Overdrive not working

Andreas Wickman

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I have a 1983 C20 with 6.2 diesel and TH700, wher the OD dont work anymore. The OD is just powered by a single green wire. Any idea what has happened? It have worked fine for many years but suddenly wont step in. Where to start fault-finding?
 

PrairieDrifter

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Brake switch, there's a vacuum switch as well as it's hose to check on the firewall, and a few wires to check running from the trans.

I guess I'm not positive on the vacuum switch because it's a diesel.
 

scrap--metal

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An issue with the brake switch would prevent the torque converter from locking up while in overdrive, but I don't think that would prevent the transmission from shifting into overdrive.

Perhaps that's what the OP meant?
 
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PrairieDrifter

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An issue with the brake switch would prevent the torque converter from locking up while in overdrive, but I don't think that would prevent the transmission from shifting into overdrive.

Perhaps that's what the OP meant?
I think he can get it into overdrive it just won't activate.
 

Matt69olds

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The overdrive on a 700 is hydraulic, nothing electrical. If your referring to the lockup converter, that is electrical.

Start with checking for power on the purple wire at the brake switch. It should have power with the key on, push the brake pedal and it should go out.

The purple wire supplies power to the solenoid, I have no clue what controls the ground side of the solenoid with a Diesel engine. There is a 4th gear pressure switch in on the valve body to tell the computer the trans is in 4th, I’m guessing there is a speed sensor and some kind of module to detect engine load to lock/unlock the converter.
 

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I’m guessing there is a speed sensor and some kind of module to detect engine load to lock/unlock the converter.
That's the vacuum switch on the firewall that @PrairieDrifter previously mentioned. That and the brake switch are the only two things I can think of that control the lock-up converter on a 700-R4.
 

Andreas Wickman

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No vacuum is involved because its a diesel. Thing is that I have added a switch witch prevents the OD to step in until I want and I just want it once on the fourth gear. Without that switch, it jumps in on every gear. The switch is just in line with the single green wire going down to the trans. I have checked that it's power on the cable. It must be a coil/solenoid inside the trans that once energized controls a valve? Anyone who can explain how this works?
 

PrairieDrifter

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Here's the solenoid, before and after I swapped in the painless kit. This would be the guy that controls lockup.
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Blue Ox

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Unless it's been fooled with there is definitely vacuum involved with diesels. They have a vacuum pump on the engine and there is usually a vacuum and/or electric throttle position sensor on the injection pump.

I don't know if that has anything to do with the lockup issue. It's been a long time since I fiddled with a 700, but GM didn't convert the whole truck to electric just because they put a diesel in it.
 

Itali83

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Unless it's been fooled with there is definitely vacuum involved with diesels. They have a vacuum pump on the engine and there is usually a vacuum and/or electric throttle position sensor on the injection pump.

I don't know if that has anything to do with the lockup issue. It's been a long time since I fiddled with a 700, but GM didn't convert the whole truck to electric just because they put a diesel in it.
There is for SURE no vacuum involvement for a 700r4. The th400 uses vacuum to control shift points and has a valve on the throttle linkage at the injection pump for control vacuum signal to the modulator at the trans.

Another thing. The OP needs to stop saying his overdrive isn’t kicking in unless he hits the switch. He’s controlling converter lockup. So from that, I’m guessing he has a converter lockup issue not overdrive issue.

Ben
 

PrairieDrifter

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There is for SURE no vacuum involvement for a 700r4. The th400 uses vacuum to control shift points and has a valve on the throttle linkage at the injection pump for control vacuum signal to the modulator at the trans.

Another thing. The OP needs to stop saying his overdrive isn’t kicking in unless he hits the switch. He’s controlling converter lockup. So from that, I’m guessing he has a converter lockup issue not overdrive issue.

Ben
No vacuum directly to the trans, but vacuum is involved for lockup to operate properly.

Also agree. 4th gear is operating it's just not locking up the converter. Which would either be the servo, pressure switch or the vacuum operated torque converter switch or related wiring.
 

Itali83

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No vacuum directly to the trans, but vacuum is involved for lockup to operate properly.

Also agree. 4th gear is operating it's just not locking up the converter. Which would either be the servo, pressure switch or the vacuum operated torque converter switch or related wiring.
My words guys. The 6.2 does not use ANY VACUUM OF ANY SORTS for the 700r4 transmission or it’s converter. The vacuum converter switch was only for the gas jobs. The 6.2’s have an electric throttle position switch on the injection pump that will lock and unlock the converter depending on throttle position.

Not saying I know everything but I’m tellin’ you. Forget about vacuum with this truck and it’s converter.

Ben
 

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Here’s my old sketch from when mine was stock with a 6.2.


No vacuum.
 

wlwarnke

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Here’s my old sketch from when mine was stock with a 6.2.


No vacuum.
Dotted line is inside the Tranny.

I believe the test terminal was under the dash.
 

Itali83

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Dotted line is inside the Tranny.

I believe the test terminal was under the dash.
Test terminal is outside of firewall by brake booster.

Ben
 

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