Fuel pump relay merc 6.2 mx horizon engine

kvduff

kvduff

Well-Known Member
Jun 22, 2011
715
Long Island N.Y.
Boat Info
1998 450 Sundancer
Engines
Caterpillar 3126 -420hp
A week ago my port side fuel pump stopped working. Was able to swap the relay from the starboard engine and all worked well, bought two new relays for the port engine and kept the good one for a spare. Took the boat out , ran for 2 hours , sleep over night took boat back home another 2 hours. This week the relay went again, thankfully I had a spare. Replaced no problem... Does any one have an opinion on this? Could it just been a crappy replacement that I bought? Or is there some other reason for engine to blow two relays in a week?


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Same engine and absolutely zero idea of what you are talking about. Do you have pix so the rest of us can learn? TIA.
 
ImageUploadedByTapatalk1433634899.082669.jpg
This is a picture of the relay, it sits next to the ecm(computer) for the engine.. There are two of them on each engine. My boat has fried/blown (not really sure what to call it) the same relay twice in the past week.
 
What is the Amp rating of the relay. Does it actually power the fuel pump?
The electric fuel pump may be going bad and drawing way too much current for the relay contacts.
 
Bt-
The relay seems to power the fuel pump. Once I replace it and flip the ignition switch to power I can hear the pump start running to pressurize the system. Without the relay or when it's blown no fuel pump noise. Interesting thought about it drawing too much. Don't want to start throwing parts at it when these relays are so cheap!
Wonder if there is a way to determine if the pump is drawing too much.
 
Thanks bt.
I read my pressure at the rail it was between 35-40 during start up and then through out the throttle range maintained a constant 40. I unbolted the pressure regulator as I have read that some of the paint chips collect in there. When I removed it, there were plenty of flakes. Decided to order a new module pn#8m0047215 awaiting the arrival and some peace of mind. ( at least on the port side).
 
Well I changed out the fuel cooler with the newest module merc pn# 8M0047215. Removal was pretty easy. The fuel line was stuck a bit and took some persuasion to remove the line but the reinstall was easy enough. Just for the record if anyone is looking for a fuel cooler the best price I found was on ebay for $857.00 sold buy Busters Marine in Far Rockaway New York. Easily the lowest price on the web right now.
 
Back again, well at least least I have a new fuel cooler, but I blew that damn relay again. Seems to happen right after starting the engine. I traced the wires as best as I could around the harness all seems intact. Could this be an alternator issue at the start up. Remember the relay is burning up everything I've read indicates that the burning. Is caused by over draw on the circuit, once it is running there is no problems. Fuel pressure at the rail is between 38 and 43. Totally lost at this point


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Connect +12VDC on A1 and -12VDC on A2 contact with the relay removed. Do you hear the relay clicking? If not it is the coil burning up and not the relay contacts. This would eliminate the fuel pump as the issue.
 
I have the same problem with my boat (350 mags) and only on the stb engine. I now carry like 10 extra relays. Not even my mechanic can spot what wrong. He said my be replacing the whole engine wiring.
 
You really need to figure out if it is the coil or the contacts that are failing. If it is the contacts there is probably an equivalent relay with a higher contact rating and different material for the contacts.
If it is the coil failing a flyback diode in parallel with the coil will help. Without knowing what on the relay is failing it is impossible to suggest a way to solve the problem.
 
Tarantula,
I had my mechanic come out thin pat Friday, he tested each of the wires in the harness lading to the relay and nothing was out of range. I have rebuilt the Delphi connector and also traced back the ground wire from the fuel pump to a connector in the wiring Harness that seemed suspect. I cut, rewired and sealed them. Thus far no relay issues. Just waiting for the next occasion.



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Westie,
The #30 leg of the relay is burning up. Appears to be in the base rather then the relay. At least in the last episode. I've rebuilt the Delphi connection and restored a suspect ground. Hoping that was the fix.



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Here is a pic of the ground wire in the harness coming from the fuel cooler pumps.....


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Last edited:
kvduff said:
Tarantula,
I had my mechanic come out thin pat Friday, he tested each of the wires in the harness lading to the relay and nothing was out of range. I have rebuilt the Delphi connector and also traced back the ground wire from the fuel pump to a connector in the wiring Harness that seemed suspect. I cut, rewired and sealed them. Thus far no relay issues. Just waiting for the next occasion.



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Click to expand...

I chased a similar problem for 2 seasons. 3 seasons back I had relay fail. It was replaced and the fix lasted all of 15 minutes. The local mechanic tested the voltage going to the fuel pump by shoving a multi tester feeler into the female delphi connector down at the fuel pump and in doing so, spread the opening. He determined that since the delphi plug had power then I must have had a bad fuel pump. However, what he did caused a bad connection between the delphi plug and the fuel pump. That diagnosis cost me $2000.

I purchased and installed the new fuel pump and it didn't work either and the old one worked fine on the bench. After inspecting all the connections, I discovered that the delphi connector was too wide and replaced it and got the engine running, but it only lasted a few weeks. It turns out I had almost the exact same issue as in the picture. After pulling my hair out and I found damaged wire in the housing between the fuel pump and the relay right at the where the wire splits. I cut it out and repaired it and no more problems. The bad wire was causing too much resistance and heat build up causing the relays to fail. This was more than likely the original problem, which lead to damaging delphi connectors and replacing a fuel pump that was still working.

You found a good mechanic. I wish the moron that worked on my boat 3 years ago knew what he was doing.
 
Hey Vince,
Just read your post here, which housing did u find that bad wire in? My issue has popped its head up again. This time the relay didn't blow but it became so hot it cut out and the insulation around the wire on the 87 leg of the relay was singged(like burnt but not through). Prior to this the problem had been isolated to the #30 leg of relay.

I've rechecked the fuel pressure. Bounces between 38-40 at idle and reads steady 40 through all Rpms. Rebuilt the entire Delphi connector a second time with longer pigtail wires thinking that after replacing once possibly some tension on the wires caused a bad connection.
Totally lost at this point.


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