Fuel pump relay

[griston64]

Can I test a 940 fuel pump relay with a multimeter ? Sorry if a silly question as although I'm now not bad at wielding the old spanners electrickery ( that's for those of you who remember Catweazle ) is not my strong point

I know I can sit in the car putting spare ones in to see if it starts but that seems a bit of a faff. I now have 5 of them due to breaking cars and buying a spare to keep in glove box. I know 2 work but not sure about the other 3

Cheers

[Chooch84]

You should be able to, but I can't remember what terminal is what. Might be easier to take the cover off the relay to follow the terminals to test for continuity. Plus you'll also be able to see if you have a dodgy/cold solder joint.

You could always re-flow all of the solder joints to pass time...

[griston64]

Originally Posted by Chooch84

You should be able to, but I can't remember what terminal is what. Might be easier to take the cover off the relay to follow the terminals to test for continuity. Plus you'll also be able to see if you have a dodgy/cold solder joint.

You could always re-flow all of the solder joints to pass time...

If you were to see my soldering you wouldn't be saying thatIt would certainly pass a lot of time though

I notice some have a date on them. Best before ?

[Forrest]

I think the way these work is as follows, but you might want to wait until someone confirms or corrects my understanding. I’m going by the circuit diagram on the side of a non-oem unit.

The white fuel pump relay contains two switching coils and has the following numbered pins.

30 - 12v input from car battery
87-1 - first switched live output (from input on pin 30)
86-1 - to (externally switched) earth from first coil to activate first switched live
87-2 - second switched live output (from input on pin 30)
86-2 - to earth from second coil to activate second switched live
85 - (externally switched) input 12v to activate second switched live

So to test you would attach a 12v power source to pin 30 and an earth to pin 86-2. You would then check for 0v at pins 87-1 and 87-2. Then jumper earth to pin 86-1 and test for 12v on pin 87-1. Then jumper 12v to pin 85 and test for 12v on pin 87-2.

I believe in normal operation 85 receives its switched input from the ignition switch and 86-1 goes to earth via some sensor on the vehicle.

It would be extremely dangerous to try the test I have described using the unregulated output from a car battery. You’d do better to use a current limited 12v power supply.

Personally, I wouldn’t bother. The relays are fairly cheap and I tend to carry a spare.

[Laird Scooby]

The best way of testing anything Mark is to use the best test bed available. In this case, the best test bed is the car so your "faff method" is in fact the best as it proves not only the input side but the outputs as well.

[BorderVolvo]

Originally Posted by Forrest

It would be extremely dangerous to try the test I have described using the unregulated output from a car battery. You’d do better to use a current limited 12v power supply.

A 9v battery, like the type you'd put in a smoke alarm, should operate the relay for test purposes.

[Forrest]

So having worked my test plan out from the circuit diagram I have now tried it and it works. I too have got a box of suspect relays. It is quite hard to see the pin numbers on the OEM relays. Get it right and you can hear the switches operating when you complete the circuit.

What I will say about this approach is that it will not tell you whether a relay will prove reliable in service. Of the two I tested, both previously removed and replaced with new, one had one coil inoperative and the other appeared to work properly. However, the one that tested good had previously intermittently failed in the car which was why I replaced it.

So, the Laird’s right but I’d add that you ought to leave the relay in the car fow a few weeks to check it works consistently.

[Rversteeg]

You can test the relays like this, but you are not actually load testing them. A dodgy soldering point (quite common on the fuel pump relay and MCC AC relay) will only show if you run a large current through it. A false positive, to stay in Covid-19 testing terms...

[TonyS9]

The relays can fail internally (invisibly) to the coils as well as cracked solder joints.

You also need to run the normal load from the pump to get the heat up, as cracks often stop conducting at certain temperatures, hence thumping the dash can help.