Transmission during engine break in question

[Mrturner1]

I have a 1970 c10, just rebuilt the engine and while I was in there I did a stall converter and a deeper trans pan, new trans filer and a new vacuum shift modulator. During the engine break in I let it run for 20 minutes and it varies from 2100-2500 rpm's. It never went out of Park obviously for a break in.

My question is this. Can you ruin a transmission while its in park? Does it heat up even when not being used (in gear)?

It's a TH400, now with a 2600rpm stall, and it had some fluid in it during engine break in, BUT it wasn't full by any means. I added 5 quarts just now after shutting it down and it finally showed on the dip stick. Now, I don't know what these transmissions hold as far as fluid, but it's also got a deep pan so it'll need more than stock.

[Alex V.]

I don't see it harming the transmission just because it ran for a long period in Park - I'd think the heat transfer from the transmission cooler (assuming it has one in the radiator tank) would warm it more than anything else, but still nothing near a dangerous level. Automatic transmissions generate their heat from the friction created while driving and converter slippage under load. As to low fluid level or not shifting through the gears and priming the torque converter right off the bat, there's a chance depending on just how low the fluid was. IIRC the factory deep pan 400's hold 12 qts., most of which was likely lost since you changed the converter. If it took 5 qts. to bring it to full it was roughly half full, assuming the capacity of the pan you installed is close to a factory HD pan.

[BigRed76]

Although I've never done it this way, the best method I've heard of is to leave the torque converter unbolted from the flexplate during engine break-in. Then after engine break-in is done, bolt up the torque converter to then run the transmission through the gears and add fluid.

In the past when I have done this I would just add 5-6 qts. to the transmission, then once the engine was fired up I had another person add 3-4 more qts. to the transmission so it was at a safe level, and there is no harm just leaving it in park as long as there is a sufficient fluid level.

[swamp rat]

No, if it did harm we would not have told you to do it..

[Mrturner1]

Originally Posted by swamp rat

No, if it did harm we would not have told you to do it..

My linkage is all jacked up from PO converting a stick to auto matic. Having trouble figuring it out

[rcazwillis]

The only concern I have is the low fluid. The pump in the trans is run by the torque converter. If it was running dry, there is the possibility for damage. Once full, check the fluid on the stick. The fluid should look and smell like it just came out of a bottle. If it does, I would not worry about it. If it smells burnt, is brown, or has tiny shiny particles, there is a problem. Good luck.

[68is4me]

^x2
You never want to run and engine with low transmission fluid. Did you add any fluid after the convertor and pan install?

[Mrturner1]

Ya quite a bit. Quart and a half in the converter and I'd say 6 after pan install. The fluid on the dipstick is nice clean light red, and as Swamp Rat was saying I've had a lot of good help on my build thread. I was in a pinch and needed help so I also posted other places. I don't think tranny heats up without friction though, and there's no friction in park if I'm reading this right

[Davidf]

So long as there was enough fluid for the transmission oil pump to pick up, you are good. Otherwise, you risked damage to the pump as it spins with the torque converter. Those two notches in the snout of the torque converter engage/drive the transmission oil pump.

[BigRed76]

The transmission builds heat any time the torque converter is spinning. The torque converter is what creates most of the heat in an automatic transmission.

[rcazwillis]

Originally Posted by BigRed76

The transmission builds heat any time the torque converter is spinning. The torque converter is what creates most of the heat in an automatic transmission.

Exactly true. The only way the other parts create heat is by slipping. If they are slipping, they will be destroyed very quickly. The higher RPM converters create more heat.