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Old 06-14-2018, 08:46 AM   #1
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Join Date: Mar 2016
Location: near Christiansburg VA
Posts: 692
Coachwork: Thomas
Chassis: Freightliner
Engine: Cummins 5.9
Rated Cap: 44 or 66? 11 rows
Lift pump low pressure warning? (5.9)

I recently purchased a 2002 Freightliner/Thomas with a Cummins 5.9 engine. I was told by the seller (a bus mechanic) that the lift pump had been replaced and a low-lift-pump-pressure warning light or buzzer had been installed. I see from the service records that it has had a half-dozen lift pumps.

But I don't know about the warning light/buzzer that allegedly was installed. Does anyone know what this would look like? I see an unlabeled red warning lamp to the left of my instrument cluster, but is has never lit. One would think it would be lit when starting if it indicated low lift pump pressure.

Thoughts? (photo will be added when I get the bus back from the shop)

And should I install a pressure gauge in the cab, connected to the Schrader valve, given that low lift pump pressure on a 5.9 tends to kill injector pumps?
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Old 06-14-2018, 09:40 AM   #2
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Join Date: May 2014
Location: West Ohio
Posts: 1,128
Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
Rated Cap: 65
Investigate what the light runs too. When I've done this, I've ran a positive wire from the key switch through a light, grounding it out through a 5psi transducer mounted after the fuel filter. You can verify it works a few seconds after the pump quits running after initial key on.

5.9 lift pumps will operate at key on for a few seconds to prime the system.

A warning light or gauge is highly recommended for the 5.9 engines with the vp44. That and an upgrade to an airtex lift pump. The stock carter unit wasn't really designed for the application it's put in, and therefore performs inadequately.

IIRC on bluebirds with the 5.9 and vp44 the check engine light will turn on if there's is low lift pump pressure. Not sure about thomas though.
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Old 06-14-2018, 12:02 PM   #3
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Join Date: May 2018
Location: topeka kansas
Posts: 570
Year: 1954
Coachwork: wayne
Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
low pressure

Doesnt seem to matter, bosch rotary or p7100 low pressure will kill either fuel injection pump. Low pressure will also take away fuel and that means less power. I will have both, low pressure warning light and gage. the gage will be off to the side, the light right out front. I also will have a pressure switch on the engine cooling system... if the cooling system pressure light comes on, then check for leaks and watch temperature gage. Exhaust gas temperature gage, exhaust manifold pressure, intake manifold pressure. There will be two oil pressure warning lights, one that is a low pressure- idle oil pressure, and one for higher speed, 1200 rpm and up higher pressure. oil temperature gage too.

From reading about the 12 valve cummins, you have a problem with less than 15 psi - this is the pressure of the fuel as it is fed to the main injection pump. Pump damage may occur from low pressures. It is reported that power will drop with pressure at this level. Many report that at idle 20 to 25 psi and at 2500 to 3000 look for 30 to 35 for most power.

I hope this is of service to you.

william
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Old 06-14-2018, 12:37 PM   #4
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Join Date: Mar 2016
Location: near Christiansburg VA
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Coachwork: Thomas
Chassis: Freightliner
Engine: Cummins 5.9
Rated Cap: 44 or 66? 11 rows
Quote:
Originally Posted by magnakansas View Post
...From reading about the 12 valve cummins, you have a problem with less than 15 psi - this is the pressure of the fuel as it is fed to the main injection pump. Pump damage may occur from low pressures. It is reported that power will drop with pressure at this level. Many report that at idle 20 to 25 psi and at 2500 to 3000 look for 30 to 35 for most power....
William -thanks for your comments. Wish I had your instrumentation! Any idea what 24v Cummins' pressure might be?
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