| Fuel pump voltage [message #287270] |
Mon, 14 September 2015 16:29  |
petemosss
Messages: 221
Registered: August 2008
Location: Jacksonville, Florida
Karma: 0
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Sorry for the non gmc post but there is more brain power here than anywhere. The rednecks on the truck forums drive me batshit. My 1994 c2500 5.7 TBI gets no fuel. Pump not running. The pump relay connector at firewall and connector at tank show 12v for about 2-3 seconds before dropping to 8-9 volts while cranking. Is this normal or do I have an ecm problem? Already tried new relay same result. Will a pump run on 8-9 volts?
Thanks folks!
Pete
74 Canyon Lands 26' "Emery"
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| Re: Fuel pump voltage [message #287282 is a reply to message #287270] |
Mon, 14 September 2015 17:57  |
tphipps
Messages: 3005
Registered: August 2004
Location: Spanish Fort, AL
Karma: 9
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There should be no voltage drop. I have no idea if the pump will run on lower voltage.
I had a similiar voltage drop problem on my Honda generator installation. Push the switch to start, voltage would drop in a very few seconds from 12 v to about 6 volts. Not enough to do the function.
Turned out to be a "floating" ground problem. When a proper ground connection was restored, it worked perfectly. Down side, I replaced the starter first, not last. Anyone need a Honda EV starter?
Check the ground(s).
Could also apply a 12v source directly to the pump and see if it works.
Tom, MS II
2012 Phoenix Cruiser model 2552
KA4CSG
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| Re: Fuel pump voltage [message #287284 is a reply to message #287270] |
Mon, 14 September 2015 18:20 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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On the GM cars there is a body wire connector close to the pump and can have loose connections. I found one that a po had probed the connection with a VOM and opened the holes for the prongs. The 2-3 sec should be the pump prime circuit then the drop would be from primary. Could may be a bad ground on primary side?
petemosss wrote on Mon, 14 September 2015 17:29Sorry for the non gmc post but there is more brain power here than anywhere. The rednecks on the truck forums drive me batshit. My 1994 c2500 5.7 TBI gets no fuel. Pump not running. The pump relay connector at firewall and connector at tank show 12v for about 2-3 seconds before dropping to 8-9 volts while cranking. Is this normal or do I have an ecm problem? Already tried new relay same result. Will a pump run on 8-9 volts?
Thanks folks!
C. Boyd
76 Crestmont
East Tennessee
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| Re: Fuel pump voltage [message #287303 is a reply to message #287270] |
Tue, 15 September 2015 04:30 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Check the voltage across the battery while cranking. It could be a weak battery and the voltage is dropping from the heavy current load while cranking.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Fuel pump voltage [message #287304 is a reply to message #287270] |
Tue, 15 September 2015 06:26 |
petemosss
Messages: 221
Registered: August 2008
Location: Jacksonville, Florida
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Thanks for all the good leads. Daybreak and 69 degrees in North Florida. Woohoo!
Pete
74 Canyon Lands 26' "Emery"
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| Re: Fuel pump voltage [message #287316 is a reply to message #287270] |
Tue, 15 September 2015 09:16 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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A lot of GM have 2 paths to energize the FP. ECM to relay and oil pressure switch direct to pump I would check the primary side of the relay signal. If that proves good then trace the secondary side of the circuit. Sounds like the 3 sec pump run KOEO is good followed by the drop. Disconnect the oil switch and see what symptoms you have. Or there should be a loose connector underhood to direct power the pump on assy line. Appling 12v there would rule out everything there back to the rear if that gets it working. Most GM need full pressure (full voltage) to start. You need the FSM wiring diagram.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Fuel pump voltage [message #287815 is a reply to message #287270] |
Thu, 24 September 2015 08:17 |
petemosss
Messages: 221
Registered: August 2008
Location: Jacksonville, Florida
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Completely baffled. If i use starting fluid or a squirt of gas it starts and runs (10-15 min.) Turn it off and hit key on and starter and nothing but turning over. Tried 2 different relays. new pump and filter even though old one worked fine with 12v applied. Can hear prime but cannot tell if pump is running wile cranking. Redid ground at frame by pump. Oil pressure switch is the only thing i didn't replace. It supposedly engages when pressure reaches certain point and completes circuit while cranking.(new 5.7 has big pressure) It supposedly cant keep things from starting according to multiple posts on truck forums even if bad. Any other ideas? Drove this home over 200 miles after engine swap. ????
