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Old 05-08-2017, 11:55 PM   #1
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Location: los angeles ca
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Gear Vendor holding shift point too long?

I have a GV in my 84 dually diesel, 6.2, 400th and it works great all shift points seem within reason of it's given rpm.

I found what appears to be a Laycock GV in a boneyard. Seems GV bought out Laycock from what I have read. This one I found was in an 88 c1500 with a 5.7, 400th. I have a 90 GMC, 5.7, 400th. I installed it and had the drive shaft shortened and balanced. I am using the synthetic oil in the GV so I do not have to change it as often as the conventional oil...per the GV tech on the phone.

Problem seems that when I start out in 1st, I need to get the truck up to 35 this point the engine is at a high rpm (sorry no tack) but I can tell, trust me. When it finally shifts into 2nd, it just about puts my head through the rear glass...clearly this is kinda cool on one hand but not as a daily driver.

It will then seem to shift at an appropriate rpm into 3rd (about 45 mp) and then on the freeway, it becomes a rocket ship with the engine at a very low rpm while I can cruise easily at 75....but do not.

Anyone know what the deal is? I did try to switch the other control box out of my dually but is sorta behaved much in the same way. I even swapped out the electronic solenoid but made no difference. I did this because I read that the gear oil can pass by the o rings on the solenoid shaft and cause shifting problems. My filter screen is also clean as I dropped the small pan to clean it out before adding new oil.

Any help would be great so thank you ahead of time.
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Old 05-09-2017, 10:50 AM   #2
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Location: San Diego, CA
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Re: Gear Vendor holding shift point too long?

When I had my M1010 with the same 6.2L diesel and T400 trans (no gear vendor) I had this same problem. Ended up being a bad vacuum modulator on the trans and broken vacuum line. Once I replaced those items, it shifted great. Without these working, the T400 trans will use the mechanical governor to shift, which is at high RPM.
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Old 05-14-2017, 10:22 AM   #3
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Location: Phoenix, AZ
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Re: Gear Vendor holding shift point too long?

Yeah, that does not sound like an issue with the overdrive...that is something in the trans itself. Vac modulator and lines are a good place to start.
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