Fuel pump relay

[iainand]

ok guys my car is a 1993 525 TDS, please read my problem, im sure someone can help even though my car is a diesel.

as many folks may be aware these cars suffer a hot starting problem, i have noticed that this only happens when the car is warm enough that the glowplug light does not come on. i have also found that the intank fuel pump does not prime unless the glowplug light comes on too. - i believe this may be the problem (fingers crossed). it runs ok with engine running.

now i have read the wiring diagrams for both petrol and diesel models and have probed voltage at the fuel pump relay (this is brand new by the way).

i found that the wire from the glowplug relay to the fuel pump relay only has 12 volts when engine running or when engine cold and ignition on, it shows 1.7v when engine warm/ignition on, the wire from alternator never has battery voltage (which it should have surely??)

im going to wait until engine cold again and recheck voltages at each terminal on fuel pump relay again and again once engine warmed up.

hope someone on here with some electrical knowledge can help piece this together......................

these engines will run without the fuel pump relay installed, but starting can take a lot of cranking for the main injection pump to pull the fuel through and build up any pressure, - this is what seems to happen when the engine is warmed up and intank fuel pump does not prime

PS sorry for long post

[Javier]

i found that the wire from the glowplug relay to the fuel pump relay only has 12 volts when engine running or when engine cold and ignition on, it shows 1.7v when engine warm/ignition on, the wire from alternator never has battery voltage (which it should have surely??)

This device is mysterious to me, but notice there is a fuse in that relay, guess you already checked it. How this relay receive information on engine temperature, trough the water coolant sensor? Is it working fine? Also, may be the intention is not to have pump on under warm circumstances, have you a pressure leak in the fuel system that may be causing the need to prime the system under warm conditions?

the wire from alternator never has battery voltage (which it should have surely??)

The wire comes from D+ that has voltage only if alternator is working and sending charge. For that, engine should be running.

Drawings for Diesel DDE are here:

http://www.e34.de/tips_tricks/schaltplaene/1360_1/index.htm

And for glow plug heater are here:

http://www.e34.de/tips_tricks/schaltplaene/1220_0/index.htm

And for alternator here:

http://www.e34.de/tips_tricks/schaltplaene/1230_0/index.htm

Just in case anybody want to jump in this issue.

Javier

[iainand]

thanks for reply,

i had another check on the wiring and voltages at each terminal

i found a mistake in my first check, below is what i found when rechecked and really took my time to check properly.

wire from alternator has battery voltage with engine running, 0v when off.
wire from glowplug relay has battery voltage with engine cold for about 15 seconds. with engine warmed up, (ie glowplugs not required) there is 0v.

i have added in some temporary wiring for me to effectivly 'hot wire' or 'jumper' the fuel pump relay, and what i am going to do is monitor the starting with engine warm (ie no glowplug heat required) with the fuel pump relay bridged.

I hope to find the engine starts fine - this would prove my theory that the in-tank pump does indeed need to run with engine hot or cold. then just the problem of finding out why this is not the case and a practical permanent solution.

im not using the car all the time at the moment so havent had a chance to get it fully warmed up on a decent run and check my theory. i have left the car idling until warmed up, switched off and left car 15 minutes. then bridged the pump relay for 10 seconds, turned key and it seemed to start much quicker ie. less cranking than normal.

thanks for reading all this babble! hope someone can help or give me some pointers, or hope this will posibly help someone else with same problems with their TDS once i hopefully find a solution.