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1987 740 Oil light[700/1987] posted by Phillip Warren on
Tuesday, 23 March 1999, at 11:05 p.m.
This 740 has 121K miles on it and was bought not running. We got it running and it was running well, so we drove it home. Upon arrival about 5 miles later, the oil light came on. The oil level was good when checked, but the next day the car was driven around the block a few times (about 3 miles) the oil light came on again. Do you know what causes this if the oil looks good and the car runs good?
--Phillip Warren
Re: 1987 740 Oil light[700/1987] posted by Don Foster on
Wednesday, 24 March 1999, at 6:03 a.m.
Phillip --
Two possibilities come immediately to mind:
- The oil light is telling the truth.
- The oil light is not telling the truth.
Most worrisome is if the light is telling the truth. That is, your oil pressure is dropping.
You, or your mechanic, should pull the oil switch and install a conventional (mechanical) oil pressure gauge on a long hose. Run the hose into the car so the gauge is on the seat while you drive around the block. Your engine should maintain 15-30 psi at idle (hopefully higher) and 45-70 psi while driving and at normal temperature.
The pressure should not falter or vary, except in response to engine speed.
If you don't have this pressure, then you have an engine problem -- probably a bad pump, bad delivery system, or maybe a plugged pickup screen (not unheard of).
The plugged screen problem often arises after years of very poor service. The sludge in the pan gets picked up as the engine warms up, blocks the screen, and the pump starves. After sitting overnight, the sludge drops back down, and the pump works again for a few minutes.
But perhaps the idiot light is not acting intelligently.
The light goes on when the oil switch completes an electrical circuit to ground (engine block) allowing current to flow through the bulb. Proper oil pressure opens this switch, extinguishing the light.
A wiring problem, such as a chaffed or bare wire, could intermittently touch the block, illuminating the light. This is a common problem on Volvos, particularly with the wires around the oil filter (just behind the alternator). This is also a common problem Volvos of this vintage due to crummy (crumbly) engine wiring harnesses.
Or maybe the switch is defective -- and shorting when the engine warms up. This, too, is not uncommon.
In my opinion, the most important test you should do is the oil pressure measurement with an independent gauge. The quickest thing you can do is "eyeball" the wiring, and perhaps substitute a new switch.
Wednesday, 24 March 1999, at 10:49 a.m.
Ours did exactly what the original poster describes. The single wire from the pressure switch was chafed just as it ducked under the right front of the engine, against the power steering pump bracket I think. I simply shoved the woven sleeve that's over the wire at the point down a bit further, and the problem went away and stayed away.
The oil pressure monitoring test is probably very wise, even if you can make the problem go away.
--Brian