Fuel Control Issue

[mh86]

Does the ecm initiate the 20 second fuel pump run when the key is turned on? 1993 C3500 HD TBI to 1986 carb motorhome conversion with 1228747 ANRS.

[fastacton]

With an '8747, it should be a lot shorter than 20 seconds, but yes.

[mh86]

Thanks for your answer. The 1986 setup only ran the in tank fuel pump in the crank position to assist the carb. Possibly after oil pressure built up also. And yes it is only a 2 second run. I got the 20 seconds from my 88 manual. thanks again !!

[1project2many]

The 1986 setup only ran the in tank fuel pump in the crank position to assist the carb. Possibly after oil pressure built up also.

I would think the in tank pump would run at all times the engine is running, even with the carb. Even if there's a mechanical pump, the work required to draw fuel through an inoperative in-tank pump can be high. Low or negative pressure in the line from tank to engine driven pump combined with high heat often found under motorhomes creates a situation where fuel can easily boil in the line and create vapor lock. Pressurizing the line by making the pump operational could reduce the chance for vapor lock.

Either way, you'll want to connect the in-tank pump wiring to the ECM controlled relay. Making sure the vehicle wiring and configuration matches an existing wiring harness can help in the future if diagnosis is needed.

[mh86]

I would think the in tank pump would run at all times the engine is running, even with the carb. Even if there's a mechanical pump, the work required to draw fuel through an inoperative in-tank pump can be high. Low or negative pressure in the line from tank to engine driven pump combined with high heat often found under motorhomes creates a situation where fuel can easily boil in the line and create vapor lock. Pressurizing the line by making the pump operational could reduce the chance for vapor lock.

Either way, you'll want to connect the in-tank pump wiring to the ECM controlled relay. Making sure the vehicle wiring and configuration matches an existing wiring harness can help in the future if diagnosis is needed.

You are right on with my situation. After bench testing the 93 wiring harness I've concluded that I will disconnect the the 86 FP relay and run solely off the 93 relay and ecm controls. Also making notes of every detail change. By the way, before I got to this point I rebuilt the 86 454 and installed it in the 93 with a carb and discovered as you mentioned that the mechanical fuel pump would'nt pull thru the inop intank pump. I ended up removing the pump and extending the pickup tube with a sock on the end. Once I get a little further along, I'll be back to talk about the Drac or no Drac, but I don't want to overload myself right now. Thanks for the support !!!

[mh86]

You are right on with my situation. After bench testing the 93 wiring harness I've concluded that I will disconnect the the 86 FP relay and run solely off the 93 relay and ecm controls. Also making notes of every detail change. By the way, before I got to this point I rebuilt the 86 454 and installed it in the 93 with a carb and discovered as you mentioned that the mechanical fuel pump would'nt pull thru the inop intank pump. I ended up removing the pump and extending the pickup tube with a sock on the end. Once I get a little further along, I'll be back to talk about the Drac or no Drac, but I don't want to overload myself right now. Thanks for the support !!!

I've got the fuel control wiring mostly complete and now looking to send the vss signal to the 8747 ecm A10 terminal. I do have a drac available but hope to use the original Dana cruise control regulator to achieve the same results as the drac. The attached Dana diagram shows all the in and outputs, but I'm not sure which one to send to the ecm, or if this would even work. I could sure use some suggestions at this point. thanks

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