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Old 09-13-2021, 04:14 PM   #1
Bus Crazy
Mountain Gnome's Avatar
Join Date: Oct 2017
Posts: 1,222
Year: 1999
Coachwork: BlueBird
Chassis: TC1000 HandyBus
Engine: 5.9L 24V-L6 Cummins ISB
Rated Cap: 26 foot
Cummins ISB lift pump wiring circuit

I've got a 1999 Cummins 5.9L 24-valve ISB.

It's got a check engine light, and the code is (or was, haven't checked it recently) "low voltage to fuel pump".

I located the lift pump and disconnected the 2-wire connector, and checked for voltage and ground.

One wire has less than 1-ohm of resistance to ground. Looks good.

The other wire gets 8-9 volts with KOEO. Looks bad.

With a separate meter connected to the "air intake heater relays" (they get battery power through thick cables and I just cleaned them and replaced their fuses and fuse-holders, so they have good continuity, and are a good place to check battery voltage at the top of the engine) I see over 12 volts at the same moment I'm checking the lift-pump voltage. (This is an important check, as I have daytime running lights that draw the voltage down a bit with the key on).

I checked my above-dash fuse panel, and none say "fuel pump". Some say "relay M" or "relay U", etc.

The relay panel (hidden behind the windshield wiper door in my TCFE) does not label the relays, and it's just a guess..... (somewheres on this site I think I saw a picture that had them labeled, but I have never been able to find it again - since then I copy pics like that and put them in a folder on my laptop).

I checked the wiring diagram I could find for my bus (from BlueBird, I believe) and it shows a fuel pump relay and a fuel pump control/timer unit, but does not say "relay M" or something. I could likely find the correct relay, if it's in that panel, though the long process of elimination, but where is the controller/timer? I've been all though the bus, and find new stuff all the time (just noticed something new yesterday) but I don't remember anything like that.

Any clues would be greatly appreciated. I'll post any info I discover myself.
Attached Files
File Type: pdf wiring(obsolete).PDF (124.8 KB, 2 views)

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