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Transmission relay location?

 
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Old 02-22-2019, 06:15 PM
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Default Transmission relay location?

1994 4.0 XJS coupe. I read that theres a transmission relay that could be responsible for intermittent transmission light coming on and resulting in hard shifts. Anyone know where I can locate the relay?
 

Last edited by Brewtech; 02-22-2019 at 09:15 PM.
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Old 02-22-2019, 10:52 PM
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I found it! Its inside the boot on the right quarter panel, but its tricky to find because, the fuel pump, reverse light, and transmission relays are lined up behind a panel that lines the trunk, close to the butress. Theyre not in plain view.
I moved the transmission relay in and out of its seating and plugged it back in order to reestablish a proper connection. I think it worked, because I went for a good drive and i havent experienced a problem yet. Cleaning contacts, are now on my list.
 

Last edited by Brewtech; 02-23-2019 at 12:26 AM.
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Old 02-23-2019, 01:11 AM
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At the age the relays are at, the fix might be temporary. New relays are cheap, any auto relay with the same pin numbers will work.
 
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Old 02-23-2019, 03:30 PM
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Originally Posted by Brewtech View Post
I found it! Its inside the boot on the right quarter panel, but its tricky to find because, the fuel pump, reverse light, and transmission relays are lined up behind a panel that lines the trunk, close to the butress. Theyre not in plain view.
I moved the transmission relay in and out of its seating and plugged it back in order to reestablish a proper connection. I think it worked, because I went for a good drive and i havent experienced a problem yet. Cleaning contacts, are now on my list.
What does the relay actually do? Is this for V12 as well as AJ16 4.0?
 
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Old 02-23-2019, 06:20 PM
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Originally Posted by carsnplanes View Post
What does the relay actually do? Is this for V12 as well as AJ16 4.0?
The relay is for the 4.0 only. It provides power to the transmission control module.

 
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Old 02-24-2019, 10:41 PM
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Correct. The 4.0 came with a ZF4 and it uses a control module which is powered via relay. If the module is not working, then shifts dont match engine speeds. It results in hard shifts or limp mode. The V12 uses a GM transmission. You might find your fuel pump, and reverse light relays in that same location though.
The fact is, that a shop could recommend a service🙄 or a rebuild to no avail if the module is not being powered by the relay. Sometimes the module does go bad, but thats rare. I drove the car about 400 miles today and it held steady. Shifted like it should. So smooth, that the suspension became my point of interest,1 hour into my drive.
 
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Old 02-25-2019, 06:04 AM
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Originally Posted by Brewtech View Post
Correct. The 4.0 came with a ZF4 and it uses a control module which is powered via relay. If the module is not working, then shifts dont match engine speeds. It results in hard shifts or limp mode. The V12 uses a GM transmission. You might find your fuel pump, and reverse light relays in that same location though.
The fact is, that a shop could recommend a service�� or a rebuild to no avail if the module is not being powered by the relay. Sometimes the module does go bad, but thats rare. I drove the car about 400 miles today and it held steady. Shifted like it should. So smooth, that the suspension became my point of interest,1 hour into my drive.
Interesting...So the module is not always powered when there is normal ignition power? So if the module detects a difference between shift commands and engine speeds, it takes itself out of the picture via cutting power to the relay?. Interesting that it commands an outside power source (relay)to do the job rather that itself internally to the module. Most modern car's modules go into limp mode internally, trigger a CEL light and limit gear shifts to one gear all within the module,but never cutting power to itself.
 
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Old 02-26-2019, 01:28 AM
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Originally Posted by Jagboi64 View Post
At the age the relays are at, the fix might be temporary. New relays are cheap, any auto relay with the same pin numbers will work.
youre right. Im going to figure out what replacements are best for this day and age. Im sure the blue hella relays have been revised several times.
 
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Old 02-26-2019, 03:37 PM
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Originally Posted by Brewtech View Post

youre right. Im going to figure out what replacements are best for this day and age. Im sure the blue hella relays have been revised several times.
I have been going to the junkyard and using the high beam relays out of various Fords. I figure few people use their high beams, so the relays will have relatively few cycles on them.
 
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Old 02-27-2019, 02:44 PM
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Originally Posted by Jagboi64 View Post
I have been going to the junkyard and using the high beam relays out of various Fords. I figure few people use their high beams, so the relays will have relatively few cycles on them.
what do those look like?
 
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Old 02-27-2019, 05:56 PM
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They are just standard cube relays. I use them because they are easy to find at the self serve junkyards.


 
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Old 02-27-2019, 07:28 PM
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Thanks!
 
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Old 03-03-2019, 12:36 PM
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Everything was fine until, I went fo a spirited drive and then a transmission light popped up again. So... I replaced the relay, but the transmission keeps going into hard shift mode. If I shut the car off it will reset, but its only a matter of time before it trips again. What could be causing my issue? Transmission has been serviced
 

Last edited by Brewtech; 03-03-2019 at 12:54 PM.
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Old 03-03-2019, 01:20 PM
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I just returned from a short trip and it went into limp
mode on my driveway. I checked the relay and it was hot. Probably more than it should be. TCM issues?
 

Last edited by Brewtech; 03-03-2019 at 01:29 PM.
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Old 03-03-2019, 08:43 PM
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Very interesting stuff... So I decided to use a different relay from the engine bay. But cleaned up everything really good, and now everything is as it should be, but Im going to order the JLM21842 relays and replace all of them. Im sure the old blue relays are not up to snuff for the transmission module anymore due to mileage. The car does have 175,XXX miles on it, I bet those blue relays are half or close to that. When I ordered the first replacement, I was given a cheap part, and its not built for the demand. So before I go into the module, Im going to use a JLM21842, and see if this fixes my issue
 
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Old 03-03-2019, 09:00 PM
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I also order relays from Digikey, there are some good ones there. It's nothing Jag specific about the relays. If you really wanted to go upscale, you could get 40 amp relays instead of the usual auto relays that are 30 amp.
 
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Old 03-04-2019, 08:22 AM
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Take a look at the diagram. Seems there is an input to the TCM called "Fluid temp Sensor". perhaps your fluid is getting too hot and going into limp mode. This may be caused by a blocked transmission cooler line to the radiator or something to that effect. Is the relay you keep replacing the power relay or the warning relay?
 
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Old 03-04-2019, 08:28 AM
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Your power relay controlled by ign switch (52) out of the Inertia switch
 
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Old 03-04-2019, 10:48 PM
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thanks! Im going to spend some time looking at those layouts. I see the transmission warning relay in the diagram. Where is that located in the car?
 
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Old 03-05-2019, 12:11 PM
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Originally Posted by Brewtech View Post
thanks! Im going to spend some time looking at those layouts. I see the transmission warning relay in the diagram. Where is that located in the car?
Here ya go...


 

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