1990 Jaguar XJS Convertible...
I have the differential out/off of the car...

Any ideas on how to properly shim a diff mounted speedO sender unit? I have a "new" one from a friendly eBay guy. He sent sender and 2 shims. My old one is still available, parts line up size wise, but with different number of shims. More,,, for reasons I have not pinned down, this sender has never worked. Don't know if it's a gap thing,,, or a problem further up the line to dash. This will be the last part of the system I have not replaced... I've been chasing it.

How does one measure the gap,,, when one can't see the bloody gap? Do I lightly hold the sensor and spin wheels by hand and shim until I cannot feel the diff gear tapping (lightly and slowly) the sensor tip??? Adding a shim at a time JUST until contact stops? This is a question that has been keeping me up at night...

Thank you ALL in advance for your time and attention 😊

 

I don't know the answer but I will be needing to know myself...as I have been chasing an erratic speedo in my 90 convert.

 

Use a vernier calliper with the depth feature ( the rod that comes out the end of the calliper). Measure the distance from the toothed reluctor wheel to the pickup sensor mounting face and record this number. Then measure on the sensor the depth from the tip of the sensor to the mounting flange on the sensor body where it would sit on the diff face. Add the required clearance to this length, the max clearance is about 0.050". Aim for a clearance of about 0.020-0.030". Then subtract the depth you measured first. The difference is the required shim pack thickness.

 

Nice. Actually have one of those, lol...

Question: is the reluctor of a shape that allows an easy measurement with such a short margin for error. It's round and toothed, correct? Guess I'll just be looking to center the gage as best i can with my probing... trial and error. I just got want to get to close and destroy the sensor OR be too far and have to go digging around thru those holes in the trunk.

 

The wheel is a toothed wheel made from metal. You'll see it through the hole in the diff cover, although you might have to rotate things to get a tooth to measure from. For the sensor, it's round, should be easy to measure.

 

Got ya!!!
Thanks for the info!

 

Use a bit of Plasticine stick it to the end of the sensor. Clean the sensor first with IPA or Metho so it does not fall off. Insert the sensor and the Plasticine will compress measure the thickness

 

But the sensor "should" come into contact with the reluctor,,, right? If so, the sensor tip will bottom out in the r eluctor and the messurent would be zero, no?

 

How can I test before reinstalling the rear cage??? Is there a way?

 

No, there should be a gap.

 

Yes, right - a gap. I get that... My question was more directed at warrjon for his suggestion of using a putty type substance to gain an estimate of the gap...

What I am hoping is that BEFORE getting the differential up and back in I can get a sense of whether the damn thing is functioning or not. Once the cage is in,,,, measuring, testing, and modifying is damn near impossible without dropping the cage again.

Is there a way to test the sensor?

 

If your multimeter has an AC volts setting, connect the meter across the wires of the sensors and give the diff a spin. You should get a few volts output, depending on how fast you can spin it by hand. As long as it's non zero you should be good.

 

I'll give it a shot. Thank you!

 

If the sensor bottoms use a washer to space it out so you can get a measurement , you want the gap to be about 0.020" and no more than 0.040"

My diff has IIRC 3 thin plastic shims for the speed sensor.

 

I have a few shims... I got 2 with this set up. Then, there are the shims on the old (original)...

Is there a sure fire way of testing the sensor BEFORE I mount the cage back up into the body? Spinning the wheels? Although I found a pretty effective way at getting to the sensor (pryed off the paint can style covers under the tank shelf,,, in thru the boot) it would be a devil to try and take measurement or do anything by feel... What a thing!!!!

 

JayJagJay,

I have an outboard disc 94 convertible but I think the cage structure and diff access should be the same as your car.

After I refurbed the rear end and diff and reinstalled the cage, I immediately noticed that my speedo fluttered significantly at 30mph, even though I'd reinstalled the sensor with the same number of shims.

I removed the sensor and changed the shims for 3 new ones, after which the speedo was much more stable.

So, it does seem that the sensor gap is really quite critical. You therefore may need to do final adjustments once the cage is back in the car and the speedo operating. Although a bit fiddly, it is possible to remove the sensor with the cage in situ but just be careful that you don't chip the nose of the sensor as you remove it.

Good luck

Paul

 

Hey Paul... Thanks. Thanks to you all!

Seems as though there is no easy answer to this situation.

I'm going to have to stop projecting (although I'm only projecting a week out) and see what happens. I'll take close measurements of the old and new sensor (old never worked), try to get as best measurement I can in general with the gage, try a putty (although I don't have the putty Warren suggested, and see what happens. The "new" one came with 2,,, I don't know how many are on the old oft the top O me head.

I dread the idea of figuring out when all is in place. I do. I've spent a lot of time under this car. It ain't always purty...

 

If the sensor is too far away from the reluctor the voltage it generates will be lower and may not trigger the interface unit reliably. It also needs to be at the correct angle oso the shims need to be flat.

The only way to test the sensor is to make a reluctor wheel and install it in a drill, mount the drill and sensor so that the sensor is 0.020" from the wheel and measure the AC voltage, it should a at lerast 3volts or more. Don't need to worry about frequency as if the sensor is generating voltage the frequency will be correct.