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Tractor Talk Discussion Forum

O/T Seeking GM transmission advice

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MeAnthony

07-28-2005 12:29:49




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Ok, here goes. '86 Blazer, 350, 700R4. The engine's been changed. When driving, you only feel 4 shift points. Being a 4 speed with a lock-up converter, shouldn't there be 5 shift points? Also, when idling, it bangs pretty hard when shifted in to gear. I'm wondering if there's a vacuum hose incorrectly hooked up and maybe the torque converter is staying locked up? Along the lines of this theory, there are two vacuum switches on the firewall, one behind the brake booster and one above the right valve cover. They both take a vacuum hose on one end and a 2-wire plug on the other end. These two switches are plumbed together; vacuum hose runs from one to the other. Shouldn't these switches be hooked to a vacuum source?

The book I have on this vehicle, and the underhood emissions schematic, haven't been helpful in solving this problem.

Thanks for your assistance,
Anthony

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Joe(TX)

07-29-2005 07:33:45




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
Like Hossier Pete described, The 700 transmission is a 4 speed but the 4th is overdrive. Its not a 4 speed plus od.



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MeAnthony

07-28-2005 14:28:44




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
The idle is approx. 700 rpm. Does this qualify as high?
The u-joints, transfer case and differentials don't have excessive slop.
Reading through y'all's replies, I remember that when my BIL drove it home for me the other day, he mentioned that he had to get on it kinda hard, then let off to actually get the 4th shift. Accelerating normally from a stop would only get you as far as 3rd gear. Does this point to a too-tight kickdown cable?

In terms of Allen's response of it being engine vacuum over electric solenoid engagement, does this mean that those switches on the firewall should be attached to an engine vacuum source? As opposed to being only connected to each other?

Once again, I appreciate everyone taking the time to respond.

Anthony

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Robert E. Smith

07-28-2005 18:08:51




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 14:28:44  
MeAntony
There is a really good Yahoo Group called automatictransmissionproblems, the group's moderator is a ASE Certified Trans Mechanic and will answer any questions you may have, I ask him about the tranny in my K3500's 4L80E , it was hunting in overdrive dropping in and out,in and out! Told me to check the throttle position sensor and bingo! 20 bucks at the parts house and trannys fixed!
Just go to Yahoo's home page click on "Groups" after the groups page comes up do a search for automatictransmissionproblems and join the group!
These's guy's will be able to really point you in the right direction.
Good Luck Bob Smith

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Allan in NE

07-28-2005 15:23:26




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 14:28:44  
Anthony,

This switch isn't going to look right to you because I "built" this overdrive unit. But anyway, this is the vacumn switch for the torque converter solenoid.

What this switch does is "unhook" the converter when you pour the coals to it. It works in conjunction with, and is in series with that brake switch that the other boys are talking about.

Your truck does not have a "kickdown cable" per se. But rather a throttle cable, which works the throttle valve against governor pressure to determine which gear the truck should be in.

Usually, a frayed or sticky cable will stop your shifting or at the very least delay it.

Another important item on those 700s is to make sure the silly fuel filter is clear because it can really mess up the shifting if it is even partially plugged.

Allan

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MeAnthony

07-28-2005 15:51:51




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 Re: O/T Seeking GM transmission advice in reply to Allan in NE, 07-28-2005 15:23:26  
That sure looks like it; I have one in the same place. Where does that vacuum hose go? Mine is routed straight across the firewall to an identical switch, also on the firewall, above the passenger-side valve cover. Yours looks like maybe it runs down to an engine vacuum source?

I can check the wiring; there's been some "remanufacturing" done on the harness on the outboard side of the brake booster.

Thanks again,
Anthony

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Allan in NE

07-28-2005 16:25:31




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 15:51:51  
HI again,

Yes, just "T" it into a "hot" (not ported) engine vacumn source.

Allan



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Hoosier Pete

07-28-2005 14:09:54




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
You should only feel 3 shift points, 1-2, 2-3 & 3-4. The torque converter clutch comes on in third and fourth. The harsh engagement can be caused by too high engine idle speed. Also the afore mentioned u-joints etc. When shifting to reverse is the engagement also harsh? A misadjusted TV cable, (sometimes called a detent cable) can also cause a harsh engagement as can a stuck TV valve in the valve body or a stuck pressure regulator valve. Are the shift points correct in relation to engine speed? Go to the library and get a GM service manual and rent yourself a hydraulic pressure gauge and follow the instructions in the service manual. Good Luck!

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Brokenwrench

07-29-2005 09:50:25




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 Re: O/T Seeking GM transmission advice in reply to Hoosier Pete, 07-28-2005 14:09:54  
Looks like we must have been thinking and typing the same thing at the same time. :)



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Brokenwrench

07-28-2005 14:07:08




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
When driving from a stop up to highway speed you should feel a 1-2 shift,2-3 shift, 3-4 shift and lockup. The lockup can come on in 3rd or 4th depending on the load on the vehicle. A quick way to test the lockup system is to get up to 55, and while holding steady on the gas, slightly press the brake pedal. you should see the tack jump alittle, or here a slight RPM change. When you let off the brake, the RPM should drop back down slightly. I doubt your harsh engagement, is related to the converter. As stated before, if the converter were locked up you"d kill the motor.

Your harsh engagement could be something simple like your new motor idling faster. Especially on a GM, where the trannys engage quick to begin with, idle speed can make a big difference. You could also have driveline slop, (ujoint, transfer case chain, differential).

If this engagement problem showed up after the motor swap, I"d chase the idle. Good Luck Brokenwrench

Just a thought, as Allen said (I think), The 700 doesn"t run a vaccuum modulator, so this thing lives and dies by the throttle valve cable. If that thing got way misadjusted or the cable got frayed it could be hanging the throttle valve open, that could make for harder engagements as well. but for that to be true you"d have high hard upshifts as well.

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buickanddeere

07-28-2005 13:24:40




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
Sould only feel three shifts and torque lock-up. Is the engine idel speed too fast? Is it a very low stall speed towing torque convertor? Is there anything loose in the driveline that lets everything turn, then jolt to a stop when stifting into 1st? If the torque is locked and in gear she will stall the engine if the rear wheels are not turning. Those switches should be in service. Some backyard tinkerer has "Improved" the system. Unless your book is the GM serice manual, it's only good for a door stop. Being an 1986? It's time to quit pouring money into an old bottomless pit. Find a 1 or 2 year old vehicle coming off a lease.

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Redmud

07-28-2005 13:19:47




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
Forget the part about the vacuum pump in the post down below, my Blazer has the pump..



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Allan in NE

07-28-2005 13:17:41




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
Anthony,

Those two switches on the firewall are what control your lockup on the converter. It is an engine vacumn over electrical solenoid arrangement.

The banging into gear is most likely slop in the drive train, as that transmission uses no vacumn and is controlled via a throttle cable working against governor pressure.

Allan



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Redmud

07-28-2005 13:14:05




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 Re: O/T Seeking GM transmission advice in reply to MeAnthony, 07-28-2005 12:29:49  
If the converter stays in lockup, it becomes a standard trans without a clutch pedal and really hard to stop. the switches should have a hose from the vacuum pump. Check the brake pedal switch under the dash, make sure the contact bracket on the pedal is working the switch. if brake lites stay on, no torque converter lockup. Bad ujoints and slak in the rear axle make for hard shifts into gear. {just got thru replacing my joints} and my truck shifts 3rd and lockup at the same time. My vacuum is also disconnected from those switches, and no trans problems. One switch is for the EGR valve under the air cleaner, can't remember bout the other.

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