930 fuel pump relay problem

robertb · 11-30-2005, 02:48 PM

hope you guys can help, I'm having a problem with the fuel pumps on my 79 930. When I go to start the car I have about 50% shot of the fuel pumps starting with the first turn of the key. The other 50% of the time I hear the relays click and no sound of the fuel pumps turning on. Sometimes I will sit there for a minute turning the key back and forth until I get lucky and they turn on. Kind of puzzles me that sometimes they work fine and other times they don't. Once they're on they stay on and the car drives fine. So I guess Iv'e got a short somewhere but don't really know the best way to find it. I assume it's either in the ignition or I have a bad ground. I bought one new relay and tried swapping it around with the other two, didn't help. Any ideas on how to diagnose this would be greatly appreciated.

dean · 11-30-2005, 03:52 PM

The fuel pumps shouldn't come on in a CIS car when the key is on. They come on while cranking and then once your motor has airflow they stay running.

Dean

hobieboy · 11-30-2005, 04:38 PM

Do you have any starting or running problem that traced back to lack of fuel? As Dean said, the pump doesn't come on everytime you turn the key on. It depends on the residual pressure in the system. I believe if you turn it to ON once, hear the pump, turn if OFF, then wait ~15min, the pump should then turn ON again for sure.

dean · 11-30-2005, 05:48 PM

On CIS cars, the pump or pumps are turned on by the starter cranking or by a switch on the air flapper. When there is airflow the flapper moves and closes the sw and the pumps should run. They shouldn't run at all with just the key on.

Dean

john walker's workshop · 11-30-2005, 06:00 PM

if they normally run with the key on, the sensor plate switch has been unplugged. most likely. there is another relay on the panel to the left of the engine that turns off the pumps on overboost which can be partially unplugged, or which often has a broken solder joint. it's yellow. remove the yellow cover and look at the solder joints on the bottom, where the pins come out. a magnifying glass helps to see a small circle in the solder which is a crack. resolder if needed. that relay is worked by the overboost switch which is on the upper intake section near the air cleaner just below the breather hose. white wire to it. the wire needs to have continuity through the switch until overboost opens it. the front relays often overheat their sockets and melted rubber insulates the pins of the relay.

robertb · 12-01-2005, 09:01 AM

Sorry guys I should have been more clear, the pumps don't come on if the key is just in the on position, they come on at the point when tension begins in the key, right before the engine starts cranking. If they don't come on right before the cranking starts the engine will just crank and crank and will not start. The front fuel pump went out a few months ago and I replaced it and it worked fine since then. I don't think this is related to my new problem as my new problem just started a couple of weeks ago but I could be wrong. When this new problem first started the pumps would sometimes cut off when I was driving and I would have to pull over and restart the car although now once the car is on it drives fine. I will look at the relay in the back panel tonight. Thanks for the help guys.