I'm working on putting a Dakota Digital cruise control in my 1989 XJS with the Vehicle Speed Sensor in the rear diff plate not in the transmission. I'm guessing it has 8,000 Pulses Per Mile but does anyone know if the signal is a square wave or a sine wave?
I guessed it was a square wave but I'm not certain and if anyone knows I'd rather not have to rely on a guess.

Appreciate any thoughts on this.

 

There are a couple of threads on this over on Jag-lovers. I'll summarize:The speed sensor interface interface unit feeds two different signals, one to the speedo and cruise control, and a different one to the trip computer. Supposedly, one signal is 5v, the other 3.5v, not clear which is which. One guy used a signal generator and fed a squarewave 8000 pulse to the trip computer, and it registered correctly. I'm assuming the speedo is also a square wave. ​​​​​​​This guy went to a GPS speedo adapter, but he ended up;using 16000 pulses per mile (!).​​​​​​​https://forums.jag-lovers.com/t/gps-...olution/416778 ​​​​​​​Hope this helps some. ​​​​​​​Jon​​​​​​​​​​​​​​

 

That thread is super helpful! 6 pulses per rotation of the wheel, and my 235/60-15 do 773 rotations in one mile, so that comes out to 4,638 pulses per mile.


I also like how he's using the GPS to drive his speedometer. My backup plan is to use the GPS to drive the cruise control but it's something I want to avoid if at all possible. I have a GPS speedometer in my MGB and it drops out if you go under bridges - I think the cruise needs something with better speed monitoring.

 

I'm still working out the rest of the settings for the cruise control, but in my car it looks like the pulse per mile setting is 16,000.

 

That number of pulses is apparently more modern. The GM 400 driven pulse generator is definitely 8000 ppm, but it is highly likely that this changed when the rear axle signal generator was introduced.

 

Following this. Thank you Thorson.

I was working on my speedometer (again) last night for a little before the sun went down. What a thing. Your facts and figures help. The only thing I walked away with was that I was getting 3.08V from the Blue or U wire at the sending unit and at the #7 WY White and Yellow wire at the dash connector - same vales exactly. It seemed to vary with wheel speed (I had the wheels off the ground 3 or 4 inches so I could spin them in D).

The V seemed to vary by same value at both points with wheel speed (no way of guessing how fast the wheels were turning) by letting off and applying the brake. Got NOTHING on the actual speedometer dial. Very frustrating.

I'll have questions once you've figured this all out. I have 2 speedo's,,, which may both be dead. Speedo's have always jumped when turning the key,,, doggone odometer even turns a little. It's nutts... I wish I had some sort of voltage potentiometer so I could vary voltage from the 0 to 5 volt range to see if the speedos actually work it not 😖

 

JJJ
The signal generator send pulses, not a continuous voltage; I suspect the variable voltage you are seeing is actually the pulses varying with wheelspeed. Do you have a trip computer? if that is working, then the signal is working, as it does both speedo and trip.

 

Good Morning, Greg... Thank you.

I'll answer here right quick (which will lead to one question I guess), but I think I should start a new thread so as to not get in Thorson and others way, who are thinking about alternatives to the OE system.

My trip computer works and seems to be working better since I found the electrical gremlin in the connector under the dash 2 or 3 days ago... Measures milage quite accurately really, and has since installing the current sensor...

I'm on my 3rd diff sensor. The original was toast, I broke the second as it was mounted to far in and came into contact with the diff. Anyways,,,

My understanding was that the sensor in the diff created a signal (sine or square) and then the transducer located in the boot translated that signal (number of pulses) to a steady V that the speedometer, trip computer, CC ECU could use...? Smooth up down variations in voltage(s) as control after the transducer. Is this correct, or no?

I am definitely no expert on the thing.

 

The trip computer and speedo only share signals if it is a car with the transmission speedo transducer. The differential transducer type goes thru a signal conditioner box, which puts out two separate types of signals, one for the trip computer, and a separate one for the speedo and cruise control. The speedos are also different depending on what type of transducer is used.
The signal conditioner boxes are known to fail. It's possible that just the output for the speedo failed, if the trip computer still works. If the cruise works, then it is the speedo itself, or the signal wire to it from where it splits with the cruise signal wire.
He really needs to look at the speedo signal with a scope. Any cheap scope would be able to pick up that signal.

