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I have a very early series 2 1982 635CSI, manual. I'm trying to get the fuel pump to come on with position 2 of the ignition (new switch) but it won't.

The main relay and fuel pump relays are good and are properly connected with 12v supplies, however, the main relay is not switching the fuel pump relay. I have done a continuity test and they are connected and the wire to pin 20 of the ECU is fine. So, could it be the ECU?

The fuel pump won't come on until the ECU "sees" the crankshaft turning through the "pulse generator" (aka the crank position sensor).

Thanks for the reply. Done some investigation into the ECU and it's correct for my year etc. According to the circuit diagram the pump should come on at position 2 of the ignition - this is what's I'm used to with Audis of the same era (though mechanic FI).

Doesn't the flywheel sensor control the firing timing? If you're right then maybe there's no problem - seems a bit late in the day to call the pump on!

Hornhospital is correct. The fuel pump will not activate until the engine is cranking.

The diagram you are using is correct for when the car is running.

Take a look here for troubleshooting questions:

http://www.hiperformancestore.com/motronic.htm

That link is no longer active unfortunately. I have it in electronic form at home but won't be back till next week.

OK I found it. Everyone should read this and credit to Hiperformance Store.

tschultz wrote: ↑Thu Jan 25, 2018 2:33 pm Take a look here for troubleshooting questions:

http://www.hiperformancestore.com/motronic.htm

Thanks for sharing this link Tom!

Many thanks, I've been reading through this over the last few weeks - very useful. For some reason the fuel pump isn't pumping when I crank the engine - there is an issue between the main relay, fuel pump relay and ECU - there's continuity between all and the relays are fine. It's possible that the flywheel reference tag has come off - I'll test for this tomorrow...

Done lots of testing this weekend, I have narrowed the no start fault down to the fuel pump relay coil only getting 8v between terminal 85 and 86 upon cranking.

The speed sensor is working, relay itself is fine. As I understand it 12v is supplied direct from the Main Relay upon cranking - then to the Motronic Unit - could the Motronic unit cause low voltage and if so how?

Below is something I've been working on to help test the wiring - I'm ignoring emission controls as it for a UK car - 1982 - 1984 ECU 011. If you can see any mistakes or can fill in the blanks please let me know.

Check for a voltage drop (when cranking) across the relay terminals on the main relay (30 and 87). If a voltage drop of more than 1v is registered, there could be high resistance on the main relay contacts.

Bit of a major breakthrough - I swapped back my old OBC and there is now 12v at the relays and injectors - fuel is flowing. It must be the switch in the OBC - it only allowed 6v through - is this typical of a failed unit? Strange thing is, the OBC display now works fine - which it didn't before.

At one point last week all of the injectors were buzzing - the fuel rail was very loose - I tightened it all up and now no buzzing, but should they buzz?

The symptoms you mention seem consistent with the OBC security switch "buzzing", which could cause the main relay to "chatter". That is, the OBC is causing its relay to chatter, which could cause your measured 8v at the fuel relay due to main relay being only momentarily closed, and also explain buzzing injectors. If this happens again, a good idea is to bridge out the OBC relay to test.

Nice fault finding though. A 12v circuit is simply that. A set of switches (could be relays) operate a device (usually a coil on another relay or light or motor etc). You should see a total voltage drop across the circuit (when in energised state) of 12v. The device or load itself should see 12v or close voltage drop across it. Switches should see less than 1v voltage drop across them. Everything in the series circuit adds up to 12v. If there is 8v drop across a relay coil, you know that there must be 4v drop across somewhere else on that circuit. This is one of the fundamentals of fault finding.

Thanks for the reply, feel like I'm becoming familiar with the whole setup!

I've tested the coil, it's fine but no spark is going to the distributor - HT lead measures over 2k ohms - what should it be?

When attempting to start, are you getting 12v on the coil primary ? (Green wire). Check all grounds, but especially G103.
Edit; Sorry. If your HT lead is an inductive resistor type then 2K ohm should be OK.

Thanks for the reply!

I replaced the leads as they were original - but still no start! I again checked the HT lead with a spark plug against the manifold and I got a nice spark - but couldn't repeat it?!? I cranked the car over many time - no fuel is being sprayed into the cylinders - the injectors are getting 12v. I tested the outputs from both flywheels sensors and they read 0.2v and 2.0v - half what they should be - slow cranking reducing the output maybe?

Could there be two faults?

1. OBC intermittent 'Start Lock' playing up - I can and will jump the connections to rule this out.
2. Faulty fuel pressure regulator - there is flow but maybe too much so pressure is too low. The car was standing for 9 years - when I first got the fuel circulating the FPR made a creaking noise - rusty spring? I'll order a new one...

Soon I'll have replaced half the car!

randall977 wrote: ↑Thu Feb 01, 2018 11:28 pm the injectors are getting 12v.

That's only half the equation. The injectors are supposed to have 12v continuously any time the key is on. It's the ECU that has to pulse the injectors to make them work.

Problem is, it's very difficult to measure pulses - is there a way of doing it? Is there a diagnostic tool available?

Noid light. Simple as can be. Unplug one injector and plug in the noid light, spin the engine and observe. It should flash with each revolution of the engine. You can make a noid light out of a #194 bulb. Straighten the wire contacts to fit the injector harness plug. It won't be as bright as an actual noid light, but it will prove whether the ECU is pulsing the injectors or not.

Noid light for sure. Just want to clarify. You've established you have no fuel and no spark ?

Thanks for the replies - looked it up and I can see what to do - I'll make a noid light.

Shame I didn't pick up your reply before removing the inlet manifold etc to clean up the injector holes etc - I got carried away!

I did test each injector though and they all fire. It seems that the injectors are not getting a pulse signal or there's no spark. Could it be an ECU fault? My ECU is a recon unit - not by me - could have been damaged by the previous owns attempts to start by jumping the car...

It's quite expensive to test an ECU and I suspect if it's fine they will still tell me they had to repair it for £250!

To help us help you, you need to answer the questions put to you. The "shot gun" method to fault finding is not good.

Slightly disappointing day - very nearly managed to test the injectors with a noid light but not quite due to big petrol leaks.

I cleaned all the injectors, put everything back together but (unknowingly) managed to nick one of the injector fuel rail seals causing petrol to poor down the block upon cranking. At first I though it was the previously disconnected cold start valve - then realised it was the injector above. So new seals on the way...and cold start valve (which doesn't work).

What I did learn was that I'm getting fuel pressure, injectors 4,5 and 6 injected. The gearbox seems to be dragging in neutral, if you put your foot on the clutch the cranking is about 10x faster. In fact I think it was only when I got fast cranking that things started to happen - sadly progress was cut short for the above reasons.

Bit more testing today including connecting a noid light - it lit up with the ignition and flickered as the car was cranked - but I assume this was the drop in voltage due to cranking? I have 11v at the injectors with ignition on - is this normal?

I by-passed the OBC etc but this made no difference. It seems that neither fuel or spark are getting to the cylinder - coil is fine but no spark from the HT lead. When I remove a spark plug it's dry - shouldn't the cold start at least put petrol in the cylinders? The fuel pump relay only shows 7 - 9 volts when cranking - but upon removal of the pipe from the pressure regulator it is flowing when cranking...

Tomorrow I'm going to put an oscilloscope on the flywheel position sensor and injectors as a multitier such as mine is not sensitive enough to conclusively detect pulses.

In the back of my mind is the ECU but this is clutching at straws.

To cap it all off the fuel gauge is not showing any fuel, even though it has been okay until today (when I added more petrol).

I agree with baders, no answers=no answers. Toph