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Discussion Starter #1
Hi all -

I'm in the process of some diagnostic work on a no fuel situation in my S4 Motronic Spider. The engine just shut off today on the road, coasted to the curb, got towed home. No bang, no bump, just cruising along fine as usual and it shut off - all other electrics working fine. As of now, the fuel pumps aren't turning on - however, when direct voltage is applied via a battery, they both work fine. So here's what I've done.

1) Checked grounds/connections/tubing/fuses and relays. All work fine.
2) Confirmed good, fresh gas in tank. No air leaks.
3) Confirmed strong spark from plugs and coil.
3) Car cranks, RPM tach moves, no fuel on plugs. Gauges work.
4) Squirted starter fluid in open TPS, car started, died.
5) Jumpered fuel pump relay pins 30-87, still no dice.
6) Battery is good, voltage is fine.
7) No ECU codes.
8) ECU and relays click when ignition is turned "on". No noise from pumps during cranking.
9) Rotor and dist. and wires are fine.

Wish I'd had my voltmeter with me, as well as my shop manual, but alas. I would have liked to confirm voltages. As of now, I don't believe I'm getting power to the fuel pumps. No whine from the pumps or anything (unless I connect a battery directly to them). RPM timing sensor is hooked up. Nothing obviously loose or disconnected. I keep things neat and clean. Relays all were checked in the horn relay position. Checked out OK. Fuses near the ECU swapped out too. All connections there tight and clean.

A few years back I had an ECU issue that resulted in no voltage to the ignition coil. The last thing we suspected was the ECU, but I'm curious if this is another ECU problem, except this time it is killing voltage to the pumps. Again, nice fat sparks from coil. I'm following the same troubleshooting path I did a few years ago. Agh.

I'm perusing my S4 shop manual, hoping to come with some ideas. Dammit, why can't I ever get off easy - like an in-tank pump or something??? I'm just so jaded to this process already. Murphy's Law is in full effect on me this week!

Thanks in advance for your ideas.
 

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I had a similar problem with my S3, it turned out to be the inline fuse that is hiding in the rear deck behind the passenger seat. I am guessing that you also have the same fuse..

Ed
 

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Discussion Starter #3
Thanks Echo...boy, I wish it was one of those fuses, but it's not. I'm just plain old stumped.
 

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O NOES!!!

Hope you don't have the same problem I have. I've got good relays, signal from the coil, pumps work when voltage is applied directly (the only way I can drive the car for the time being), voltage coming to the relays, fuse holder is missing so that's not it, I'm even getting voltage at the part of the circuit up by the inertia switch in the engine bay. The pump-side of the inertia switch circuit up there is showing a direct connection to ground, which doesn't sound right, so maybe I've got a short somewhere. Funny thing is I can plug in the relay and I only have like less than a volt coming out of the pin that's supposed to supply power to the pumps (#87 I think), so the relay contacts might be burnt. But shorting 30-87 doesn't make the pumps go either.

I'm totally stumped myself. I'm just hoping that as I work on other parts of the wiring harness and correct damaged wires and such, I'll spot the problem. We'll see.
 

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A word of advice about troubleshooting with a voltmeter. You can have high resistance connections in the circuit that will only show up when the circuit is loaded. For example, there is a switch of some kind (inertia, temperature, pressure, a relay contact) in the circuit. You disconnect the load and measure a good voltage. That is because the resistance of your test meter is much higher than the resistance in the circuit, there is very little current flowing and there is only a tiny volt drop across the bad contact and you cannot measure it. The way to trouble shoot these circuits is to switch them on with the load and measure the volt drops in the circuit. If the 12 volts at the pump wire drops to 1 volt when you try to operate the pump, then 11 volts are being dropped in the circuit. Measure the drop across each device in the circuit. One of them will be high and that is the culprit.
Ed Prytherch
79 Spider
88 Milano Verde
 

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Discussion Starter #7
Thanks everyone for your responses. I'll have my digital voltage meter with me tomorrow when I work on the car and my shop manual. I'll check voltages at the fuses by the pump relay, and also voltages to ground, as well as the fuel pumps. The S4 has no intertia switch. I'll also be able to get some sort of reading (I hope) from the appropriate pins on the ECU (20 and 35).

Anyone have any suggestions when I'm getting the appropriate voltages to everywhere BUT the fuel pumps, which I believe is the case. I mean, I could take voltages all over this car, but really, what could be the culprit when everything else is working - relays, fuses, wires, etc. The ECU...again.

Gee, sure would like to relax this weekend... :)

I looked at RML ignition systems, but alas there is nothing for the Motronic S4. Anyone use Megasquirt? I would really like something that would allow me to completely rip out this PITA Bosch ECU.

Thanks
 

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What is the last device in the circuit before the fuel pump? The pump relay or a fuse? If you have voltage going into that device but not coming out, then you have it. You just have to take measurements until you see where the voltage is being dropped. If you want to measure the voltage on a wire without unplugging it, push a pin through the insulation until it makes contact with the conductor and measure it. It is not rocket science. All the volt drops in the circuit must add up to the battery voltage. You want almost all of it to be across the fuel pump terminals. If it not, then it is somewhere else. The voltmeter will enable you to find it.

I had a problem with the fuel pump cutting out on my Milano. It was harder to find as it was an intermittent problem. I first of all ran a jumper from the ignition switch to the fuel pump so that it ran all the time that the ignition was on. The problem went away so I new that it was not the pump. I removed that jumper and put it to the coil of the fuel pump relay. Again, the problem went away. So it had to be one of the two devices in the circuit to the relay coil. I guessed at the AFM switch and jumped it out. That fixed the problem so I knew that I had an intermittent fault on the AFM switch.

I had a very similar problem on my spider. In that case the inertia switch went open circuit or high resistance when it got hot. It "went good" when it cooled down. I proved it by jumping it out.

Permanent problems are a lot easier to find than fleeting ones.

Ed Prytherch
 

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Hrrrmm, do S3's have the fuel pump switch on the AFM? I hadn't thought of that.....I was under the impression the pump on the S3 was only cut out by the inertia switch, and got its "on" signal via the signal line from the coil.
 

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Discussion Starter #10
Thanks for your respones - Friday afternoon I'm going to be going to work on the car, armed with your questions and advice and we'll see what I come up with. I'll be sure to post either way. Hopefully I'll come up with some real evidence on Friday.
 

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Discussion Starter #11
Finally resolved the issue. It was a bad power connection at the fuel pump. First thing I did when the car stalled was check them, and only after I removed and cleaned them did the car start right up. Man, talk about a simple fix, but what a PITA to troubleshoot!

Thanks everyone for their replies!
 

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Maaan, if only mine were that simple.....but I know it's not that, in my case, since putting juice directly to the power lead in the cargo bay going to the pump makes them work. Oh well.....dash removal commenced last night. =O
 
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