700r4 lost third gear and overdrive

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Hey everybody, I got a 1988 K1500 with a 5.7 and a 700R4. After a year and a half of getting this beast back to roadworthy, I finally did it. The wiring harness caught on fire and I resoldered it. Timed it perfectly according to my buddy with the shop and timing light. Got it on the road and the fastest I can drive it is 45 MPH in second gear. Try to get it in third or overdrive and it'll just surge when I hit the gas. Had a code for high tps sensor voltage, no oxygen sensor connected (there isn't one on the manifold I've got connected, just deadheaded the wire), and a bypass circuit being detected (probably gray wire for the fuel pump module on the firewall, I jumped that over to the hot lead from the positive post). It's OBD1 and my experience with that is limited. Any possibility it could be in limp mode? I mean, I try to take it above 45mph it'll start to pop, and even sometimes when I have it around thirty or jam the gas pedal.
Dude at the garage I was working on it at told me to get a new IAC motor. Plugged the diagnostic tester in and I could hear it whirring a few times. What confuses me is that before I had to take it out of commission, I drove it in third gear and overdrive, and I was pretty careful with the transmission.
Any advice would be appreciated.
 
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studigggs

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Backup spark and fuel won’t impact your transmission. Only electronics in that thing are the lock up solenoid and an overtemp sensor I believe. All controls are mechanical/hydraulic.
 
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Backup spark and fuel won’t impact your transmission. Only electronics in that thing are the lock up solenoid and an overtemp sensor I believe. All controls are mechanical/hydraulic.
Thanks. Know if I can put a 4 wheel drive TH350 or 400 in it's place or if there'll be some tinkering to do before hand? It's got an NP241C transfer case.
 

Schurkey

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Connect a scan tool, find out what the computer is trying to tell you. Codes can be useful, the data stream is ESSENTIAL.

Verify fuel pressure under load.

How long has it been since the plugs, plug wires, cap 'n' rotor, air filter, fuel filter, EGR, and spark advance has been verified?
 

yevgenievich

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Being that it surges when in 3rd, it might be that engine is not running correctly to where it can't pull at the lower rpm. Transmission might still be fine.
 

Hamps

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Just to verify. Is the transmission shifting into third and by surging do you mean the engine free revs? There is no reason it shouldn’t shift into at least third. From there we can help diagnose it a little better. Thanks.
 
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