Engine/Transmission (1994 - 1998) Low fuel pressure light

[DonTX]

I want to add a low fuel lift pump pressure warning light on my 97. Anybody have some experience on what switch pressure rating I should use and a good place to graft it in at?

 

[tp0d]

Well, the mechanical P7100 pump doesnt really need it, as low fuel pressure will not hurt it at all. It is lubed by engine oil, not fuel. You will notice a lack of power and possibly white smoke if you have a real loss of fuel pressure.



That said, if you still want to add a light, you will need a pressure sender with a normally closed contact, open on pressure rise of 15psi. (min pressure for P7100). You will also need a snubber in the line to protect the sender, and the sender itself should be capable of handling max 100psi just to be sure you have no diaphram explosions. I have no idea where to source such an item, just dont go cheap on the sender, or your engine will get a diesel bath. You can tap into the lift pump pressure on the back of the injection pump, at the banjo fitting. They sell pre-tapped banjo's just for this purpose.



good luck



-j

 

[Joe G.]

I use a marine pressure sensor. It's normally used on a marine alarm system. It is open as long as pressure is above 10 PSI. Below that it closes the contacts. I have it on a T fitting to the sensor for my fuel pressure gauge. I also have one on my oil pressure. If either pressure is lower than 10 PSI a "look at your gauges" red light will come on. I'm using an LED for that now. I plan to add more things to the input to that alarm light. Any sensor that gounds the circuit will turn on the light. I also plan to change to a light that will shine on the windshield to give me a HUD display to alert me to look at my gauges.



I recently had an overflow valve fail. Pressure got as low as 6 PSI before I got home. At first I thought that I had a fuel pressure gauge problem because it ran so good with the low pressure. When the pressure dropped below 10 PSI the alarm light cam one. When I turned it off it would not start again.

 

[DonTX]

Thanks guys for all that info. I am thinking a red LED in the three gage pillar mount would get my attention as well as any place else.

The lift pump was kaput on my motorhome when I bought it, had been for some time, and putting on a new lift pump was accompanied with a rebuild on the VP44, not sure if there was a connection between the two. I thought I had read that low pressure on the 7100 destroyed it as well. Regardless, I STILL want a warning light.

 

[Joe G.]

TI thought I had read that low pressure on the 7100 destroyed it as well.

Nope. The engine quits if the pressure is low enough. The P7100 pump is engine oil lubed. I had about 12 PSI fuel pressure when I left San Francisco. It had dropped to below 10 PSI by Santa Rosa. The last hundred miles was at 6 PSI. I thought I was going to walk, but it made it all the way.



The VP44 is fuel lubed. That's why low fuel pressure kills it.