Pete
74 Canyon Lands 26' "Emery"
[Updated on: Thu, 24 September 2015 08:22] Report message to a moderator |
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| Re: Fuel pump voltage [message #287819 is a reply to message #287815] |
Thu, 24 September 2015 14:53 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Make sure the ECM is getting an RPM signal from the distributor or crank sensor. Best way to check that is to use a scanner and watch the RPM reading on a live data readout. Some systems use cam signal, some use crank, and some use both. Since it ran with substitute fuel supply to start it, the crank sensor may be out of adjustment and the ECM is only seeing a signal after the engine is running and the signal is repeated hundreds of times per minute.
Apply battery voltage to the pump lead out of the relay to bypass the ECM's control of the pump...Does it start and run when bypassed? Apply battery voltage to the supply side of the relay so the relay contacts are in the circuit...does it start & run? Apply ground to the ECM control wire for the relay to check the relay coil (if the ECM supplies a ground to engage the relay)...does it start & run? Test the voltage (or the grounding ability) of these circuits before applying power or ground. The ground control wire from the ECM should pass enough current to light a test light. DO NOT randomly test wires from the ECM with a test light! Some circuits will not handle that current (Especially Chrysler products). Get a schematic so you know what you are looking for as far as wire colors and with these simple tests and logical thinking you should be able to zero in on the fault.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: [GMCnet] Fuel pump voltage [message #287822 is a reply to message #287815] |
Thu, 24 September 2015 15:54 |
Guy Lopes
Messages: 499
Registered: April 2004
Location: Sacramento, CA
Karma: 3
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Pete, you didn't mention anything about fuel filters. Is that a possibility?
Guy Lopes
76 Birchaven "Orion"
Sacramento, CA
W6TOL
www.GMC-Guy.com
-----Original Message-----
From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of pete
Sent: Thursday, September 24, 2015 6:17 AM
To: gmclist@list.gmcnet.org
Subject: Re: [GMCnet] Fuel pump voltage
Completely baffled. If i use starting fluid or a squirt of gas it starts and
runs. turn it off and hit key on and starter and nothing but turning over.
Tried 2 different relays. new pump even though old one worked fine with 12v
applied. Can hear prime but cannot tell if pump is running wile cranking.
Redid ground at frame by pump. Oil pressure switch engages when pressure
reaches certain point and completes circuit while cranking.(new
5.7) It supposedly cant keep things from starting according to multiple
posts on truck forums even if bad. Any other ideas?
--
Pete
74 Canyon Lands 26' "Emery"
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Guy Lopes
76 Birchaven "Orion"
Sacramento, CA
W6TOL
www.GMC-Guy.com
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| Re: Fuel pump voltage [message #287823 is a reply to message #287815] |
Thu, 24 September 2015 16:06 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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petemosss wrote on Thu, 24 September 2015 07:17Completely baffled. If i use starting fluid or a squirt of gas it starts and runs (10-15 min.) Turn it off and hit key on and starter and nothing but turning over. Tried 2 different relays. new pump and filter even though old one worked fine with 12v applied. Can hear prime but cannot tell if pump is running wile cranking. Redid ground at frame by pump. Oil pressure switch is the only thing i didn't replace. It supposedly engages when pressure reaches certain point and completes circuit while cranking.(new 5.7 has big pressure) It supposedly cant keep things from starting according to multiple posts on truck forums even if bad. Any other ideas? Drove this home over 200 miles after engine swap. ????
I'm guessing that once you get it running with stating fluid, the alternator pumps up the voltage to 14V or more which may be enough to keep the pump running. Then when you shut it down and crank, you'll be back in a marginal low voltage condition again.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: Fuel pump voltage [message #287830 is a reply to message #287823] |
Thu, 24 September 2015 19:23 |
petemosss
Messages: 221
Registered: August 2008
Location: Jacksonville, Florida
Karma: 0
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Senior Member |
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Thanks for all the solid leads. New ACDel filter and Ive even been testing things with battery chg hooked up. Even with the old motor it took more than few seconds of cranking before it would kick. Hmmm...OP switch is new too installed with swap.
Pete
74 Canyon Lands 26' "Emery"
[Updated on: Thu, 24 September 2015 19:28] Report message to a moderator |
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| Re: Fuel pump voltage [message #287845 is a reply to message #287270] |
Fri, 25 September 2015 10:20 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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At KOEO, the pump should run 3 seconds then shut off via ecm. At crank it should again energize. If you do KOEO you say you get 12v then drops to 9 cranking. If you do KOEO and wait for 3 second time out no crank, does it drop to zero as it should?
John Lebetski
Woodstock, IL
77 Eleganza II
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