Speedos and speed signals

[dave88sw]

Hi,

I have no idea where to post this, as it relates to multiple different vehicles and trying to combine parts from different cars...

I'm in the process of fitting a 3.9V8 with the 14cux management to my 1986 110.  I want to get everything working as it should, so i've been looking into giving the ECU the speed signal it needs.

To that end, studying wiring diagrams, it would appear that the speed signal comes straight from the speed sensor (transducer) on a Range Rover/Discovery.  Please, correct me if im wrong but Discovery/Range Rover speed sensors provide 8200 pulses per mile?

I've also found posts from 1 or 2 people who have used a TD5 speedo and transducer with the 14cux and they say it works, although, i've read that the TD5 speed transducer only produces 4100 pulses per mile. (theory being that the 14cux only really needs to see a signal of some description when it's moving, nothing too precise).

So, looking into TD5 speedos, i stumbled across this post:

https://nas-row.com/index.php?threads/td5-speedo-in-a-300tdi-truck.868/

3rd post in.

The image they've attached suggests that pin 1 doubles the signal to send it to the ecu (when used in a TD5).

 

So, my question is, if i were to use a TD5 speedo, would it be best to wire the 14cux speed signal directly to the transducer output, or wire it to pin 1 on the speedo.  It would seem that pin 1 outputs exactly what the 14cux is looking for, or is it not that simple?

 

Thanks

Dave

[western]

Td5 speedo takes its signal direct from the transducer on the gearbox, the transducer green YBE100530 as used in Td5 defenders] is wired with 3 pins, signal, 12v feed & earth. the signal wire goes to speedo, 12v feed from a ignition switched feed, & a suitable eath point, works in my 1989 ex TD now a 200tdi 110CSW with a td5 instrument pack

[Eightpot]

The 3.9 speed transducer is cable operated - a short speedo cable runs from the speedo drive on transfer box into one side of transducer,  a longer cable comes out the other side and into speedo as normal - just keep the standard speedo 

The transducer is used to regulate idle at low speed from memory - I've done the same instalation without a transducer and all was well, maybe a little fidgety when dropping to idle when getting to traffic lights etc. 

Edited August 30, 2021 by Eightpot

[dave88sw]

Thanks,

I know on the earlier range rovers the transducer was inline. The issue with that is that the cables are hard to find these days and the transducer is expensive. (I'd also quite like to have the later electronic speedo)

The discovery 1 v8 had a transducer (ybe100540) on the transfer box, this provided the 14cux with the correct speed signal (which as far as I can tell was 8200 pulses per mile)

As western says though, the TD5 uses ybe100530 which is 4100 pulses per mile. It is said that this is fine when used to provide a speed signal to the 14cux but if the post I linked to above is correct, the TD5 speedo can provide the correct 8200 pulses per mile.

 

Can anyone confirm that the td5 speedo does in fact output 8200 pulses per mile on pin 1?

Thanks

 

[western]

CwazyWabbit on here was doing lots of Td5 speedo work,his thread is in here somewhere within this thread, how to adjust the mileage reading LEGALLY so it will match the vehicles existing reading prior to fit into a pre Td5 90 or 110.  https://forums.lr4x4.com/topic/69070-td5-speedo-mileage-run-up-circuit/

 

[dave88sw]

That's useful to know, thanks, it would be nice to get the mileage correct as mine only has 90000 on the clock currently.

 

I found this thread:

https://forums.lr4x4.com/topic/51712-def-td5-speedo-transducer/

The second post includes a paragraph from Rave which states that the speedo does double the speed signal, so it sounds like it would provide the perfect signal for the 14cux, has anyone ever used it before?

 

Then, to muddy the waters even more, i found this thread:

https://forums.lr4x4.com/topic/102859-speedo-transducer-input-for-14-cux/

This suggests that i need to put a 120 ohm resistor inline on the wire to the 14cux.  Can anyone confirm that to be the case or not?

 

Sorry for all the questions, i'm just the type that likes to know what i'm doing, rather than giving something a go and being disappointed when it doesn't work.

[dave88sw]

So, I've decided to try the output from the TD5 speedo, it would appear to be exactly what the 14cux needs. I'll report back if it doesnt work.

Can anyone advise whether the resistor is necessary/important. I cant find reference to it in any wiring diagram but I understand its built into the original range rover transducer. Is it to protect the transducer/speedo?

Electrics are not my strong point...

Thanks

Dave

[Eightpot]

Resistors are used to reduce voltage or current in a circuit - could be there to reduce the ~14v input to the transducer to a more desirable 12v to the ecu, or just a little current protection, or to smooth the signal flow.

[dave88sw]

Thanks, that's useful.

I've managed to clarify that the resistor is needed for the Discovery spec YBE100540 if connected directly to the 14cux, otherwise the speed sensor will burn out fairly quickly.  (I also own a 96 V8 Discovery, that uses the YBE100540 as standard with 14cux, i did once find a wiring diagram for it and the speed signal goes to the speedo first and then from there to the engine ecu, i can only assume that the speedo head on a disco contains the necessary circuitry to look after the speed sensor).

I think for the sake of a quid or 2 to order a 120 ohm resistor from ebay, it would be worth wiring it up with the resistor inline and trying it, i don't want to kill the TD5 speedo.

If this works, i'll report back as i'm sure it would be useful to others.

[dave88sw]

So, I can finally wrap this up. 

It works perfectly, I've got the TD5 speedo wired as per all the various guides available on here. I then connected pin 1 of the speedo (which takes the 4100 pulses that the transducer outputs and doubles it to 8200 pulses that the 14cux needs) to the speed signal input on the ECU (pin 6) via a 120ohm resistor. 

Hopefully this is of use to somebody doing a similar conversion.