Sears point update
07-08-2014, 10:20 AM
#16
Rennlist Member
A clogged fuel filter will reduce flow and increase pressure at the pump-- causing an increase in current demand by the pump. But again that would be continuous, and the engine would be going lean at the top end.
When the fuse blows, how does it look? If the fuse element is vaporized then it was a sudden high-current short to ground. If it has a "melted" look then it happened slowly, a small continuous over-current. Have a look at the blown fuses and post a picture if you can.
07-08-2014, 04:30 PM
#17
Rennlist Member
Thread Starter
Agreed, unlikely. The fuel pump doesn't care about your throttle but it does work a little harder at WOT, because the fuel pressure is tied to engine vacuum.
A clogged fuel filter will reduce flow and increase pressure at the pump-- causing an increase in current demand by the pump. But again that would be continuous, and the engine would be going lean at the top end.
When the fuse blows, how does it look? If the fuse element is vaporized then it was a sudden high-current short to ground. If it has a "melted" look then it happened slowly, a small continuous over-current. Have a look at the blown fuses and post a picture if you can.
A clogged fuel filter will reduce flow and increase pressure at the pump-- causing an increase in current demand by the pump. But again that would be continuous, and the engine would be going lean at the top end.
When the fuse blows, how does it look? If the fuse element is vaporized then it was a sudden high-current short to ground. If it has a "melted" look then it happened slowly, a small continuous over-current. Have a look at the blown fuses and post a picture if you can.
i think i tossed the fuses., but ill see if i can find one.
07-08-2014, 04:33 PM
#18
Rennlist Member
Thread Starter
Just a passing thought, but speaking for myself, I would replace the entire wiring harness in a 928 if I were to go racing. Why? As a starter, the stock wiring harness weighs about 43 pounds in total. Secondly, the wiring 928 harness(s) I have been into, have all had corrosion in strange places, broken wires here and there, a bunch of unnecessary wiring for racing, on and on. Companies like Painless or Watson Street Products make some very robust kits for wiring race cars, or anything else. This would yield modern wires and modern connectors, of the necessary current capacity, while eliminating the entire CE panel. The tough part of the process would be connecting the computer(s) as I don't think there are modern connectors to readily / directly replace those connections. Those would require careful dis assembly of the old connector, removal of the old pins, finding the correct replacement pins, connecting new wires to the new pins, and then putting them in the old connector "blocks". Not rocket science. I have done it more than once in old electronics, some dating back to WWII or older. But, when finished, many of the electrical issues we have with 928's should be minimized if not eliminated. Not sure I would try this with a street car where I wanted to maintain all the various functions and features, but for a race car it could be made far more simple and most likely far more reliable. I have done this on several cars, admittedly all were more simple electrically speaking than a 928, but it is pain quickly forgotten when done...
07-08-2014, 04:39 PM
#19
Rennlist Member
Thread Starter
I was studying the wiring harness last night via the wiring diagram. did some tests too. obviously, the relay 30 feeds power to pin 87 when closed and then feeds to the fuse 42, and then off it goes to the fuel pump. however along the way after the fuse 42, it hits the 02 sensor. if that has a short, theres the issue!! however, that wiring looks solid, unless something is happening in the o2 sensor itself? likely? the worst case, is its intermittant. I dont need the 02 sensor when racing, so maybe disconnect when racing? but if the short is on the backplane connection for the 02 sensor, then same problem will exist. BUT, i can run a separate wire , ignition activated directly to the pump. but ill need a switch on the stock fuel pump wire, so if that blows the fuse again, i can remove the short from the circuit (if 02 related), and use a separte power line to the pump. (have to break the stock circuit because if that short is still there, it willl effect the alternatve line, as i have already seen at the track! 
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07-09-2014, 04:16 PM
#20
Rennlist Member
Thread Starter
went for a 20min hyway run with the stock system and a current meter on the fuse 42. current was 6.5amps, no matter how fast and what the WOT RPM. so, certainly no clogged fuel filter , or pump issue. the return trip, i used the alterate wired from switched power, directly to the fuel pump, and disconneced the stock power terminal from the circuit. (so of there was a short, in the wired, backplane of the fuse panel, or o2 sensor, it wouldnt show up)
5.7amps was the current read, no variance.... BUT, this new wire doesnt power the 02 sensor, so its not reading and controliing mixture at idle. (o2 sensor power comes after the fuse as the power is then applied to the fuel pump positive terminal in the stock set up)
so, still nothing found..... But, im thinking there is an exposed piece of a spade terminal on the 02 sensor tap, that if was near the fuse panel cover metal studs, it could have shorted there. But, its the black wire, not the two white wires from the 02 sensor, so can that provide a short?
this is frustating......can a 02 sensor have an internal short?
i think ill run it again at the track before the laguna event, and make a two switch set up to remove the stock circute and use the new clean wire from power to fuel pump.
5.7amps was the current read, no variance.... BUT, this new wire doesnt power the 02 sensor, so its not reading and controliing mixture at idle. (o2 sensor power comes after the fuse as the power is then applied to the fuel pump positive terminal in the stock set up)
so, still nothing found..... But, im thinking there is an exposed piece of a spade terminal on the 02 sensor tap, that if was near the fuse panel cover metal studs, it could have shorted there. But, its the black wire, not the two white wires from the 02 sensor, so can that provide a short?
this is frustating......can a 02 sensor have an internal short?
i think ill run it again at the track before the laguna event, and make a two switch set up to remove the stock circute and use the new clean wire from power to fuel pump.
07-09-2014, 05:07 PM
#22
Rennlist Member
Thread Starter
ill pull the 02 sensor and see what i see.
does the sensor wire have ability to short out to ground and pop the fuse?
thats the only moving around, exposed connection at the fuse panel area.
07-11-2014, 02:50 PM
#24
Rennlist Member
Thread Starter
I think ill have to pull it and see if anything is frayed and remotely able to contact the chassis. I have a suspicion that that o2 sensor wire (not the power or ground, was arcing out to the fuse panel studs or floor. but I don't know if that is possible. is the sensor wire able to draw big current if it touches ground? ill have to get a magnifiying glass and check out the exposed surface to see if it has evidence of arcing on it. otherwise, I might have to deploy plan B...... separate circuit in parallel with two switches to toggle between the two, if the primary does this again.
07-17-2014, 03:18 AM
#26
Rennlist Member
Thread Starter
I cant simulate any issue. as it was fine in stuck in 3rd gear around sears. something about 120mph and a hard bumpy left turn, seems to trigger it, and only that.
if the 02 sensor wire can short to ground and blow the fuse, i think its that exposed terminal. if not, I think ill have to make a dual switch network to break the normal power line to the pump and turn on a separate line to the fuel pump. frustrating!
if the 02 sensor wire can short to ground and blow the fuse, i think its that exposed terminal. if not, I think ill have to make a dual switch network to break the normal power line to the pump and turn on a separate line to the fuel pump. frustrating!
