| Electrical question [message #371493] |
Wed, 18 October 2023 11:33  |
Greg C.
Messages: 224
Registered: October 2019
Location: Knoxville, TN
Karma: 0
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Senior Member |
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I've built an all new dash in my coach. Every since I installed the Autometer gauges, the oil pressure has shown alarmingly low. A few months ago, I installed, at different times, two different mechanical gauges plumbed directly into the engine. Pressures shown there appear to be normal, with about 40-50 psi cold and 30 or so hot at idle. The coach has about 68,000 miles on it.
The Autometer electric gauge shows 25-30 cold and 5-10 hot at idle, running synthetic 10W30 motor oil.
After consultation with Autometer, I installed a new Autometer gauge. Same readings. Same gauge bench tested with air pressure and a mechanical gauge plumbed in for a baseline shows the gauge to agree with the mechanical gauge. Autometer says that the gauge is very likely to be inaccurate if not correctly grounded.
Today, I removed my ground cable, cleaned it, and checked the resistance. It's right at .5 ohms in 39" of cable. Looking online, that seems to be about 600 times higher than it should be. It's #4, I just ordered a new custom made #2 cable with an auxillary wire on it. I found the transmission ground strap on the coach, but I can't find the front one. There was only the one cable on the engine block stud.
Does anyone have an opinion on this? Am I on the right track?
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
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| Re: Electrical question [message #371494 is a reply to message #371493] |
Wed, 18 October 2023 22:21  |
bwevers
Messages: 598
Registered: October 2010
Location: San Jose
Karma: 5
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Senior Member |
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I have a stock oil pressure sender that measures 20 ohms with no oil pressure, engine off.
The stock sender resistance is 80 ohms with 80 PSI air pressure.
I'm running 15W/40 and get 60 PSI cold at idle, 1500 miles on new engine. It drops down to 35 PSI when hot.
This is with a stock oil pump, not the Melling M-22FHV high volume pump.
Autometer shows these two Pressure sender units, where high resistance = lower pressure (opposite from GM).
But 0.5 ohms would not seem to affect the reading much at 20 PSI.
2241 (0-80 PSI)
PSI-----Ohms
0-----73
20-----30
40-----21
60-----15
80-----10
2242 (0-100 PSI)
PSI Ohms
0-----250
25-----158
50-----111
75-----75
100-----43
Regards,
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
[Updated on: Wed, 18 October 2023 23:06] Report message to a moderator |
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| Re: Electrical question [message #371495 is a reply to message #371493] |
Thu, 19 October 2023 01:40 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Let's look at the basic grounding of the engine system. The master ground for everything run buy the engine alternator and battery is the engine block. So the ground side of the sending unit itself can not be bad unless you covered the threads on the sender with Teflon tape.
The grounding of the dash mounted gauge is to the aluminum plate that the gauge is mounted in. So if you want to check that ground return path from the gauge. Meter the gauge ground with one lead of your meter and connect the other lead to the engine block. This should read as close to zero ohms as possible.
The ground path routing for everything mounted in the dash aluminum plate, including this gauge, is from the gauge to the plate and then via a wire that goes from that plate to the aluminum plate that the isolator is mounted on under the right side hood. After that a second wire goes from that plate direct to the negative battery post. From the negative battery post the circuit runs through the #2 or #4 heavy battery cable direct to the engine block.
The jumpers you were looking at on the trans to the frame or the one from from the body to the frame are NOT in the circuit for those gauges.
From your description I suspect that you have a loose connection on one of those wires or that you grounded that dash plate to something other than the engine block. It is common for the wire connections to the dash aluminum plate to come loose or oxidize.
Ken B. .
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Electrical question [message #371496 is a reply to message #371493] |
Thu, 19 October 2023 08:27 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Everything Ken said so well!
From your OP it sounds like your negative battery cable may be missing the second thiner black wire ”Y’ed” from the battery cable molded terminal end and going up to the firewall aluminum plate. Check for that.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Electrical question [message #371497 is a reply to message #371493] |
Fri, 20 October 2023 07:22 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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One addition. Before you measure plate to block, short your meter probes and set the meter to zero if it's analog, or note the resistance and add it to the measurement is the meter is digital.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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