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Speedometer and Sensor Compatability
Posted: Fri Oct 02, 2020 4:04 pm
by FarnhamStag
Hi guys,
I've just converted my manual Stag over to and Auto box and I'm having difficulty getting the speedo to work properly. The original system worked and consisted of a R380 manual box with a Rover SD1 electronic sender fitted into the mechanical drive. This then feeds the Rover cruise control and trip computer in parallel with a smiths speedo modded by speedy cables to take that signal. All worked well for 20 years and can be calibrated using rotary dip switches
The new auto box has no speed drive, so I'm now connected to the speed sensor in the BMW E28 Diff in the hope the sensor is compatible.
It works until about 40mph when it becomes very unstable, get to 60mph and its jumped to 135+.
The BWW sensor is 2 wire and is fed with 12v, moving the car a few metres back and forth with the sensor disconnected from the speedo toggles between either 6 or 12v output continuously so I'm wondering what sort of sensor it is and why it would toggle between the two voltages.
Looking at the speedo/cruise input I have about 420 ohms to ground.
Googling suggests the sensor is either a reed of hall, my original thoughts were to fit a parallel resistor to try and clean up a possible messy signal but if it's a reed switch this might overload things.
Any ideas on what I need to do to get this working with the Rover based bits?
Thanks
Re: Speedometer and Sensor Compatability
Posted: Fri Oct 02, 2020 5:22 pm
by DaveEFI
A hall effect needs power to work - a reed switch not. If you get an on/off resistance reading from the sensor disconnected when you turn a wheel, it will be a reed switch - not that I've come across one being used for this.
Do you know what signal your device expects? IIRC, the SD1 unit pulses between 12v and 0.
Re: Speedometer and Sensor Compatability
Posted: Fri Oct 02, 2020 5:53 pm
by stevieturbo
The Jaycar module allows you to adjust pulse counts, and certainly the old discontinued kit version allowed you to switch between either a square wave output, or simulated VR type output.
https://www.jaycar.co.uk/speedo-correct ... t/p/KC5435
Re: Speedometer and Sensor Compatability
Posted: Fri Oct 02, 2020 5:54 pm
by stevieturbo
Re: Speedometer and Sensor Compatability
Posted: Sat Oct 03, 2020 8:39 am
by DaveEFI
Yup - built one of those (old Jaycar) for a mate in the SD1 club who'd fitted a box with no speedo drive - a T5? I've not seen the sensor, but IIRC he found a kit which fitted the propshaft.
Other thing to experiment with is a pull up or pull down resistor on the sensor output.
Re: Speedometer and Sensor Compatability
Posted: Sat Oct 03, 2020 9:49 am
by FarnhamStag
Checked and it's reed switch, so I get about 1.4 ohms when closed which is about right with a reasonable length of cable. So I have a direct feed of 12v into this and this goes into the speedo/sd1 trip/cruise module and that ends up having around 420 ohms impedance.
How I get 6v/12v I don't know so will have to check that again.
I'm not sure what sort of current these mechanical switches are designed to take, my original idea was to reduce put a parallel resistor in place with the speedo to drive more current and possibly clean up the signal, but I don't want to compromise the life of the switch.
What would the Rover sd1 expect to see and what do I need to do to make up an interface circuit to make them compatible?
The Sensor in the diff is tucked up tight against the boot/petrol tank so removal involves dropping the BMW subframe/diff assembly which I'd rather not do.
Thanks again Dorian
Re: Speedometer and Sensor Compatability
Posted: Sun Oct 04, 2020 3:04 pm
by richardpope50
Why not contact Speedy Cables for a mod? They are still in Wales (next door to CAI who may also help).
Re: Speedometer and Sensor Compatability
Posted: Wed Oct 07, 2020 12:28 pm
by FarnhamStag
Thanks Richard, my speedo is from Speedy cables back in 1998! I have called and emailed them a number of times but as yet never had a response.
I've borrowed a small oscilloscope, so I'll have a play with that to see what's actually happening. It may just be a dodgy sensor
Re: Speedometer and Sensor Compatability
Posted: Wed Oct 07, 2020 5:37 pm
by richardpope50
Both Speedy Cables and CAI do not seem to answer e-mails and I remember leaving a message did not work for me once. Could be worth a try again.