 

For anyone going down this path in the future, here are the settings on my Dakota Digital cruise control that ended up working the best:
Gain Sensitivity: High
Pulses Per Mile (PPM): 16,090 (There was no option for 16,000 but if there was I would have used that)
Engine Timer: 8 cylinder high
VSS Source: Square Wave
Transmission: Automatic
Control Switch Open Circuit

The violet wire on the "cold" side of the brake switch caused me the most headache. I incorrectly assumed it was looking for +12 volts as the sign that the brakes were applied. It's really looking for full continuity to ground with brakes off and open circuit with brakes on. My car had about .08 ohms resistance with brakes off and this was preventing the cruise control from engaging. I used a relay to make sure the violet wire had full continuity to ground and this solved that problem. The manual even gives you a handy reference picture for how to wire it up:



And for reference, here is how I have the cruise control connected to the capstan. This picture is standing at the front left of the car looking towards the back right. The cruise cable enters from the right side, wraps around the front of the capstan, and pulls the throttles open.

 

This is awesome, Thorson.... In that, I apologize, but I have to ask - I'm trying to take and understand measurements at the speedometer and IC connector and at the blue wire POST transducer/sender (or whatever it's called) which receives its signal from the sensor in the diff... I don't mean to hijack or blur your (valuable) thread...

Like (similar to/kinda like) the voltage signal from say the TPS to the ECU, or the CTS to the ECU (which uses that voltage value to do things), does the transducer translate pulses (a value) from the sensor in the diff to a (simple) voltage that varies up and down (in the 0 or close,,, to 5v range) to control the position of the needle in the speedo? I thought that the job of the transducer was as translator...? Pulse to clean varying voltage...?

With a silly look on my face, I've looked at the guage, and it doesn't seem to have the technology built into it to do the sine or square wave translation. It looks magnetic and/or mechanical using a B+ voltage, a ground and a signal wire to balance out the voltage range from the transducer/sending unit to produce a value on the dial...

I really hope that is the case... I don't know how to use an osilliscope, nor do I have one... And, even IF it's a simple varying voltage used to control the speedo - still a little stumped. I don't want to pay 100 bucks for another dial which will ultimately be what I need to do whether GPS or OE route. SMH... Death by 1000 pin ****** with 4 cars and associated costs.

 

The input to the speedo is pulses, not voltage. The circuit board inside the speedo decodes the pulses and feeds the dial and odometer. All modern speedos work this way.

Jon

 

Jay,
I don't know the answer to your question and I don't know that anyone out there does. I did a lot of searching but the details around the specifics of the signals aren't really that well documented. I too was looking at buying an oscilloscope and learning how to use it but I got lucky and stumbled upon the settings that would work.
And I hear you on the death by a 1000 paper cuts. Between this Jaguar and my MGB, along with my 2 daily driver Jeeps, sometimes I wonder what the hell I am doing.

 

Right. But mine is a 32yo dial, not a modern one. I thought the reason for the module in the boot was to be the brains of the operation, separating out the signal from the sensor in a language the 2 or 3 different devices needed to give a reading.

To be clear I DONT know what drives it, but would like to... If it were reading waves that would make it a PWM dial of sorts...? Correct? Meaning the square signal would be received in the dash and the brains of the dial would pulse 12v to the needle? I just don't know.

 

Hahaha... For me as well,,, getting an osilliscope would mean learning how to use it. I think there is one in my future, but honestly, I'd rather not. I'm getting more and more comfortable around the machines I own. 4 1/2 years ago, 3-4 very different types of cars (with no real prior car experience) put me on ENOUGH of a learning curve without adding in an osilliscope... Anyways.

Even if I knew the answer to straight voltage or a wavey signal, I don't know what I would do with it or how it would help my current situation. I think I have 2 DUFF dials and can't afford a new one right now. GPS, phone hung on the dash kind of summer it will be!

1000 paper cuts! THATS the saying. Yeah, I get it. Poor Man that likes rich man's things, lol