fuel pump relay turns off
 

Hi, this is my first post. I have a 1986 420 sel with 100 000 km on it. Recently it has developed a problem with stalling. By following old posts I have been able to trouble shoot it quite a bit and narrow things down. I have replace the fuel pump relay and the fuel filter but am still experiencing the same problems. What I have discovered is both fuel pumps work, but once the engine warms up the fuel pump relay will turn off shutting down the power to the pumps. The car will restart but the relay will turn off again only alot quicker (which I believe is because the engine is now warm). What would cause the fuel pump relay to turn off? Can any other sensors or relays cause it to do this? Thanks, I hope someone can help.

When my '86 420 did this (repeated stalling) a replacement crankshaft position sensor did the trick. I do not know what the fuel pump relay was doing but it would almost always start right back up and drive a few more miles then stall again. Once it cooled off (I got to the wedding) it ran fine on the return trip, until it got hot again. Of course it might be something like a bad ignition switch or a bad ground on the battery cable, or something else. Lots of people on here know more than I about this.
While you are at it, take the ignition control module off the driver's side inner fender; underneath you will find some white stuff, probably dried out. This is silicone heat transfer paste, it helps the module use the inner fender as a heat sink. Scrape/clean it all off and replace it with some fresh. You can get this at Radio Shack or an electronics store. It will help the ignition module live longer.

Thanks Randy, I did some research and the crankshaft position sensor is possible as it sends the rpm signal to the fuel pump relay. If the fuel pump relay does not see the rmp it shuts the fuel pump off. I don't understand though why it runs fine until it heats up? Does anyone know a way to test the CPS to see if it is at fault. I don't want to spend money on a new one if it isn't at fault. Thanks Drew

They are all good points.. Did you check the fuel pressure?? The car has 2 fuel pumps.. The CIS injection system works with the fuel pressure from both fuel pumps. The fuel pumps should be tested seperately for the right fuel pressure.. If you don't have the right fuel pump pressusre the RPM drops and the crankshaft position sensor shuts off the power to the fuel pump relay.