Ed Petry

A quick question with maybe an easy answer.
On my 81 5-speed, is the fuel pump always hot when the key is in the run position or have I missed something in the wiring diagram?

Thanks in advance.
Ed

Alan

No it isn't.

If the fuel pump relay is working correctly* when you switch to ignition you will get the fuel pump to run for approx 2 sec to build pressure - then it will stop and only start again when it detects the engine is rotating (e.g. while the starter is cranking it). The fuel pump will then run as long as the engine is rotating. If the engine doesn't start or ever stalls the fuel pumps will stop. So in a serious accident the fuel pumps will turn off on their own when the engine stops.

*On some cars the priming mechanism of the relay seems to have failed, they do therefore take slightly longer to start but otherwise they work normally...

Alan

Ed Petry

Alan,
Thanks for the information.
Where does the second (engine rotating) signal originate? After you release the starter how does the pump know the engine is rotating?

On a side note. What do the designations "30" - "15" - "X" - "31" & "50" on the top of the wiring diagram stand for? Appears "30" indicates constant power. What do the others indicate?

Thanks in advance.
Ed

WallyP

The numbers refer to the DIN standard descriptions for the circuits.
30 is battery power.
15 is switched battery power.
31 is earth/ground.
50 is starter switching circuit.
If my feeble memory is correct, X is relay-switched battery power - sometimes for load reduction on the ignition switch, sometimes for retained power.

Alan

What Wally said - but X is for accessory (on before ignition - the first key turned position). There is an X-bus relay to repeat this signal to handle extra loads - but the repeated signal isn't technically called X anymore.

X - like 15 and 50 - come from the ignition switch direct.

Alan

Alan

Starter running or not - it is the same.
The signal comes from the tachometer signal to the ECU (ignition unit). Only the relay rotation detection (of pulses) is required to keep the pump running.

Alan

s54venture

According to this image I found a while back, X is accessory.

Alan

Hmmm - somehow that looks familiar...

Alan

Speedtoys

Alan:

What runs this circuit for 2 seconds, could it be extended?

mark kibort

isnt it a timer part of the circuit in the relay?

Speedtoys

Being a standard relay...?

jpitman2

The pump relay in these models is NOT a std (53) relay - it has an extra narrow pin that receives the tach pulses. The timer is another special aspect of this relay. You need a pretty quiet environment to hear the 2-second pump run...or a noisy pump, which is usually a bad sign.

jp 83 Euro S AT 56k

dr bob

The fuel pump relay looks for a pulled-low pulse from the tach circuit at least every second or two. When it's first powered up, the relay closes, but since it doesn't see the tach signal change, it opens after that second or two, and waits for a tach pulse. The decision to generate that initial relay closure depends solely on the state of the tach pulse when power is first appled to the relay. So they close most of the time, but it isn't guaranteed. With no recurring pulse from the tach, the relay opens anyway. So the fact that the relay closes sometimes is a 'bug', not a 'feature'. Later relays are powered by "smarter" electronics and don't have the same 'bug'.

Speedtoys

What would one do, if they wanted the pump to start with the key, no delay on an S4?

Alan

Think again - it is a safety feature

Alan