4Crawler

iTrader: (3)

The Fp - B connection is not really an output signal you can test with a test light. Depending on the model year, it is either a connection in parallel with the AFM Fc contact or in later models, it bypasses the CO Relay output contact. So a test light will sort of give you some confusing readings. The resistance of the test light can likely pass enough current to turn on the CO relay in an early model truck. And once the AFM contact closes, the light will go out as you have shorted out the two terminals that the light connects to. Likewise in the later model trucks, when the AFM and CO relay close, you'll have no voltage across the CO relay (as the contacts are closed) and the test light will go out.

Read how the CO Relay is supposed to work below:
- http://www.4crawler.com/4x4/CheapTri...shtml#FuelPump

Basically the CO relay has two coils and works like a logical OR function. So if the starter is running OR the AFM Fc contact is closed, the CO relay closes and sends power to the fuel pump. So if the CO relay and/or fuel pump is shutting off with the starter going, figure out why. The AFM can't turn the CO relay off, it can only cut power to one of the two coils. If there is not power to the secondary coil (via the STA signal), the CO relay will turn off.

jlc84cam350

i kinda get what your saying lol, (i have later model btw 92)
and from what i can see my plan of wiring up a small l.e.d.light say the the ground or power for the fuel pump should work since that part of the circuit should always be functioning after the the truck is running and AFM contacts are closed, unlike if the bypass wich will only work if the afm and starter contacts are open

am i correct?

I thought that the bypass would have taken over the afm and starter circuit completely even if they were functioning but guess i was wrong.

4Crawler

iTrader: (3)

I think in the later model trucks, the test jumper just bypasses the CO Relay, connects the +B terminal to the Fp terminal on the CO relay, just like it was closed. You could basically simulate that by pulling the CO relay and shoving a wire into the +B and Fp terminals:
- http://www.4crawler.com/4x4/CheapTri...ORelayLocation