No oil pressure, New sending unit & still no pressure

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JeepManiac
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No oil pressure, New sending unit & still no pressure

Postby JeepManiac » Thu Sep 20, 2018 3:05 pm

So I've been working on cleaning up several discrepancies on my new to me 1987 J20 with a 360. When I bought it, it showed no oil pressure. It ran fine and in my experience that's an indicating issue since you obviously can't run very long with 0 psi of oil pressure.

I bought a new sending unit from BJ's and replaced it. Still nothing although I did get a few flickers of gauge movement and then nothing after that again. I put a piece of wire into the connector and grounded it against the chasis. The oil pressure gauge pegged out at 80. So it would seem that everything from that wire connector to the gauge is good right?

Should I replace the sending unit again assuming I just got a bad one? Is there a threshold of accuracy where if it's actually kinda low it will just read zero? My next thought was to go ahead and buy a mechanical oil pressure gauge and check it with that. Not to mount, but just the tool for troubleshooting and diagnostics.

Fast79Chief
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Re: No oil pressure, New sending unit & still no pressure

Postby Fast79Chief » Thu Sep 20, 2018 4:28 pm

I've read that the 360 oil pumps can be a real problem after even 40 or 50 thousand miles. Seems the oil pump gears wear the internals of the pump badly enough to affect pressure to the block. There's a oil pump rebuild kit for the 360 that looks to be a fairly easy job. After you verify with a mechanical gauge, if no pressure or very low pressure, I'd rebuild that oil pump.
1979 Cherokee Chief S, 360, MSD Pro Billet Distributor with Factory Ignition Box (Wow), HEI Wires, Edelbrock 1406, Edelbrock Performer manifold, 3.54 gears, Tru-Trac Locker in the rear, Turbo 400, Quadratrac, Dual tanks, new 32 x 11.50's, big winch, Homemade 2.5" full length exhaust ... and a lot of work to do. :)

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Cecil14
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Re: No oil pressure, New sending unit & still no pressure

Postby Cecil14 » Thu Sep 20, 2018 5:34 pm

You should really just ditch the factory gauge and go with a good aftermarket gauge, either electrical or mechanical. The new aftermarket electric gauges are WAY better than the stuff our rigs ever came with. Even a functioning factory gauge should not be trusted: for entertainment purposes only.


aa
Anthony

1983 J-10 - 4.6L(MPFI)/CS130D/Hydroboost/NV3550/D300/44/44/3.54/Disc-Disc/32s/42 gallon 'burb tank

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tgreese
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Re: No oil pressure, New sending unit & still no pressure

Postby tgreese » Thu Sep 20, 2018 5:58 pm

If you actually had zero oil pressure, the lifters would not pump up and the engine would clatter loudly.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
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Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green

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babywag
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Re: No oil pressure, New sending unit & still no pressure

Postby babywag » Thu Sep 20, 2018 7:03 pm

JeepManiac wrote:
I bought a new sending unit from BJ's and replaced it. Still nothing although I did get a few flickers of gauge movement and then nothing after that again. I put a piece of wire into the connector and grounded it against the chasis. The oil pressure gauge pegged out at 80. So it would seem that everything from that wire connector to the gauge is good right?



You have a wiring issue. On ‘86-up disconnecting the sender wire should peg the gauge 80psi+ grounding it should zero gauge.
-Tony
'88 GW (aka Babywag) '90 GW (aka JunkBucket) both fuel injected.

JeepManiac
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Re: No oil pressure, New sending unit & still no pressure

Postby JeepManiac » Thu Sep 20, 2018 11:23 pm

babywag wrote:
JeepManiac wrote:
I bought a new sending unit from BJ's and replaced it. Still nothing although I did get a few flickers of gauge movement and then nothing after that again. I put a piece of wire into the connector and grounded it against the chasis. The oil pressure gauge pegged out at 80. So it would seem that everything from that wire connector to the gauge is good right?



You have a wiring issue. On ‘86-up disconnecting the sender wire should peg the gauge 80psi+ grounding it should zero gauge.


I was trying to understand this so I went out and just checked it disconnected thinking that maybe it wasn't a good ground anyway. Sure enough unplugged the gauge showed like under 80 but most of the way up. Although I swear when I checked the other night it legitimately pegged all the way. Still seems a little screwy

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babywag
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Re: No oil pressure, New sending unit & still no pressure

Postby babywag » Fri Sep 21, 2018 6:43 am

Sender/gauge works on resistance
0psi = 1 ohm, 40psi = 46 ohms, 80psi = 87ohms
So if gauge & wiring are good
grounding sender wire = 0psi
disconnect sender wire = 80psi

You can also measure resistance of sender to verify it’s working.
-Tony
'88 GW (aka Babywag) '90 GW (aka JunkBucket) both fuel injected.

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fulsizjeep
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Re: No oil pressure, New sending unit & still no pressure

Postby fulsizjeep » Fri Sep 21, 2018 6:48 am

babywag wrote:You have a wiring issue. On ‘86-up disconnecting the sender wire should peg the gauge 80psi+ grounding it should zero gauge.


I didn't know this! Thanks for that tidbit Tony. I had not got around to why my 88 is pegged at run time.
Sent from my IBM 5150 using TN5250 emulation.

Flint Boardman ~ http://jubileejeeps.org
Embracing the Borg Warner QuadraTrac since 1980 http://jubileejeeps.org/quadratrac
88 GW, 401/727/208, 5" lift, D44s/4.10s/locked up, 35s, a few Evil Twin mods, http://eviltwinfab.com and a few TT's Fabworks mods, http://www.ttsfabworks.com
76 401 Wag, 77 401 Wag, 77 401 J20

rocklaurence
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Re: No oil pressure, New sending unit & still no pressure

Postby rocklaurence » Sat Sep 22, 2018 8:01 am

Im not sure what year the Oil gauges changed but I have had to add reisistors to some gauges to get them to read correctly. I know because I added the resistors to get the correct reading and then years later I replaced the instrument panel and had to remove the resistors. I salvage a lot of FSJ and refuse to buy new for my vehicles--I guess Im a Cheap-A$$ 8-)

letank
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Re: No oil pressure, New sending unit & still no pressure

Postby letank » Mon Sep 24, 2018 6:54 am

the change was 86, with the new gauge design
Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (226 Kmiles... the running test lab)
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