Speedo Corrector with aditional parts, Confused! Help...

Hello everyone..

I have pulled apart a circuit because a friend reported it stopped working..

I already know Jaycars Speedo Corrector Circuit from past experience,
thats what I found inside, tho it contained a small circuit board also.

This circuit is used for a Truck. Something about changed engine, and need ECU to recognise real speed to stop the truck maxing out at 50km/h etc...

I have used this circuit in my cars previously and allow my Speedometer to change 99% +/-

All parts have been sprayed BLACK to not able to duplicate this circuit, so repairs are more difficult.

But this circuit has an additional Board, containing (2x Small relays, signal Diode I think..., inline speaker transformer... I think.. it's a tiny transformer.)

The Diode - It looks like a small signal diode, but without a black stripe to the side... if anything the stripe looks in the middle

The 2 relays - these are used for disabling the correction and allowing sensors to bypass the Circuit,

Transformer - The speedo sensor wires when connected to the speedo corrector go throu the diode and transformer.
The transformer is 12mm sq, and has 5 pins..... 4 pins being used.
Basiclly like this..
. .
. (this is not used... the middle)
. .
it appears Sensor Input and Output run throu the transformer.
can anyone tell me why the transformer and diode is there?

thanks so much,
Happy Holidays.

I think we need photos.

So, probably the xformer is used for isolation.

The diodes are probably across the relay coil to absorb back EMF's.

May be this will help;
https://www.siliconchip.com.au/cms/A_108157/article.html
It is the same as the Jaycar kit

photos

Thanks for replys guys,

The Diode has the sensors going throu it.
and attached some high res pix

It almost looks like the transformer is being used to triger one of the relays. Do both the input and the output of the Jaycar bd. go to the relay bd?

the relays are connected to the Sensor plugs Male / Female..
the relays when turned off are bypassing the speedo circuit

when 12v is applied both relays turn on allowing sensor plugs connection with speedo controller.

Speedo Controller Input -> Relays -> Sensor Plug! (Not passing throu any diode or transformer)

Speedo COntroller Output
the speedos board output is connected throu the transformer.
but cannot follow path as there are 5 pins...
I dont know it's wiring setup. I do know that there is low ohms between the both pairs of pins.. (ie: 1 & 3 / 4 & 5) counting as 1,2,3 side by side and 4,5 opposing the 3 pins.
if that makes sence.. here is a diagram!

I think you have the two windings on the transformer right. 1&3 is the input to transformer from the output of the jaycar unit. It gose thru a signal diode maybe to rectify it or thru a zener diode the drop the voltage and then to one set of relay conntacts. The output of the transformer pins 4&5 go to a set of contacts on the other relay. Because your ohm meter reading are so close the transformer may be a 1 to1 isolashoin transformer. The hole relay board maybe for racing? a power boost or for getting thru emissions.

can you guys help to show me what I can do to test what type of diode or transistor they are?

i need to duplicate this circuit, IE: Rebuild it as it is reported damaged