Gear Vendor holding shift point too long? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72timemachine · 05-08-2017, 11:55 PM

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 mph...at 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.

chevy_mike · 05-09-2017, 10:50 AM

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.

DieselSJ · 05-14-2017, 10:22 AM

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.

05-08-2017, 11:55 PM	#1
72timemachine Registered User Join Date: Oct 2008 Location: los angeles ca Posts: 210	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 mph...at 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.

05-09-2017, 10:50 AM	#2
chevy_mike Never Ending Projects Join Date: Jul 2006 Location: San Diego, CA Posts: 3,836	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. __________________ . 1965 C10 Panel, Tiki Express http://www.67-72chevytrucks.com/vboa...d.php?t=506580 SOLD 1968 Chevy C10, Long, Fleetside, Hot Rod Hauler http://67-72chevytrucks.com/vboard/s...d.php?t=313233 SOLD 1965 Chevy C10, Long, Fleetside, Hot Rod C10 http://67-72chevytrucks.com/vboard/s...d.php?t=415702 SOLD We were given two ears and one mouth for a reason... listen twice as much and speak half as often...

05-14-2017, 10:22 AM	#3
DieselSJ Registered User Join Date: Jun 2016 Location: Phoenix, AZ Posts: 574	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. __________________ 87 R30 CC Dually, TBI 454, NV4500, zero rust barn find 87 Jeep Grand Wagoneer, 6.5 turbodiesel, 700R4

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