5.8 EFI/c6 Not my swap, bought the truck this way for a parts truck but after spending some time with the ol'bug-eyed-beaut I've grown quite partial. It shifts flawlessly but I can't help thinking the engine is in a neutral mode since it has 0 wires save back-up lamp hooked to the C6. The speedo is waaaaay off, shows about 45 at 60 but swapping to a 3.55 rear should help no????

Anyway I'm keeping the pile so I'm just getting my barrings as how to proceed. Will the efi have fits? Should I ditch the EFI since the biggest hick-up has already been overcome?

 

Are you planning to swap to the C6 or the AOD?

If it's arleady got a C6 in it I'd leave it alone. The C6 is a non electric transmission. It should only have the back up lights coming from it. If it's working properly I'd just leave it alone.

As far as the EEC, it won't care. The AOD is also a non-computer controlled transmission, and as such, the current EEC you have doesn't care if it's an AOD, C6 or ZF5.

As for the speedo, a simple, cheap speedo gear swap will easily and quickly rectify your messed up speedometer.

 

The aod did not come behind the 5.8... that got the c6, and in 91 the e4od. engine probably thinks its always in neutral. As far as speedo, that is run off a gear in the tailshaft. Different gears were installed for different gear ratios. find the right gear, and its an easy swap

 

Unless you can find a mess of wires tied up out of the way and the check engine light is on that truck always had a C6 in it.... or the PO did an extensive swap to change the wiring harness and computer. The electronically controlled E4OD is the only overdrive trans option for an HD truck.

 

As I said b4, it has a swapped in C6... and I believe it thinks it's in neutral.

There's a bundle of wires (harness) along side the trans, a 1 piece drive shaft, and an O/D button on the dash like all the 91's I've ever seen. The engine starts and runs as it should with no CELs or warning lamps. The cable driven speedo is off with 4.10 gears and that's why I suggested 3.55's.

I'm trying to get an idea of long term probs with the truck seeing a MPH from the reluctor ring as it is still hooked up, and seeing heavy fueling and heat from O2 sensors and dash instruments. Will these normal operation experiences in N cause a prob in the ECM?

 

OK now we're getting the picture. What you describe is the accepted method of fooling the EEC into thinking the electronic trans is still there. There are a bunch of other trans sensors that obviously aren't connected that will throw codes but they are lower priority and won't set the check engine light, the engine will run fine and the EEC itself is in no danger operating this way.

The speed error will definitely affect engine fueling so that should be fixed sooner rather than later, and it can be accomplished by changing the driven gear on the speedo cable.. though you're probably gonna want/need 3.55 gears for somewhat reasonable fuel milage now that overdrive is gone.