Re: Speedometer and Sensor Compatability
Posted: Wed Oct 07, 2020 6:20 pm
by Ian Anderson
Call them and they are usually extremely helpful
Ian
Re: Speedometer and Sensor Compatability
Posted: Thu Aug 05, 2021 12:52 pm
by FarnhamStag
Thanks guys. I have did make contact with easy cables and they were helpfull but came to the conclusion that I need to send the speedo to them with a sensor to check compatibility and re-work if needed.
I have tried all sorts of pull up/down resistor and wiring combinations and nothing works particularly well (I get 50% of the speed in the early range and correct by around 60-70 MPH. Sometimes it also reads correctly, but mostly unstable.
Ideally if I knew what the output signal type of the Rover LT77 electronic transducer was this would allow me to source/make an interface that turns my reed sensor (Diff mounted speed sensor) signal into what ever the speedo, cruise control and Trip computer needs.
If I convert the speedo those other part still wont work.
Does anyone know what the Rover transducer gives out or how it works?
Unfortunately I left it fitted to the gearbox I sold.
Searching the internet seems to offer conflicting opinions
Thanks, Dorian
Re: Speedometer and Sensor Compatability
Posted: Thu Aug 05, 2021 2:36 pm
by DaveEFI
I'd use the reed switch to switch a transistor. Something easy to get like a 2n2222. Read switch to 12v and a 10k resistor from the other side of the reed switch to base. Emitter to ground. 1k between collector and 12v. Output from the collector will then switch between approx 12v and zero (depending on load). If what you're driving needs more current, you can reduce the 1K resistor to about 100 ohms.
Re: Speedometer and Sensor Compatability
Posted: Thu Aug 05, 2021 8:02 pm
by ChrisJC
I know about the Range Rover Classic speedo transducer. It is 8 pulses per revolution, and is either open-circuit or 120ohms.
Chris.
Re: Speedometer and Sensor Compatability
Posted: Thu Aug 05, 2021 8:35 pm
by stevieturbo
FarnhamStag wrote: ↑Fri Oct 02, 2020 4:04 pm
Hi guys,
I've just converted my manual Stag over to and Auto box and I'm having difficulty getting the speedo to work properly. The original system worked and consisted of a R380 manual box with a Rover SD1 electronic sender fitted into the mechanical drive. This then feeds the Rover cruise control and trip computer in parallel with a smiths speedo modded by speedy cables to take that signal. All worked well for 20 years and can be calibrated using rotary dip switches
The new auto box has no speed drive, so I'm now connected to the speed sensor in the BMW E28 Diff in the hope the sensor is compatible.
It works until about 40mph when it becomes very unstable, get to 60mph and its jumped to 135+.
The BWW sensor is 2 wire and is fed with 12v, moving the car a few metres back and forth with the sensor disconnected from the speedo toggles between either 6 or 12v output continuously so I'm wondering what sort of sensor it is and why it would toggle between the two voltages.
Looking at the speedo/cruise input I have about 420 ohms to ground.
Googling suggests the sensor is either a reed of hall, my original thoughts were to fit a parallel resistor to try and clean up a possible messy signal but if it's a reed switch this might overload things.
Any ideas on what I need to do to get this working with the Rover based bits?
Thanks
It is very unusual for a simple speed sensor to be 2 wire, AND 12v. Some ABS sensors may operate under similar conditions ( usually current sensing, weird ), but not simple speed sensors for say a dash or speedo.
But presumably it is not in the BMW anymore ? So some details of the sensor itself are needed.
How have you it wired ? Typical 2 wire sensors should not be powered, they generate a signal themselves from the metal passing over the sensor. If your SD1 sensor had 3 wires....the two signals are very unlikely to be compatible.
Ideally you need to scope each sensor to see the output so you know what each does, and also what you need.
This kit whilst allowing pulse adjustment, also allows you to switch between a digital square wave, and a simulated sine wave output ( accepting either input )
https://www.jaycar.co.uk/speedo-correct ... medium=web
But you need to know one way or another what you need rather than guessing.
Re: Speedometer and Sensor Compatability
Posted: Thu Aug 05, 2021 10:59 pm
by DaveEFI
The SD1 sensor is shown in the schematic as two wire - one to 12v, the other to the speedo. And is definitely an active device. Of course, it may pick a ground via its body, but the schematic doesn't show this. Someone in the SD1 club unpicked the potting to get inside, and IIRC it contains 3 transistors.