'99 748s fuel pump won't run w/troubleshooting so far. - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 26 (permalink) Old May 2nd, 2017, 6:17 pm Thread Starter
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'99 748s fuel pump won't run w/troubleshooting so far.

Hello all. I've tried to do as much research and searching before posting as I can. I've been troubleshooting a weird issue with my fuel pump not running.

1999 748s (Yellow, the correct color...lol.)

I have taken the following steps:

1. Tested fuel pump off bike (still in tank,) with 12v battery - good.
2. Tested for 12v at wiring harness - good (I get 12v for ~1sec, then 12v during cranking.)
3. I've tested the fuses at the fuse box and the 2 on the tail of the bike - good
4. I've jumped the bike side of the plug to the tank side of the plug with test wires - FAIL (Wanted to make sure it wasn't the plug itself.)
5. I've connected the - side of the tank plug to the - side of the bike harness with a jumper, then connected the + side of the pump plug to the battery of the bike - good
6. I can hear and feel the relay under the tail clicking on and off for 1sec at key ignition turn on.

It's bizarre to me that the pump is good, the 12v at the bike side harness appears to be good, but I can't get the pump to run when connected via the plug or when jumped through the plug with test wires. Yet, when I jump the - side of the harness / plug and connect the pump directly the battery it runs. It just doesn't make sense.

Could I have fried the relay so that it's OK without load, but when it's getting a load put on it it's failing? If so, is there a place to get a reasonable replacement other than going to the stealership? It looks to just be a Bosch relay right?

Thanks all.

'99 748s | '06 FJR 1300 | '92 GS500e project

Last edited by JayRodRay; May 2nd, 2017 at 8:37 pm.
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post #2 of 26 (permalink) Old May 2nd, 2017, 6:39 pm
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Likly the fuel sending unit / wire harness assembly that passes through the pump flange. Held in place with the big nut. What happens if you go from the bike side plug directly to the pump?
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post #3 of 26 (permalink) Old May 2nd, 2017, 6:46 pm Thread Starter
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Quote:
Originally Posted by SS904 View Post
Likly the fuel sending unit / wire harness assembly that passes through the pump flange. Held in place with the big nut. What happens if you go from the bike side plug directly to the pump?
Thanks for the reply.

EDIT: I haven't removed the pump from the tank yet. I've been testing through the plug on the harness at the tank. So IMHO the sending unit wiring "should," be ok?

Like I said. I tested jumping the - side of the sending unit plug to the - side of the bike side plug. Then connected the + side of the sending unit directly to the battery and that works.

Unless I'm missing something in what you've said. Or if you're saying the fuel level wiring in the sending unit may be coming into play?

'99 748s | '06 FJR 1300 | '92 GS500e project

Last edited by JayRodRay; May 2nd, 2017 at 6:54 pm.
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post #4 of 26 (permalink) Old May 2nd, 2017, 7:20 pm
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The fuel pump runs under the control of the computer. The pump is turned on initially for a few seconds to pressurize the system and then powered off again by the computer (via a relay) if it doesn't get a signal from the crankshaft rotation sensor that the bike is running. Even though you can hear the relay click when you hit the run switch, if the pump doesn't prime, the relay itself may be faulty.
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post #5 of 26 (permalink) Old May 2nd, 2017, 7:32 pm Thread Starter
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Quote:
Originally Posted by Strega View Post
The fuel pump runs under the control of the computer. The pump is turned on initially for a few seconds to pressurize the system and then powered off again by the computer (via a relay) if it doesn't get a signal from the crankshaft rotation sensor that the bike is running. Even though you can hear the relay click when you hit the run switch, if the pump doesn't prime, the relay itself may be faulty.
Wouldn't that have been ruled out by this though?
Quote:
2. Tested for 12v at wiring harness - good (I get 12v for ~1sec, then 12v during cranking.)
I don't want to be argumentative, just thorough.

Unless the relay is failing under load as compared to when just tested with a volt meter?

'99 748s | '06 FJR 1300 | '92 GS500e project

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post #6 of 26 (permalink) Old May 2nd, 2017, 7:51 pm
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Quote:
Originally Posted by JayRodRay View Post
I've been testing through the plug on the harness at the tank. So IMHO the sending unit wiring "should," be ok?


Missed that part.


Well, yes and no on the sending unit / pump power harness that goes through the tank flange. They have been known to fail. You may try connecting everything normally with the tank lifted enough at the rear enough that you can wiggle the wires right there when turning the key to on. Might be better to pull the flange and properly test it. Easier, and in the end, safer. A spark here, near the tank fill port, could pretty well ruin your day.


Based on your description of the pump cycling on and then off when it is working, leads me to believe the ECU, chip and relays are good. For now.

Dan.
2001 900SSie (gone, but not forgotten)
2003 ST4s (The Truck)
1995 916 (Junk Yard Dog)
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post #7 of 26 (permalink) Old May 2nd, 2017, 7:56 pm
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Originally Posted by JayRodRay View Post
Unless the relay is failing under load as compared to when just tested with a volt meter?

The relay, regarding the contacts and were it matters here, doesn't care if it's under load or not. It's a simple mechanical switch that is an open or a short. The coil of the relay that controls the state of the contacts is a separate circuit from the contacts that supply the pump with power.

Dan.
2001 900SSie (gone, but not forgotten)
2003 ST4s (The Truck)
1995 916 (Junk Yard Dog)
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post #8 of 26 (permalink) Old May 2nd, 2017, 8:41 pm
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To add: relay clicking doesn't mean power is getting to the switched contact. Change the relay, it's only a few bucks.
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post #9 of 26 (permalink) Old May 2nd, 2017, 8:58 pm Thread Starter
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Thanks for the help everyone.

Gonna go with a new relay first, then I'll tackle the tank sending unit wiring assembly if needed. If it's not the relay, at least I have a spare for the track.

12065115 is the pump relay just in case anyone ever searches and comes across this post. It was $12 from Ebay new.

'99 748s | '06 FJR 1300 | '92 GS500e project

Last edited by JayRodRay; May 2nd, 2017 at 9:05 pm.
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post #10 of 26 (permalink) Old May 3rd, 2017, 3:34 am
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Any auto parts store will have them. Just take your old one for a sample.
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