Hot fuel pump fuse

[Map63Vette]

So way back a few years ago when I first installed my Megasquirt in my 67 Dart 5.7 Hemi swap, I had an interesting issue come up. I put in a brand new fuel tank with built in fuel pump (typical Walbro 255 or equivalent), and took it for the first test drive to the gas station. I got back home and decided to run another errand real quick and backed out of the driveway when suddenly the car just stopped. I checked the relay board and noticed the fuel pump fuse had melted the plastic casing, but not necessarily blown the fuse. I popped another fuse in and it worked fine again. However, every once in a while I'd put my finger on it to see if it was hot and it would burn my finger easy if I left it on for even a second. I upped the amp rating of the fuse (thinking it was right on the edge of it's ability and thus generating some extra heat), but it didn't make a difference and still ran really hot. At that point, someone suggested I rewire the ground on my fuel pump from the chassis up to a more solid location like the transmission. One really long wire later, and I still have the same issue. To this day the fuse still runs quite hot (will still burn a finger, but would take a few seconds). I also tried swapping to a breaker style fuse since I thought that might soak a little less heat than a typical blade fuse, but again nothing has really changed. The power and ground wires to the pump are both brand new wire and fairly hefty gauge (I think something like 8-10 off the top of my head, would have to check again to be sure) and around maybe 6 feet long each. The wires off the pump itself are fairly small gauge, so the mismatch seems way off to me. I don't recall any OEM installation I've ever seen have as large of wire as I'm running, so something just seems off. Any ideas where to look? I bought the whole MS setup (computer and relay board) used from someone for what it's worth, so I didn't have any hand in building it. From what I can tell, it was all "factory" assembled stuff (v3.57 board), so I tend to think it was not a "user error" during buildup, but I'm not sure where to start the diagnosis.

[trakkies]

It's more likely to be a poor connection between fuse and holder - or holder to PCB etc. Is the fuse a firm fit into the connectors - or easy? Does the soldered connection between fuse holder and PCB look good?

[Map63Vette]

It feels like a pretty solid fit for the fuse. Definitely doesn't just fall out. Haven't pulled the board apart yet to have a better look at the backside for the holder. It's got a little wiggle in if you push on the fuse sideways, seems like a little more than the other ones on the board. Guess I'll see about yanking the whole board out and checking out the backside.

[kjones6039]

Guess I'll see about yanking the whole board out and checking out the backside.

That is well indicated IMO. I had a similar experience with a cheap after market fuse block/holder. The contacts were simply not strong enough to maintain good contact, which caused the contacts to arc ever so slightly. In my case however, the contacts became burned which manifested itself in an unpredictable ignition miss/failure.

Ken

[DonTZ125]

+1

This sort of thing was my introduction to the vagaries of automotive electricals, lo these many years ago. The clip for the glass tube headlight fuse on my '83 RZ350 was snapped but not completely broken off - one of the four arms was basically hanging on by the skin of the plating. I went through several fuses, not because they were blowing, but because the solder in the one end cap was melting!

[Map63Vette]

Well, took it all apart and nothing looked too terribly bad. The bottom side of the fuse holder terminals were a little corroded looking, but nothing seemed burnt. Went ahead and pulled the whole fuse holder out and reinstalled it. One of the solder sleeves (or whatever you might call the part that presses into the board) came loose though, so I'm thinking maybe I don't have the best connection to the traces on the board. Either way, did what I could to put it back in and it still runs hot, but pretty comparable to all the other fuses on the board. Also added some solder to the back side of my main power lines. Turns out they were only soldered on the top side. Now to sort out my noisy tps problems.

[Map63Vette]

Unfortunately the problem has reared its head again and this time with a fairly severe outcome. The screw terminal on the relay board that the fuel pump wires to actually melted and was bridging over to my fast idle line, which controls my fans. I first found something was wrong when I turned the key on and heard my fans click on when the engine was dead cold. Never have truly figured out the issue, but have decided to just bypass it entirely by running a new relay in the engine bay and running the trigger circuit from the MS to the new relay. I'm guessing there must be some kind of internal fault in my relay board that's causing the issue, but it's hard to say. Nothing looks obviously bad, but I did purchase the unit used, so I don't know what sort of previous life it may have had.

[kjones6039]

The screw terminal on the relay board that the fuel pump wires to actually melted and was bridging over to my fast idle line, which controls my fans.

Just one of the reasons I tend to shy away from the relay board! To be fair: I used one a long time ago and didn't have too much problem with it. But, my main concern is that it introduces just that many more mechanical connections and, consequently, that many more potential failure points! On all of my subsequent projects I wire everything direct with soldered connections!

I'm guessing there must be some kind of internal fault in my relay board that's causing the issue, but it's hard to say. Nothing looks obviously bad, but I did purchase the unit used, so I don't know what sort of previous life it may have had.

IMO.... Probably not.... All it takes is one connection to loosen up and the arcing can create enough heat to cause the kind of damage you are describing!

Ken

[Map63Vette]

Yeah, it is hard to figure out exactly what was wrong. I may have a somewhat flakey fuel pump as well that's just drawing a lot or something. I tend to agree some on the use of a more dedicated robust power board, but I do find the relay board very handy for the sensor wiring at least. Makes it a fair deal easier to get wires through a firewall and strung to the computer. Ideally I'd have just put a bulkhead connector in my firewall and wired direct, but I didn't have the parts at the time and still don't really have a good location for a big enough bulkhead connector on my firewall. I really should have just gutted the car wiring from the beginning and started fresh, but I was being cheap and in a hurry to get the car back on the road as it had been in the way in the driveway for over a month due to some other projects. The harness I picked up from work has another bulkhead that I may see if I can use for the computer one of these days.