What's an Tachymetric relay and where can i find it ?
It’s an relay for fueling system control commonly used on early Bosch L/LE/LU/LH - Jetronic analogue engine management systems . The same type of relay can also be found in other car manufacturers during the 80's (like Alfa Romeo ,Citroen , BMW , Opel/ Vauxhall , Volvo etc.) .
In the 205 GTI it can be found on Ph 1 and Ph 1.5/2 models with both the 1.6 and 1.9 8v engines .
Late Ph2 205 GTI models usually do not have tachymetric relay , rather that that they have an double separate system relay , one for the fuel pump and another for injection system in conjunction with digital Bosch Motronic 1.3 engine management .
It’s prime function is providing voltage supply to all the fueling system and components , mainly - injectors , air flow meter , throttle position sensor , supplementary air device and fuel pump . It also serves as an safety measure component for the fueling system .
On Ph 1 models , it’s located in the engine bay , under the battery in a small black box attached to the LH chassis leg .
On Ph 1.5/Ph 2 models , inside the cabin under the dashboard near the steering column .
On Late Ph 2 models with Motronic engine management , near the ECU .
How does it work then :
When starting the engine by cranking it via starter motor , relay being connected to the starter motor positive circuit uses supply voltage from the starter motor for energizing the relay .
Once energized , when the engine reaches cranking speed above 350-400 rpm. it then uses pulse signal from the ignition coil negative terminal for closing the relay contacts thus sending power supply from the relay to the fueling system/sensors and maintaining engine running until switched off by key .
Safety measure side of this type of relay is based on the pulse signal from the ignition coil , while it needs minimum of 350-400 rpm. in order to close the relay contacts for supplying power , when or if the engine rotation speed drops below that value the relay contacts will open cutting of the power supply to the fuel system and fuel pump .
This type of relay has seven terminals :
30 – Battery supply (permanently connected to the battery positive terminal)
15 – Ignition supply (connected to the ignition key ON circuit)
1 – Ignition coil signal (pulse signal , connected to the ignition coil negative terminal)
50 – Starter signal (connected to the starter motor positive circuit)
31 – Earth (connected to the battery negative terminal)
87 – Fuel system power supply (connected to injectors , air flow meter , throttle position sensor , supplemetary air device)
87b – Fuel pump power supply (connected to the fuel pump positive circuit)
Common faults caused by tachymetric relay :
No start: difficulty starting , no power supply to the fuel system components , engine cutting out after a few min. of driving , fuel pump permanently ON .
Causes : Loose or melted soldered joints , oxidized outer relay contact terminals , broken relay coil winding wire , burnt diode or resistor , sticky/oxidized main relay energizing contact points .