P0778 Transmission Fault Explained, illustrated
 

My daughters 2004 Jaguar has a check engine light with and the display reads transmission fault. Using the Torque app on my old android I was able to pull P0778 Pressure Control Solenoid "B". But what does this really mean, and can this be applied to other problems. I'll put this into multiple posts so I don't accidentally close the window and lose everything.

http://nylyon.com/Jaguar/2004_Jaguar_XType.png



** since I had < 5 posts there will need to be moderator approval before all of the posts show up.

Most of this information was derived from the Thermo in the following thread: [here] however, I found that some of the illustrations were incorrect or unclear so I put this together to hopefully clarify.

First, what we are looking at is an electrical error code. Essentially what the P0778 (and other codes) mean is that the TCU (Transmission Control Unit) has found an open or a short between the ECU and the identified solenoid. It is that simple. I don't know if this is a complete list, but in my opinion this open / short can be:

1. The solenoid
2. Wiring between the solenoid and TCU
3. The TCU (these get wet internally, corrode and can give errors).

I can't think of any other cause for this, but how do we check? First, we need to see what we're looking at, here's a chart of the codes and what we're looking for, note P0778 is highlited.
http://nylyon.com/Jaguar/codes.png

This is saying that 2-4 Brake DUTY Solenoid has an open or short, but where is the 2-4 Brake Duty Solenoid (it is part #10 in the following diagram):

http://nylyon.com/Jaguar/valve_body.png

I used the description from this table:

http://nylyon.com/Jaguar/valve_body_reference.png


If you can't read the diagrams, here is the source: [clickie]

So now we know what we are dealing with, let's show why this is straight forward. Looking at this illustration you can see the 2- Duty Solenoid (pin 16) is a single wire running from the solenoid to the TCU.

http://nylyon.com/Jaguar/trans_tcu_wiring.png

So somewhere between the solenoid and the TCU there's a problem. In my case I had removed the TCU to check for water damage first. I've done this before and it's not a difficult job once you do it once. The TCU was dry, so onto the next step.

Remove the battery and battery tray. Looking straight down there's a bunch of wires entering the transmission. Follow those wires down to the transmission cover and you will find 2 connectors on the cover. Remove the bottom (larger) connector. For reference the cover looks like this:

http://nylyon.com/Jaguar/front_cover.png

You have to remove the plate holding the connectors to the front cover. Those are 10mm nuts. Gently bring the wiring to the top of the car. Once up top, disconnect the larger connector, to remove the connector, there's a "handle" on the side. Gently pull this handle out while wiggling the connector free. The handle works like a lever and if everything were shinny and new, pulling the handle by itself would likely release the connector. Since the cars are now old, it needs a little help. *BE CAREFUL* not to pull too hard, it will come out, patience here.

We are testing the circuit between that connector and the internal solenoid. This is the part of the connector which still has the mounting bracket on it. You can also tell because the wires run from the connector to the transmission itself.

Remember this is a single wire and it works with the ground. Since this is a solenoid, there should be some resistance between the wiring feeding it and ground. We are going to test for resistance, so for this test you will need a multimeter. If you don't have one, get one, digital is better.

Here's the diagram of the connector you just removed. Notice the tabs on the outside of the connector, use these as reference so you know which way to hold this.

http://nylyon.com/Jaguar/trans_connector.png

For the P0778 we're interested in pins 18 <-- ground, and pin 16 <-- 2/4 Duty Solenoid. Set your multimeter on ohms, and place one probe on pin 18 and the other 2 pins to the left, pin 16. The ohm value on the right shows 2.6 to 3.2 ohms.

On mine, the meter didn't change from the value of 1. This indicates an open circuit. In my case the open circuit can be either a defective solenoid or broken wire between the connector and solenoid. So we have now narrowed it down.

The solenoid is an electrical component. Even if you test for and get resistance it does not necessarily mean that the part is good. Since the solenoid is internal to the transmission, it could be heat dependent and only fail when hot (or cold).

In my case it's showing as an open circuit, so I am assured that the front cover needs to come off.

I've ordered a new solenoid from eBay. Here's some illustrations on the solenoid:

http://nylyon.com/Jaguar/solenoid_description.png

We are looking at the DUTY solenoid, bottom picture top solenoid. From what I have read, the duty solenoid has a white connector while the brake timing solenoid has a black connector. These are NOT the same type of solenoid, the P0778 deals with the top, DUTY solenoid, white connector. Notice the relationship with the bracket to the electrical connector.

I've not removed the front cover yet (I will update this when I do) BUT have found some really critical information which is MUST to understand. There is a "DO NOT REMOVE" bolt at the bottom of the trans which seems to be mistaken for a drain plug. See these diagrams:

http://nylyon.com/Jaguar/drainplug_warning.png

http://nylyon.com/Jaguar/dont_do_this.png

Drain the transmission fluid from the DRAIN plug. Capture the fluid so you can measure it. If you measure exactly what you remove, there's little guessing what to put back in.

Once the fluid is out, remove the front cover. I haven't done this yet, but before removing the solenoid, I plan to ohm the solenoid directly at the source to ground. If it still shows as an open, then I know for sure that the solenoid is bad. If I get resistance, then I will ohm the wire from the solenoid to pin 16. I should get a closed circuit (0 on the ohm meter).

to be continued.............





In the mean time, some thoughts. When we tested the transmission connector and I got an open circuit. Had I gotten resistance, I would have tested the wiring between the chassis connector and TCU. To do this, the TCU would need to be removed and you would need to check the continuity between pin 16 and the corresponding pin on the TCU plug. This pin varies with year, and I will not recommend a pin to test on the TCU side. If you find that the wiring is good, the solenoid is good then the next (guess) would be the TCU itself. Hopefully you have a spare laying around, if not they can be found on eBay for $100 and change.

Had the time to take care of the P0778 today. Put the car up on ramps, pulled the battery and battery box and removed the wiring from the front case. Climbed underneath and removed the 24mm drain plug and drained the old fluid. I put it in a bucket which had markings on it so I know how much I drained.

Once drained, removed the 20 10mm bolts holding the front cover on the trans. There wasn't any gasket, just a very thin layer of silicone sealant. I then tested the solenoid at the solenoid and it still tested open. 2 bolts and the solenoid was removed. The connector just slides off, no locking catch. After removing the 2 10mm long bolts, the solenoid wiggles out and the new one wiggles in.

Rechecked the resistance test at the plug and got 3.2. Buttoned it all back up, got the handy turkey baster and filled up with 3 1/1 quarts of Castrol Multi-Vehicle import.

Went for a drive and the P0778 is now gone! Total cost including fluids was about $100 USD.

X-Type 2007 P0778 JB155 pin confusion
 

Jaguar X-Type 2007 petrol Canada P0778.
Thanks
https://cimg7.ibsrv.net/gimg/www.jag...ff6a33b9dc.jpg
https://cimg8.ibsrv.net/gimg/www.jag...0fe04f2860.jpg
everyone for the posts giving me the courage to attack this problem. JB155 connector pin 18 tests good to ground. The resistance between pins 18 and 16 is 38 ohms, not the less than 5 ohms that it should be.

BUT, I'm not sure that I have the correct pins. I think it was member "nylon" who posted a yellow sheet showing the pins (see my photo), numbered from 1 to 20, and my connector (see my photo) has 18 pins. Odly that yellow sheet has a handwritten 36.4 for pins 18-16.

Can anyone confirm that my pins colored in green are the correct ones? Thanks for any help on this.