I am helping someone install a 1227747 and 7.4 TBI w/TH400 in a 56 chevy pickup. He provided the wiring diagram from I don't know where but it is more of a multiple choice type thing sos suspecting it is not a GM or Helms document. From what information I do have it is a light duty as it has EGR, at least that is what the wiring diagram list if EGR then light duty.The original harness got hacked beyond recognition during engine removal and install so flying blind other than what little info I currently have. The good news is I have found most of the missing connectors and pins to create a new harness. If it is a help the OEM ECU has a broadcast code of ANTY. Here are the questions...

There are 3 connections to the fuse panel
ECM fuse 1 that goes to A6 and EGR and Injectors
ECM fuse B that goes to C16 and the fuel pump relay and Oil pressure switch
Gauge/Idle fuse that goes the the AT kickdown relay
Are these all live all the time with the key off?

Next the IAC
I have pins listed as follows
C3 LT GRN/BLK
C4 LT GRN/WHT
C5 LT BLUE/WHT
C6 LT BLUE/BLK

What pins do they go into in the IAC connector labeled A B C D?

Last at least for right now LOL the oil pressure switch..
I have seen it wired in other places as an auxillary to the fuel pump relay so the pump has power if the engine has oil pressure. This one is wired with one side of the oil pressure switch going to ECM fuse B and fuel pump relay input and pin B2 fuel pump input. Has anyone ever seen this and what purpose does it serve?

Any input or help would be greatly appeciated if this is the wrong place to post my apologies and where to post helpful too.
Dave

 

First thing you should do is get a real wiring diagram you ae confident in. Why guess at something that could be so crucial. Plenty of places on the net should provide a diagram possibly even one on this site. Over at diy-efi.org I believe you will find a good diagram in no time. But to answer your question from memory, egr/inj's and tranny relay are key on and the fuel pump is bat. From my 8D 730 ecm, gr/blk, gr/wh, blu/blk, blu/wh are a,b,c,d. Note that GM sometimes reversed these depending on mask even with the same ecm. And I believe that OP switch/FP wiring is normal. I believe the ecm uses the fuel pump volt input for diagnostic purposes and maybe even in the engine run routine.

 

I found the wiring diagrams at DIY-EFI and there were 7 pages of ecu but no indication of the switched wiring. They also had different fuel pump /oil pressure sender wiring too. The engine is specifically 89 GMC Suburban 2500 Lt duty 7.4 liter. None of them listed which wire is which in the IAC. I hate to buy a FSM for the truck to do the wiring but may have to. The diagram I got was from a friend that has a Mitchels online account. Maybe I will take a look on the net for a used one. Thank you for your help!
Dave

 

Yeah I wouldnt want to buy the FSM for just the ecm wiring also. I think you'll be fine with the responses I gave you but it wouldnt hurt to get at least one more opinion.

 

None of the circuits you mentioned need power w/key off. A blu/blk B blu/white C grn/white D grn/blk

 

I found a 89 Suburban wiring book around 128 pages on the net for 25 bucks so hopefully answerers everything and a future resource too. I was thinking one of the battery wires must be powered all the time to store fault codes and other things like blms etc.. Thanks for the info on the IAC wiring though as was mentioned it might change according to different masks and I have been forwarded two different ones so go figure.

 

I guess that confirms reversal info, my 7747 ran $42 & ANTY. I believe I stand corrected on fp relay, this is true, (I checked mine to be sure).