Guys, Help please. Does anyone know how many Pulses Per Revolution (PPR) the GM 400 speedo transducer (the electro-mechanical device on the RHS rear of the GM 400 for electronic speedos before the rear axle sender type) pushes out? I think it might be 16, which most are, but I would like to know for sure.
Alternatively, can anyone explain to me how to test it (I only have a multimeter).

 

The output to the speedometer is 8000 pulses per mile, so you can work it back from the gear ratios. I'd spin it with a drill and connect your meter across the terminals. As long as you get a few volts it should be good.

Was there a problem with your old one? They seem to either work or not.

 

The maths are beyond me! Can anyone help? Maybe Warrjon will take pity on me?

The old one works, but I had a new one in stock bought for nearly nothing. The old one has a knurled captive fixing nut that screw onto the speedo gear casting; the new one has a six sided captive nut and it is NOT the same diameter thread as the knurled one! What is more the driveshaft is NOT the original 0.104inch square, but is a 3mm square driveshaft and it will NOT fit into the speedo gear!
So as at sometime in the future my old one will fail, I am trying to locate a new transducer that will fit. Loads of USA places sell them, at either 16 PPR or a few at 8. And they have the correct 0.104 inch (2.6mm) driveshaft. So I am considering getting one as a backup, but I need to know it will be compatible with my speedo from both the PPR and the voltage/signal point of view.

 

Excuse my asking, but are you 100% sure it is 8000 per mile. I ask because this one from Jegs:
https://www.jegs.com/i/JEGS/555/41610/10002/-1
says it is correct for 8000 per mile = 8 pulses per revolution, so it look as if it would be OK. My next task is to find out if the pulse output from a Hall effect 3 wire jobbie will work the speedo like the OEM one does, which is 2 wire. Thanks for your help
Greg

 

Yes, I'm sure because I'm in the process of converting a Daimler DS420 to an XJR engine, and the later system generates the speedometer signal from the ABS system, not the transmission, so I've had to do an interface to convert them. It's also the same system the cruise control uses, and I'm adapting the Series III XJ cruise system.

I think the transducer is a unique Jaguar application, not shared with other GM applications, so I don't think a generic GM transducer will fit. That's probably why you are seeing different cable drive sizes.

I've not seen a knurled retainer, only the large nut, but my experience with most transducers is Series III V12 cars. My Daimler is the same as the SIII application.

 

Thanks for your reply, JB. As all the transducer does is generate a signal, and as long as the signal is the same, I would have thought a generic will do the job; they both attach to the same small removable casting, the gearwheels are the same, it is just the threaded part that seems to be different on the currently available Jaguar transducers. The standard GM thread seems to be 7/8 - 18, which I am pretty sure my knurled one is. The one with the hex is quite a bit smaller at about 3/5ths or so. Also the casting at 7/8ths can be bought too,
http://th400speedo.com/wp-content/up...th400large.jpg
so I hope it is just a matter of ensuring the output signal from the three wire generic will work Ok and not blow the speedo. And then wiring it up properly of course...
If you have any comments on this line of thinking, please put me straight!
Greg

 

Yep as long as the output is 8000ppm it should work fine. If not 8000ppm there are a few companies that make speed adjusting modules that go between the transducer and the speedo and adjust the pulses out.

 

Jagboi....an XJR6 drivetrain in a DS420...wow, what a project!

The knurled thunb nut transducers are for BW66 trans found behind the 6 cyl engines. The nut is for 12's

Rob

 

My 1985 V12 coupe came from the factory with a knurled transducer, and so did my spare engine/box, also 1985!

 

It's turning into a more difficult project than I thought. The DS420 is the smallest big car I have ever worked on, there is no space for anything.

 

I had a look at the wiring diagram, and it's power in and signal out on the stock Jag transducer. I don't think a Hall effect would want power in, but I'm not sure.

 

Thank you for that info. Actually the Hall effect ones are three wire, and do want power in as well as signal out. They also require a separate dedicated earth.

 

Hi Greg,

I had a look at some parts cars, and you are right. These cars had both types. \
We have to keep these archives correct.

Good luck,

Rob

 

Rob, thanks for your info. If you could pull me the speedo gear casting from a GM400 that takes the smaller diameter hexagon type thread, I would gladly pay for the postage and any scrapper's costs! PM me if this is a possibility.

 

What became of the jegs aftermarket sensor? Did you get one? Did it work?

 

I did get one, but my old transducer that I bodged up the wiring on is still working, so i have not fitted the jegs one yet. It will work though, I am sure.

 

I had just gotten a Dakota digital sensor, that was quality piece, but would not fit(too large would hit the trans mount/tunnel). So looking for a new jobber. Wiring the 3rd wire will be the only thing that I’m not exactly sure where to go.

 

On the OEM car's loom the green wire is 12 volt input (goes to the Jegs red wire), the other OEM loom wire is the signal output from the OEM (and the Jegs) unit and goes to the speedo. The third wire on the Jegs unit is just an earth, which that design needs, just earth it to the gearbox casing somewhere.

 

oh awesome, thanks! Got to send back the Dakota and get a jegs 👍