[Sean's Jaguar XJS Tech Pages]
Jaguar EFI ECU Adjustment Tool

Last modified 2005 APR 07 04:40:07 GMT

First, What is the ECU Adjustment for? The ECU adjustment changes the base fuel mapping. The adjustment procedure is covered in Kirby Palm's book, XJS: Experience in a Book, so I won't duplicate it here at this time. In short though, with the engine in closed-loop operation (pulling the loopback plug from the port adjacent to the feedback monitor port accomplishes this, but should only be done for the duration of the adjustment), the feedback monitor ports should report as close to 2.5V on both banks when the engine and ECU are running in some semblance of tune. If lower than this figure, the ECU is running lean, and if higher, it is running rich. On the 16CU ECU pictured here, you're supposed to connect a 47uF capacitor across to ground to adjust for the fact that the signal is actually a square wave (applicable if you're measuring with a DVM -- if you're using a scope, this isn't really necessary). Radio Shack #272-1027 for about US$0.99 should do the trick (the part should really be cheaper, but they package it in a single-unit bubblepack).

When adjusting the ECU, you should count off the tics and the direction you're turning the adjuster, so you know where the original location was should you need to return to it while making adjustments.

If your ECU DAC-number has the suffix "RB", I suspect that means "rebuilt".

Walter Petermann wrote up a good basic description of what to look for on the diagnostic feedback port. He also has a nice explanation of how to adjust the ECU as well. For more reading, you can check out Kirby Palm's book and search for "FEEDBACK MONITOR SOCKET".

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Here's a shot of the EFI ECU box itself, the top plate facing the viewer. This is a 16CU, as fitted on my 1988 XJ-SC. This can be found mounted in the boot, behind a trim panel near the battery.

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Removal of the six screws visible in the above shot will permit removal of both the upper and lower cover plates, revealing the ECU Mainboard. The mainboard is mounted component side DOWN, so we're actually viewing the mainboard from the bottom of the ECU. The Vaccuum Port and ECU Adjustment Port would both be facing outboard, with the ECU Adjustment port closest to the rear of the vehicle.

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The ECU Adjustment Port with the shaft tip just barely visible. You'd have to remove the ECU (or use an inspection mirror) in order to see this, as the adjuster is on the outboard side of the ECU when mounted.

[JPEG IMAGE/5383 bytes] This is a (crummy) shot of the ECU adjuster tool inserted into the adjustment port. Note how the adjuster handle comes in full contact with the ECU case (red arrow)-- I made the tool shaft just long enough to get a good seating on the pot shaft (tip is at the yellow arrow, seating up against the pot detent sleeve, identified by the magenta arrow), and the handle prevents over-insertion.

To construct the adjuster tool, I took a 1/2" wooden dowel and drilled a hole in it (not all the way through though), and cut a piece of handle from an inexpensive paint brushe (I've since been told by someone else that they used a similar material - the handle from a glue application brush). The brush handle was ROLLED metal (that is, it was like a sheet which was rolled into a tube - it could be expanded or contracted with a little force), reasonably stiff, which I expanded as necessary to the diameter of the adjustment shaft, then flattened one side (the one where the seam was) with needlenose pliers, re-fitted over the shaft and adjusted the diameter to fit snug. I then soldered the seam so that it wouldn't flex. Refitted, and it works great. By pressing the shaft of the metal tube into the drilled out hole in the dowel (large enought to fit, small enough to offer a LOT of friction - though you could opt to drill a larger hole and load it with epoxy instead) just far enough that the exposed shaft is just long enough to fit into the ECU and not quite come in contact with the detent sleeve, I don't have to worry about inserting it too far (if you have a long reach on your adjuster, and you slip it in .25 - .50 inch too far, you can push the detent fixture off of the detent tic).

I took some 3/4" shrink tubing, and placed it over the wood dowel and shrunk it up. This is purely optional - I did it for a few reasons: the red will be easier to notice in a toolbox than the piece of wood, it is less susceptible to grease and the like (not that there would be any around the ECU, but gringy hands have a tendancy to transfer crud), less chance of picking up a sliver, and lastly, 'cuz I felt like it. I also glopped a small bit of red tool insulator material on the butt end to complete the job (though this last bit isn't present in the pics here).

While out picking up some wire at an electronics shop, I spotted some aluminum and brass tubing (I've used the same stuff before for fashioning connectors for the window relay kit), which had approx the correct diameter. US$4.50 for something like a 2 or 3 foot section (I didn't pull it from the bin to check length).

Various shots of the constructed ECU Adjuster tool, against a metric ruler:

[JPEG IMAGE/5155 bytes] The exposed shaft is about 13mm, the handle is 37 mm, approx 8mm diameter (I used a chunk of standard 1/2" dowel).

[JPEG IMAGE/5842 bytes] The ID of the tubing is a fraction over 6mm.

[JPEG IMAGE/4892 bytes] The flattened edge is about 1mm shorter.

[JPEG IMAGE/4029 bytes] Here's a straight-on shot of the tip. Note the bottom (flat) edge is where this particular tube rolls over itself - after sizing (and after this photo), I soldered this edge to prevent it from flexing when in use.


Sean B. Straw
Post Box 751224
Petaluma, CA 94975-1224 USA

EMail to: Sean.Straw+Jaguar@mail.professional.org

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