Fuel/oil safety switch - car stops

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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gospo666
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Fuel/oil safety switch - car stops

Post by gospo666 » Sat Oct 29, 2016 5:19 am

Hi,
I
I changed safety switch: it looked ok, everything was good so I decided to go for a short treep. After 7 km, my vehicle stoped as before with the old safety switch.Good thing was that the distance was twice longer than before;)
But still this same situation, that in one moment electric pump located in a fuel tank stopped and that's why also vehicle stopped. I did, as some of You told me, short conection in safety switch conector so the fuel pump could work all the time. Vehicle started again and sucessfully I returned home.

So my observation is that electrical pump has to be in mode "on" all the time vehicle is running becouse when safety switch work properly it cuts the fuel going.
My question is what is wrong if I changed safety switch for new and symptoms are this same?
Second ask is: can I go without safety switch so electrical fuel pump would work all the time?
Thanks


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muttguru
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Re: Fuel/oil safety switch - car stops

Post by muttguru » Sat Oct 29, 2016 6:01 am

gospo666 wrote: My question is what is wrong if I changed safety switch for new and symptoms are this same?
Second ask is: can I go without safety switch so electrical fuel pump would work all the time?
Thanks

Hello Gospo, it's good to hear from you. Here are some comments that may help.

Fuel Pump Safety Switch
The purpose of this switch is to stop the fuel supply to the engine when the oil pressure drops below a safe level. If an engine runs with low oil pressure, the engine will be damaged and will fail very quickly.
It is possible that your safety switch is failing. However, this is very difficult to diagnose properly
because a switch that works correctly one minute, then fails to work a minute later, then works again....is very hard to fault-find. If a switch stops working completely, if it has power, the switch is definitely faulty and should be replaced.
There is a possibility that your switch is working correctly...and the problem may be your oil. Is the oil and filter "new"? Or is the oil old and dirty?
New oil is "thick" (viscous) but old oil is worn and thin. If the oil is very thin, the oil pressure may be too low and the switch will recognise this and turn off the engine. How does the oil get too thin? One way is through the mechanical fuel pump (you have a mechanical pump as well as the electrical pump). If the diaphragm in the mechanical pump is old, it will crack and allow the fuel to go into the sump....which thins the oil very quickly.

Year ago I had a 1945 Wills jeep that had a leaking fuel pump..... we would drive it for a few miles and then the oil pressure gauge would suddenly "drop". We would stop and walk home to get the tools. When we returned to the vehicle, the fuel in the sump had evaporated and the oil was "thick" again. We would drive home and when we returned to our garage, the oil pressure would be low again. It took us days to figure out what the problem was. The old Willys did not have a safety switch, but the M151 and M151A1 have the switch.....so it is possible that a leaky diaphragm in your mechanical pump could leak fuel into your engine sump and thin the oil enough to trigger the safety switch.

The simple check is.... when/if your fuel pump switch stops the engine, lift the hood and take out the dipstick. If the dipstick smells of petrol/gas, then the mechanical pump is failing.
If there is no smell of fuel in the oil, then most likely the fuel switch is failing and should be replaced.

In an emergency you can go a short distance using the switch by-pass but the switch is there for a good reason and if the reason is "fuel in the oil" you need to fix things quickly before the engine seizes up.
Does this help? Let me know.

ken
Kind regards....
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Re: Fuel/oil safety switch - car stops

Post by hambone » Sat Oct 29, 2016 7:58 am

Could the oil pump be failing? Could the oil strainer be stopping up? You could have grud in bottom of oil pan?


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