98 S70 won't start - main relay not energized

homerh · Post by **homerh** » 24 Dec 2009, 21:46

I just finished doing a valve job after my timing belt tensioner broke. I also replaced the radiator, oil cooler hoses, PVC system, water pump, timing belt and of course the tensioner. I had the head reconditioned by a machine shop, but did all the rest of the work myself. After putting it all back together and checking carefully that everything is correctly back in place, it now cranks, but won't start.

In diagnosing the problem I quickly determined that I had compression, but no fuel nor spark. Knowing that I must have spark before I can get fuel, I traced out the system using the engine control schematic and determined that the main system relay is not being energized when the ignition switch is in position II. All the dash lights come on and no codes are set. I checked all the fuses and have power at the relay terminals, but the ECU is not pulling the low side to ground (it stays floating). What can cause this besides a bad ECU?

98 S70 GLT
155K

linuxsuperuser · Post by **linuxsuperuser** » 24 Dec 2009, 22:46

I checked all the fuses and have power at the relay terminals, but the ECU is not pulling the low side to ground (it stays floating)

My 2 Cents...

Problems with grounding can cause all sorts of problems, using an ohm-meter check the grounding connections on the ECU ground wire to any bare metal point that is bolted to the frame. And perhaps verify that the grounding pin in the relay holder is also reading no ohms, from the frame.

Having to replace that ECU would suck, this almost sounds like a simple oversight, like not using a star-washer for ground connections, sounds very annoying.

Good Luck
I have a slower, S70 98 N/A 171K but I know that year.

Post by **Ozark Lee** » 25 Dec 2009, 10:24

Does your IAC valve buzz when you have the key in position II? In the process of replacing the radiator you might have not gotten the master fuel system relay plugged back in. It is also prone to having its wires break when it gets unplugged. It is the gray relay on the fan shroud towards the drivers side of the car.

The IAC valve and the injectors get power from that relay so if the valve isn't buzzing that is a place to start.

Also make sure that the crankshaft position sensor got plugged back in.

...Lee

homerh · Post by **homerh** » 30 Dec 2009, 21:15

With your advice I was able to find the problem, which turned out to be the big connector on the left side of the radiator fan shroud, right underneath the fresh air intake duct. I used my multimeter to verify the ECM had pwr and gnd, but when I checked for continuity between the pin that is responsible for closing the solenoid of the main relay and the corresponding pin on the ECM, I found it to be open. The schematic in my manual told me the color of the wire and I traced it to the connector. I knew I was home when I pushed on the connector and heard a click. The car started up right after that and now it runs great. Thanks again for your very helpful tips. I was a day away from towing my car to the shop.

turboman850 · Post by **turboman850** » 17 Oct 2019, 17:17

How did you know which pin to check at the ECM? Curious, as yesterday tried same by pulling out the ECM to look for pin numbers but could not find any on female or male of this Electronic Control Monitor.
Also, do I "Truly" even need the Main Fuel Relay and can't I just "hard wire" across it or will this damage ECM or injectors???