Can I test the Fuel Pump Relay with the Horn?

[scotricker]

The red fuel pump relay on my 78SC works just fine. I had a bad heater blower motor relay, and replaced it with the horn relay, and that got me to thinking..

I ordered a replacement black horn relay, and that can also function as my spare for the heater blower motor.

I ordered a spare red fuel pump relay, too, just in case...

So, if I might someday suspect that the fuel pump relay had failed, can I plug it into the horn socket location and test if it toots the horn?

Or is there another way to verify good or bad for that relay?

Just theory for now, all is well, I was just thinkin'.

[JJ 911SC]

Scot

Check Kidrock thread for the technicl difference between Red & Black relays.

Red, Black and DME relays should be part of the "On-board" spare kit among fuses, voltmeter...

stoopid relay question

[scottbooth]

The short answer is, not really.

The problem is that in normal operation the coil in the FP relay is not energized, so the current for the fuel pump goes through the "normally-closed" contact on the relay. When the air flow contact on the CIS metering plate on the engine closes (if the engine stalls and stops sucking in air), the relay coil energizes, which pulls the relay in and shuts off the fuel pump. (The "normally open" contact gets power only when the starter is active, since when you are trying to start the engine, it may not be pulling enough air to lift the air flow sensor plate and open the switch contact.)

In a sense the fuel pump relay works backwards to what you would expect.

The horn relay OTOH works just like you would expect - pushing the button on the steering wheel energizes the relay coil, connecting power to the "normally-open" contact, and tooting the horn.

So you aren't really testing the right condition when you use the FP relay in the horn relay socket.

Note that this is not related to the difference between black and red relays. The only problem with using a red relay in a socket meant for a black one would be in a case where the relay coil circuit was wired with opposite polarity as compared to the one for the fuel pump. In this case all of the current would go through the diode that is intended to shunt the back EMF spike, and the diode would burn up. However, I believe that all of the relay coil circuits in the 911 are wired with the same polarity, which means there is no harm in using a red relay in place of a black one. In fact, you gain a bit of extra protection due to the reduction of the EMF spike. (In a pinch, you can also substitute a black for a red, but then you lose the EMF bypass function, which could cause damage to the circuit that drives the relay coil.)


Scott