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Join Date: Dec 1969
Location: Dade County, FL.
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Send a message via AIM to JP Noonan
Blowing pump fuse, and hot V.R.

About 2 months ago I blew my fuel pump fuse while driving fairly hard. Figured out there was no noise, traced it to the fuse, and away we go...for about 2 months. June 28th leaving for NY NY, fuse blows again, pop in new one, get to airport, leave car until the 9th, it starts right up (actually I'm very pround of that one, considering it was outside the whole time). I drive a couple short trips, blow fuse again. Works for 10 minutes, pops again, new fuse. Now if I rev the motor it blows the fuse. I get home by removing the hot lead (worried that there is a short) and use a 30 amp fused jumper wire, directly to the battery.

Pump is a 6 month old new Bosch Carrerra 911 pump, filter is new, and I can't seem to find a short.

I don't have lots of time to work on it this week so any previous experience as to the cause is welcome. I'm thinking maybe the humidity in Miami has caused corrosion on the contacts, whould that resistence be enough to heat up and pop the fuse? I got a rock in my fuel pump? Some more mouse eaten wires I didn't find yet?

Also it seems that for the short time I drive the car the V.R. gets hotter than normal. It has an extra ground to the chassis besides the stock one. Maybe more corrosion, or the V.R. is dying and spiking the fuel pump?


TIA.

Old 07-10-2000, 04:39 PM
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Location: Hickory NC USA
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To determine if it is the pump, I recommend half-splitting the problem. Disconnect the factory electrical connectors at the pump. Then, install a temporary new fuse at the pump and connect it the hot wire to the battery + and the ground to the neg lead.

My $0.015 worth.

Oh check your return line (not dead heading the pump)

I hope this helps...

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Smoke Daddy
Old 07-10-2000, 06:52 PM
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Well the bypass I used had a 30 amp fuse in it which hasn't blown for the last two days. I cleaned up the contacts on the relay board and put in a extra VR. VR still gets hot and when I put the stock 25 amp fuse back in I got home and blew the fuse pulling in the drive.

I'm fairly set on it being a increase in resistence as the temp increases in the wiring. To that end, I've removed all the epoxy on the back of the relay board to find some of the contacts have been soldered, but not all. I think this is a "rebuilt" relay board like the catalogs sell. So I wipped out the Dremel with wire brush ("why-r-brush? Cause it works!") and cleaned the riveted contacts on the top and bottom of the relay board. I used a 9/64" drill bit in reverse to carefully clean out the relay sockets, can't say it's the safest thing to do, I think it may twist the sockets too much, there is probably a metric drill size that would work better. Anyway, I soldered the relay board to within an inch of it's life, we'll see if that does it.

Next the wiring.

Any other ideas?


P.S. Jim is that your car in Grassroots Motorsports?
Old 07-12-2000, 08:46 PM
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Yes, the red 914 seen in grassroots is mine.

That was a fun day, driving around road atlanta.

I am currently putting a 911 3.0 SC engine in.

Any luck trying to run the pump from an external wiring source???



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Smoke Daddy
Old 07-13-2000, 03:57 PM
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I not sure I would go with the resistance increasing ideal. Typically if resistance increase and voltage source remains constant, the current will decrease (Ohms law)

I would say that either the pump is going bad, or you have a short in a wire somewhere to ground... IMHO.

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Smoke Daddy
Old 07-13-2000, 06:32 PM
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Yeah, the only law I remeber from physics in "Newton's Law", like when I drop a heat exchanger it accelerates toward the ground, unless it encounters my head first, which of coarse proves "Murphy's Law"

Several others have pointed out my mistake about an increase in resistance causing enough heat to blow the fuse. I did have the pump jumpered with a 30 amp fused 10 ga wire that went directly from the pump to the battery, I first dissconected the pump from the wiring harness. With this "road side" engineering the pump ran fine, unfortunatly it also ran with the engine off.

Today things went well, I had removed my relay board, cleaned it with the Dremmel and soldered all the riveted contacts. I also disconnected the Aux Air valve. The Aux Air wires appear O.K. and that entire ignition harness has been rebuilt, so maybe it is an internal short in the AA regulator as someone else suggested?

Next week I'll figure it out, but a bad AA would explain the problem only occuring when the engine is hot.

P.S. When I say the "ignition harness" I mean the one that has the oil light, coil +12V, starter wire, reverse lights, tach wire, and +12V for the AA. I Used 16 ga wire and soldered them into the facory socket thingy after un-soldering the factory wires. Looks nice if I do say so my self.

Old 07-13-2000, 10:46 PM
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