar symptoms. It runs and drives pretty good but the tachometer doesn't operate and the transmission fault light is on. The check engine light is on with a fault code of p1775. I have replaced the crank position sensor as maintenance but that didn't do anything for it.
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Old 02-14-2016, 06:44 AM
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The crank signal is an input to the ECU. That is obviously working as the car runs.

There is a single output of engine speed from the ECU to the Tans, Instrument Cluster and AC. I'd be suspecting a dodgy ECU plug/wiring. Check for moisture and corrosion in the plug and check for signal out of the ECU.

The pins to check etc are in the Electrical guides that can be downloaded from the links on forum stickies.
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Old 02-14-2016, 08:02 AM
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My most recent X-300 and number of other reported cases have had corroded pins on the ECU. That's the module in the RH front kickpanel. I would look there first.
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  #9  
Old 02-15-2016, 07:48 PM
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Default non working tach, etc

Hello to all, I just got a couple of emails from the site regarding this post. ?

so here is what I have found, there is one wire ( purple ) that comes out of the ECU that then goes through some connectors and then into the Transmission ECU and then back out of there and then into the climate control ECU ( these are all in the passenger foot well ) and then it goes up to the tach. ( the wire stays purple the whole time in and out of each component )
This is the engine speed signal wire it is supposed to have a specific frequency at a given RPM at 5 volts and then that frequency changes with RPM ( I do not remember the numbers right now, this was quite a while ago, but those numbers are in the manual ) So I bought a small cheap oscilloscope for like $100 to see what the ECU was putting out. And guess what it was putting out the exactly right signal, as was every link in the chain, ( every connection, every component all the way to the end at the tach )
I then checked the signal on another Jag and it was the exactly the same. I then pulled the instrument cluster out of both cars and swapped them, and the non working one works just fine in the other car, and the working one stops working in the problem car.
I should mention at this point that all 3 components involved do not work correctly. The transmission will not shift into 4th gear, and the climate control system does seem to know what to do regarding temperature so most of the time blows cold air not the set temp. But if you put it in diagnostic mode everything in the system works and it makes heat.
So there you go I spent a huge amount of time on this issue and still have no idea.
But I hit a deer recently, so now the car is totaled so it is a mute point for me. Sorry I cannot be more helpful.


As an aside I wanted to let everyone know that I had a leaking diff issue, so pulled the whole assembly out and pulled it apart and found the spider gears were galling, so you cannot buy them alone ( of coarse ) so you have to buy a whole rebuilt diff $$ or a used diff to get the spider gears out of ( with no gaurentee they are any good ) and it seams nobody know anything about the Sailsbary diffs in these vintage jags, they nobody even seems to know who actually builds them. we all know that the older Jag diffs had DANA 44 guts but after 1987 it seems to be a mystery. So the other option was to buy a new after market unit for a DANA and see if it would fit. So I bought an brand new Auburn limited slip unit for a DANA 44 for $140 and took it to a locale diff shop, and told them to see if it would fit, and I am glad to tell it does. So with bearings and seals and install for another $ 400 or so. Way cheaper than $1600 for an exchange rebuilt unit, even though you nobody sells spider gears to rebuild them with.
So spread the word that in the XJ & XJR 95 to at least 97 and probably 88 to 94 as well that you can just put in an Auburn DANA 44 unit.

Darryl
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  #10  
Old 02-16-2016, 07:31 AM
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Climate Control module uses the speed signal to modulate fan speed to compensate for ram-air pressure. So it wouldn't have any sensible effect on temperature - beyond that you'll get more of what you're asking for (hot or cold) as the airflow through the evap or heat core slows down, more heat-transfer will occur....but I digress...to determine if the speed signal is "Knackered" as far as the Climate Control is concerned, look for your fan speed to not speed up when you come to a stoplight - not always noticeable, but frequently so.
I realize you are apparently done with the car, but post for anyone else fighting this problem...thanks for posting back the follow-up!
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