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Old 10-05-2017, 07:58 PM   #1
dirtfarmer22-250
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Turbo 350 In A 88 GMC???

I just recently bought a 88 GMC SWB truck with a the stock 350 setup in it. The guy I got it from put a turbo 350 in it. The question I have is, what do I need to change to make it shift right. It takes off good, shift's to 2nd straight into 3rd. I tried adjusting the detent cable and its the same. Do I need to change the cable to a older style? I have to do a little more adjusting on the gear leaver as it doesn't go into 1st and hold it. Could this cause my problem? Thanks in advance for any info.
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Old 10-05-2017, 11:50 PM   #2
Captainfab
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Re: Turbo 350 In A 88 GMC???

A couple things come to mind. First, if this truck is an original automatic truck, it would have came with a 700R4. Connecting the kick down cable for the TH350 to the same position that the 700R4 TV cable was, likely is going to not be good for the TH350. The two cables perform completely different functions. Secondly, is the vacuum modulator connected to manifold vacuum? The quick shifting is likely a governor issue. Thirdly, if there is not an external VSS sender on the speedo connection of the TH350, to send a signal to the ECM, the engine will not run right.
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Old 10-06-2017, 08:05 AM   #3
b454rat
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Re: Turbo 350 In A 88 GMC???

They did use a 3-speed auto but guess it was th375. But cable would hook same way a 700 would
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Old 10-09-2017, 11:16 AM   #4
dirtfarmer22-250
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Re: Turbo 350 In A 88 GMC???

I talked to a transmission guy here in town and he said it sounded like a vacuum problem or a modulator problem. I put a gauge on the vacuum line and as close as I could tell, it had around 20lbs till I opened the throttle and it dropped to around 14 or 15. I checked to see if the modulator was adjustable and it was, but, it just clicked like it was too tight. Either direction I turned it, it just clicked and I could tell it wasn't moving. Ordered a new one and It'll be here later today.
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