Jump to content

Welcome to 205GTIDrivers.com
Register now to gain access to all of our features. Once registered and logged in, you will be able to create topics, post replies to existing threads, give reputation to your fellow members, get your own private messenger, post status updates, manage your profile and so much more. If you already have an account, login here - otherwise create an account for free today!

* * * * *

What is the tachymetric relay?

What's an Tachymetric relay and where can i find it ? 
by DamirGTI

Posted Image
Posted Image

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 .
Posted Image​
On Ph 1.5/Ph 2 models , inside the cabin under the dashboard near the steering column . 
Posted Image
On Late Ph 2 models with Motronic engine management , near the ECU .
Posted Image
Posted Image
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 .