Gluteus Maximus Posted December 18, 2022 Report Share Posted December 18, 2022 1974 620 with automatic...My truck is running great for the most part, but I've noticed a "shudder" when it shifts from 1st to 2nd gear, especially when under a load or going uphill. Fluid looks great...Is there anything a non mechanic can do to help this, or should I just drive it till it dies at which point I'll consider having a 4 or 5 speed conversion done? Thanx! Quote Link to comment
datzenmike Posted December 19, 2022 Report Share Posted December 19, 2022 This is free.... Inspect your carrier bearing rubber surround for deterioration. This thing on the rear of the front driveshaft... If rotted torn or worn the driveshaft will become off center under load and vibrate. As in a shift of gears. Looking at the trouble shooting chart for the 3N71B... 1/ Oil level is correct? Is dip stick damaged and reading incorrectly? After driving check stick for any foam indicating over filling. 2/ Vacuum diaphragm or piping leak. This is the small hard line from the brake booster fitting on the rear of the intake that runs down behind the engine to the left side of the automatic. There is a rubber hose at each end connecting it to the intake fitting and at the transmission end, to the modulator valve. Engine vacuum is a good indication of throttle position and engine load. The vacuum diaphragm tailors hydraulic pressures for shifting. Look for cracked hose. 3/ Control Valve* 4/ Band servo* 5/ Brake Band* * Least likely as the 3N71B is a very reliable automatic. Quote Link to comment
Gluteus Maximus Posted December 19, 2022 Author Report Share Posted December 19, 2022 Thanks Mike, I crawled under the truck and the rubber doughnut looks old and dry, but no noticeable cracks or tears. I'll see if I can find the other line tomorrow and check it out too. Appreciate the tips. Quote Link to comment
datzenmike Posted December 19, 2022 Report Share Posted December 19, 2022 When done and nothing found and if the fluid is bright see through red you might drain and replace it and the filter inside. There is an internal brake band that can be adjusted. Quote Link to comment
Gluteus Maximus Posted June 24, 2023 Author Report Share Posted June 24, 2023 I tried a tube of this snake oil...surprisingly, the shudder seems about 80% better. I went to an automatic transmission shop to see about changing the filter, and they told me to just leave it alone and drive it. Quote Link to comment
datzenmike Posted June 24, 2023 Report Share Posted June 24, 2023 First thing is check the rubber surround on the carrier bearing at the rear of the front driveshaft and all three U joints. If the rubber is rotted there will be vibration or shudder under load. It might be this and not the transmission. It's free to check. Shuddering is usually worn out or contaminated ATF. Just replace the transmission fluid and if possible the filter. I don't agree with that advice. Maybe they want you as a customer. Datsun automatics use DEXRON ATF. ATF should be bright red and see through. It should not be turning brown or cloudy. What does the fluid look like? 1 Quote Link to comment
Crashtd420 Posted June 24, 2023 Report Share Posted June 24, 2023 (edited) 14 hours ago, datzenmike said: Shuddering is usually worn out or contaminated ATF. Just replace the transmission fluid and if possible the filter. I don't agree with that advice. Maybe they want you as a customer. Datsun automatics use DEXRON ATF. ATF should be bright red and see through. It should not be turning brown or cloudy. What does the fluid look like? While you dont agree with the advice, it is a thing... I fell victim to it with my pathfinder transmission.... Fluid was burnt and nasty so I decided to change the fluid and filter.... this destroyed the transmission somehow, shop said if it was burnt that bad I shouldn't have done anything.... basically it should have been rebuilt.... So fluid color matters, if it still looks and smells ok then change it and hope for the best.... Edited June 25, 2023 by Crashtd420 Quote Link to comment
Stoffregen Motorsports Posted June 24, 2023 Report Share Posted June 24, 2023 (edited) That is an old mechanic's thing. Disturbing the insides of an auto can make the problem worse. Never asked why. For the record - automatics are definitely not my strong suit. Edited June 24, 2023 by Stoffregen Motorsports Quote Link to comment
datzenmike Posted June 24, 2023 Report Share Posted June 24, 2023 It seems counter intuitive to leave bunt ATF in and just drive it. Supposedly flushing out all the abrasive clutch particles will shorten the life. I agree they say this but don't back it up. What would happen to your engine if you didn't change the oil. Sounds like BS to me. I had an old '82 Grand Prix and was descending an extremely steep long logging road so I slipped it into low for the engine braking effect. I noticed that after a while the engine wasn't helping slow the descent so at the bottom I checked the ATF and it was foamy and looked like the blackest oil change you have ever seen. Foam caused the loss of hydraulics and the clutches slipped burning off some friction material. It drove fine but that night I dumped it all out and refilled with new. Drove it for years after. Quote Link to comment
Gluteus Maximus Posted June 25, 2023 Author Report Share Posted June 25, 2023 Yes, I checked the rubber donut months ago when I first posted this and it looks okay...kinda dry, but no cracks or tears. The fluid looks bright pink and clear. Guess I'll keep driving and see if anything changes in the coming weeks and eventually I'll drain it and refill. Quote Link to comment
Ooph! Posted June 25, 2023 Report Share Posted June 25, 2023 Had to rebuild the auto in my F250 at 380k and this was its symptom prior to giving up. At low speeds like when crawling at idle in traffic it would shudder but felt fine when accelerating. Quote Link to comment
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