GM-GMC: OPS relay, racor fuel filters, auto parts stores

Question
1995 C1500 Reg Cab long bed 6.5 TD



Hey Harry,


Thanks for all of the help so far. I have to ask if you have a thread on installing a relay that is switched on by the OPS to run the lift pump.

I understand the principle, but would like to hear your input before I go splicing wires.

Should the lift pump power come from the Injector Pump relay?

What lift pump relay would you use?

I was actually thinking about installing a guage (or at least an idiot light) to monitor fuel pressure inside the cab. Where would you attach this? Would the pressure at the filter bleed valve be a good place to monitor pressure? Would it indicate restrictions in the filter as well or would it be on the lift pump side of the filter? How about the return line, would that be a better place to check the pressure?

Finally, I have heard 3-5 lb of pressure at the bleed valve is good. Can too much pressure be a problem as well as not enough?


Thanks,

David  

Answer
David,

I believe that it is a gray wire that leads from the OPS to the lift pump.  This is the wire that needs to be cut and run through the coil of the new relay (other side of coil to ground).  Find a new source of +12V DC (could be the insulated tie point on the firewall of most GMs), fuse it at 10A, and run it to one of the switch contacts of the relay (other side is where the gray wire to the lift pump attaches).

Here is a URL for the relay: http://www.allelectronics.com/cgi-bin/category.cgi?category=500700&item=RLY-351&...
You can buy these (and a matching socket)at most auto parts stores and the terminals have all the same wiring, making it easy to replace if required.  Since the relay has 88 Ohms of resistance, the current across the OPS contacts will drop to less than 200 mA and keep them from failing due to over-current conditions.

I have mechanical fuel pressure gauges installed at the inlets to my secondary Racor fuel filters.  They tell me that the lift pump is working and that the secondary filter is not restricted.  Anywhere between the discharge of the lift pump and the inlet to the fuel filter will give the same results.  I bought some small, liquid-filled, 0-15 psi gauges for fuel pressure gauge use.  I also used stainless steel braided hose, which is overkill, but make sure that you use fuel line and not vacuum hose for any fuel system modifications.  You don't need the gasoline fuel injection hose, but the hose should be marked for fuel line use.

Stanadyne injection pump seals are tested to 20 psi.  Do not take the pressure that high and you will be fine.  My Carter pumps run at 6 to 8 psi and 3-5 psi is also acceptable.

Harry