Over the past couple of months I have had two relays fail on me which resulted in a cranks-but-won’t-start situation. I just want to quickly detail the symptoms and diagnosis method to hopefully save someone some time. This may be especially relevent if you have experienced this problem after some very wet weather (as I did).
Relay 395: Located in the ‘plenum chamber’(black box that houses the ECU, top left hand side when looking under the bonnet from the front). The relay is the closest to the front of the car (small black, 5 terminals). The symptoms of failure were complete and sudden loss of power, after which the car would crank but not start.
In my case, water had gotten into the plenum chamber due to a missing screw at the rear. This was evident from a puddle of water in the bottom of the chamber. The relay was corroded on the inside, tested and showed an open circuit when the solenoid was powered on.
After replacing the relay (Audi, circa £16), the car would still not start. I tested the in-tank fuel pump which was now working and pumping fuel to the filter (note: it only pumps when cranking, not when the ignition is just switched to the ON position). It is now obvious why the car still did not start: after the fuel pump relay failed and the tank pump stopped pumping, the fuel lines were sucked dry by the high pressure pump and then the engine died once no more fuel could be supplied. My remedy for this was to crank the engine whilst spraying easystart (or hairspray) into the intake to get the engine to fire and push up the RPMs. This caused the high pressure pump to suck harder and prime itself, after which the car coughed back into life and resumed its normal running.
Relay 219: Also located in the plenum chamber and is the furthest from the front of the car (large black, 5 terminals). The symptoms leading up to this failure for me were jerky/lumpy acceleration, noticed especially when engine under load travelling up hill. Also present under normal driving conditions but less noticeable. The car also took slightly longer to start and seemed to have a rough idle, causing the car to wobble more than usual when standing still. The symptoms were similar to a failing injector or inconsistent fuel delivery (e.g. an air leak). The morning after the symptoms were first noticed, the car would crank but not start.
First I checked basic fuel delivery to rule out the in-tank fuel pump: pulled the hose off the fuel filter (top left) and cranked the engine. Fuel spurted out of the hose when cranking. Then to the relay. I took relay 219 out and tested it. Only one of the two contacts (87 or 87a on the diagram, I forget which) was making a circuit when the solenoid was powered on. Upon disassembly, the relay looked clean (the arms that make contact looked like they had heated up though, evident from the visible colour change in the copper… is this normal?). However, when moving the arm to the open and closed position, I noticed that in the closed position one of the arms was just short of making contact. I bent this in slightly to close the gap, reinstalled, and sure enough the car started perfectly.
This poor contact would explain the jerky/lumpy acceleration before the no-start situation. I can only assume that relay 219 is responsible for the ECU/injector power. Needless to say, I will be buying a spare relay..
Recommendation: Drill a hole in the bottom of the plenum chamber at its lowest point! This will allow any water that gets in to drain out before damaging any components.
Hope this can help someone. Let me know if you have any questions!
Jim
Reply With Quote
Bookmarks