No start - Fuel and Main relay problems?

[davep]

Here's a bit of info that may help when troubleshooting:

Main Relay (22) and Fuel Pump Relay (21)

These relays shown as (21), (22) in schematic below, are used in two different circuits and when ignition On is sensed they are separately controlled from ECU (1) pin 16 and pin 12, respectively. If one of the relays is faulty then the respective +12 Vdc supplies are not realised, swapping relays simply means the fuel pump supplies are made and not main, or vice versa.

Sequence of operation and supply distribution

A +12 Vdc battery (13) voltage (NG and N) is permanently applied to:
• Fuel pump relay (COM pin 30)
• Main relay (COM pin 30 and Coil pin 86)
• ECU +ve supply (pin 15), used for battery backed RAM.

When ignition is first switched On (9), +12 Vdc (White) and (White/Slate) is applied to:
• Fuel pump relay (Coil pin 86)
• ECU ignition sense (pin 19).

Once ignition On is sensed the ECU, via pin 16, earths the Fuel pump relay (Coil pin 85, Blue/ Purple), the relay once energised feeds +12 Vdc supplies to:
• Lambda sensors (2), (3) - (White/Orange) - (pin 87A)
• Fuel pump (8) - (White/ Purple) - (pin 87) via a fuse (6) and Inertia switch (7)
• Purge control valve (26) - (White/ Purple) - (pin 87) (if fitted).

The ECU, via pin 12, also earths the Main relay (Coil pin 85, Blue/Red), and the relay is energised feeding +12 Vdc (pins 87 and 87A Brown/Orange) supplies to:
• Injector supply circuits (23) - (pin 87)
• AFM or Airflow sensor (20) - (pin 87A)
• ECU internal supply voltage regulator I/P pin 2 (pin 87A), this is also where Main voltage is monitored for injection pulse width control purposes.

Dependencies:
• A permanent +12 Vdc battery voltage must be applied to pin 30 of the Fuel pump relay (21) regardless of whether the Main relay (22) is in circuit, or not; the Main relay is not wired directly in series with and does not supply power directly to the Fuel pump relay.

• Operation of the ECU pin 16 earth control line to pin 85 of the Fuel pump relay (21) does depend on whether the Main relay (22) is in circuit or not.

• With the Ignition switch (9) at position I and the Main relay (22) in circuit, pin 16 voltage level is maintained at about 0V (it does in fact rise slightly).

• The fuel rail is now primed. If Ignition switch (9) remains at position I for long enough an ECU delay counter times out, whereupon pin 16 rises fully to +12Vdc to de-energise the Fuel pump relay (21), cranking has not occurred. If Ignition switch (9) position II is selected within the delay countdown period, and starter has engaged, cranking has occurred, spark driven fuelling interrupts have commenced so fuel rail remains primed.

• With the Main relay completely removed or failed pin 85 goes immediately to +12Vdc, keeping the Fuel pump relay Off.

• The ECU (1) regularly measures the input Main voltage level at pin 2, if it is 0 Vdc or drops below a certain level either because of a faulty Main relay (22) or very low battery voltage, an 'injectors have insufficient or no voltage supply' condition is flagged and the Fuel pump (8) is switched Off by the ECU (1), via the Fuel pump relay (21).

Conclusion: the ECU senses when the Main relay is not in circuit and is responsible for controlling Fuel pump On/Off, via pin 16, accordingly; therefore:

• A failed Fuel pump relay = no fuel.
• A failed Main relay = no main voltage sensing = Fuel pump relay off = no fuel.
• Swapping either failed relay = no fuel.

Note: The above relates to a '92 TVR Griffith, where the two relays are wired and colour coded in accordance with the Land Rover 14CUX schematic diagram - dated 1990. Colour coding does vary depending on model year. It does not take into account any immobiliser circuit operation for security reasons.

[Wolvesboy]

Awesome information.
Thankyou!

[Steve_D]

A little caution required as some of those circuits are not true on later cars plus wire colours are different. The original Rover diagram is also wrong as the relays should show 2 pins marked 87 not 87a.

Steve

[davep]

Having checked many variants of the 14CUX ECU firmware, including Operation Pride, control and sequencing of the Fuel Pump and Main relays remains the same throughout.

The only differences I have found in the Fuel Pump and Main relay operation are:

- The distribution of the supplies from pins 87A and 87 of the Main relay (K116):



Where +12 Vdc supplies for the Injectors and AFM are sourced from pin 87 not 87A.

- Also the +12 Vdc voltage to the Fuel Pump relay (pin 30) is colour coded PW and is routed via the ignition switch when On via a fuse in the 1995 Land Rover version.

The operation of the ECU remains the same as described previously.

If Land Rover intentionally annotated these relays in their diagnostic schematics as 87 and 87A rather than use the Bosch data sheet annotation, so be it.

Edited by davep on Friday 21st May 11:15