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wire to fuel pump wire, fuse gets very hot


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Mad Scientists 
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Posted: March 08, 2010 at 12:17 PM / IP Logged  

 Without going back and looking at the wiring diagrams, I believe the fuel pump was controlled by cranking signal when the starter was engaged, and oil pressure when the engine was running.. the S10 pickups from the mid 90's come to mind.

 I'm still not clear why it would be necessary to move away from the ECU fuel pump control.. even if someone wanted to run a different relay, they could still use a fuel pump signal line from the ECU.

 My impression of the criteria for fuel pump shutdown was simply to shut off the pump if the ignition was on - IF it was absolutely certain the engine was not running.. I imagine the ECU is looking at several inputs to determine engine run state, but without oil pressure (on the older systems) the engine wouldn't last long anyway, so oil pressure was a fairly durable indicator. I don't see where engine preservation ever entered into the equation.. even today, I'm not aware of anything that would shut down a running engine.

 Jim

KPierson 
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Posted: March 08, 2010 at 2:00 PM / IP Logged  

The OP NEVER said he isn't controlling the relay from the ECU (or the old fuel pump power wire).  His pump is not wired directly to the battery, it is wired directly to the battery THROUGH A RELAY. 

It is somewhat common, on some cars anyway, to rerun the fuel pump wire, in a heavier gauge, directly to the battery to decrease voltage drop and increase performance of the pump. 

I have to say I agree with i am an idiot on this one.  If the problem was caused by too small of a wire then the entire wire would get warm, not just a small section of it.  Based on thermography information I would first suspect the battery terminal, because if it was the fuse holder you would expect the heat to dissipate out of both ends of the fuse holder.  Either way, the heat was a result of a poor connection and the connection needed to be fixed.

That being said, proper fuse sizing is extremely important - according to this chart (at least) https://www.the12volt.com/info/recwirsz.asp the OP is in good shape and could actually run a 40A fuse on the 12awg wire.

Kevin Pierson
Mad Scientists 
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Posted: March 08, 2010 at 5:18 PM / IP Logged  

 The OP never really mentioned how his fuel pump was wired in, but thread drift has brought it up.. and specifically mentioned the oil pressure method. GM has used that method for years, and I was wondering why it was being portrayed as a bad way to do it. My concern with sensing off something else, such as the charging circuit, would mean that loss of drive belt or failed alternator would seem to shut off the fuel pump.. which would probably be a bad thing if it happened during driving. Loss of oil pressure would also shut down the engine, but without oil pressure the life of the engine is measured in minutes, if not seconds, anyway. Regardless, I believe they (the manufacturers) have gotten away from oil pressure shutoff.. I'm not aware of a single trouble condition that would tell the ECU to shut down the engine, ie: overheat, loss of oil pressure, loss of cylinders,  or anything else.

  Ford, for example, has lost class action lawsuits for ignition modules that failed and shut the engine down while driving.. can you imagine the legal liability if a manufacturer engineered something that *intentionally* shut the engine off?

 As for the fuel pump hot wiring, doing voltage drop measurements on the circuit while it was powered would likely have shown a large drop from the battery to the fuse holder.. that would have been my first clue. Also, as was mentioned before, heat=resistance.

 Jim

oldspark 
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Posted: March 08, 2010 at 5:29 PM / IP Logged  
In the GM/Delco system I know off, the fuel pump will be powered whenever the oil pressure is up (switch closed) and ignition is on - irrespective of the ECU.
I won't argue why its fuel pumps was controlled that way, all I'm saying is that it does not meet normal safety requirements (ie, if the engine stalls, then the fuel pump must shut off).
Furthermore, for those that think - sorry..., I mean for those that believe the oil-pressure sensing in the Delco system above is to somehow protect the engine, they are wrong. Obviously! (The ECU does NOT kill the engine upon loss of oil pressure.)
Oil pressure sensing is a valid way to preserve the engine, but NOT by killing the fuel pump.
In summary, fuel pump control and oil pressure should be completely separated - they are two totally different requirements.
This has been discussed and agreed elsewhere and is considered a dead issue - except by those that haven't yet considered it, or are not able to comprehend it.
PS....
Mad Scientists wrote:
.... but without oil pressure the life of the engine is measured in minutes, if not seconds,
THANK YOU!... if only you knew....
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