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Discussion Starter #1
My 86 Spider died as I was entering driveway. It literally coasted to a stop. After a little troubleshooting I saw their was no fuel pressure at rail. I subsequently found the in-line fuse before fuel pump drive relay blown. I then also discovered that it was a 30 amp fuse and the drawings say it should be 8amps. Evidently the former owner found that a 30 worked and I just never had occasion to check it out until now.

I'm going to create a little test jumper with a fuse holder, put them on each pump (one at a time), use an 8 amp fuse and then see what blows.

I checked for continuity to ground on the non-power side of the fuse socket and found none so there is no wire shorting to ground. I figure one of the pumps is for some reason drawing a lot of current.

Before proceeding I thought I'd throw this out there seeing if someone else has had similar issue.
 

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Sounds like you are attacking it correctly. I kept blowing that fuse and changed the fuse holder to a waterproof atc type. No more problems. I would also change the fuel filter.
 

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But Mad North-Northwest
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This was on a Motronic S4, mind you, but on the later Spiders the fuse for the two fuel pumps also powers the O2 sensor heater. I had a buddy whose car kept blowing the fuel pump fuse and it turned out to be a short in the sensor heater. You might check the S3 wiring diagram to see if this is the same (I do not have it in front of me at this time, sorry).
 

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Discussion Starter #4
Thanks guys

The O2 sensor heater element indeed is part of this fused circuit.
Very good information here.
I would think the continuity test would have shown some ohms as the heater element is either
1. broken - in which case there is no current flow thus no blown fuse. or
2. shorted to the sensor casing, in which case I should have seen some ohms

I will definitely include the O2 sensor heater element in t-shooting.

I don't think there is any problem with fuel flow as when I replaced fuse and things worked for a few seconds, fuel pressure shot up almost instantaneously when I started cranking motor. But, once again I will include in t-shooting efforts.

Thanks to all. And keep those cards and letters coming....

I will post outcome.

Warren
 

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But Mad North-Northwest
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Well, you can run for a short while without the in-tank pump or the sensor heater. Unplug those and see if the 8A blows. If not, plug stuff back in one at a time until you find the culprit.

That's how we sorted out my buddy's S4. Sure beats adding a fuse to each device.
 

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Discussion Starter #6
Reply

The device would be only for testing and will cost pennies (socket, 2 crimp-on connectors, and a fuse). It will take literally five minutes to check in-tank pump.
The pump underneath car may be a problem as I can't drive it up on the ramps. I may be able to find test point in circuit on top.

I've got a small assortment of test jumpers for all my cars. Thirty plus years of troubleshooting all sorts of control systems in major electric generating stations was a great learning experience.
 

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I assume you are talking about the 8 amp fuse holder under the passenger side of the package shelf.

Do you have a meter that will read DC amps? If so, connect across the 8 amp fuse terminal and measure the current draw. If it exceeds just a few amps, you can search for your problem, otherwise, your issue could be the old fuse holder itself.

Either way, knowing the current draw would be useful information.
 

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Discussion Starter #8
I have not bothered reading the amps as it has blown two 30 amp fuses in a row rather quickly. So I know that it at least exceeds 30 amps. Knowing the exact value isn't going to be that critical in troubleshooting.

I will isolate and test each device in the fused circuit - the two pumps, the O2 sensor heater, the pump relay (could be internal issue with contacts and/or grounding), and the "auxiliary air device" - whatever that is (haven't read up on it yet).
 

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Discussion Starter #9
Gubi -thanks

Success!

After about 30 minutes I found the problem. I found that the O2 sensors wire run up to top of left fender well. The connections were staring me in the face. I did a simple test. I unplugged the 2-wire cable (to heater element). Then I put a 10 amp fuse into FI fuse holder. I started car and all was OK.

I shut car off, replugged connector to heater element, started car and the fuse immediately flashed open.

I'm pretty sure that I would have eventually found the problem, but thanks to Gubi, I saved a lot of time.

I have already ordered another O2 assembly.

This also explains another "issue". When driving at night with the headlights on and fan motor on, the dashboard voltmeter always looked just a little low to me. This constant current draw from the element was probably the gremlin that was straining my electrical system. It was probably drawing close to 30 amps with ignition on and, evidently after further degradation of the element, the amps increased to now exceed 30.

Thanks again to all. Aren't these forums great!

Warren
 

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But Mad North-Northwest
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Glad to be of assistance. Now go find the stupid PO who put a 30A fuse in there and smack him with a tire iron!
 

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Discussion Starter #11
Problem solved

Replaced sensor and all the problems went away. The old sensor looked like toast anyway.

I probably increased my mpg significantly. When the voltage signal from a bad O2 sensor decreases, the computer misinterprets this as an indication of high excess O2. To avoid running lean it increases duration of fuel injectors causing a richer condition thus wasting gas.

I'm still keeping an ample supply of spare fuses in the trunk.

Thanks to all and special thanks to Gubi.

We'll talk again when my next problem arises.

Warren
 
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