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1968 Ford Mustang — service lifetime transmission fluid or leave it alone?

RustBeltLearnerCommunity StarterMay 19, 20261968 Ford Mustang FastbackTexas
1968 Ford Mustang Fastback, about 161,500 miles. The manual treats the fluid as lifetime under normal conditions, but the vehicle is getting older and I plan to keep it. What are owners doing in practice, and why choose drain-and-fill versus an exchange?
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13 replies

OldSchoolWrenchCommunity StarterCommunity StarterMay 19, 2026permalink
This is usually cheaper to test than to guess. Get a real battery/charging test on the 1968 Ford Mustang, not just resting voltage, before blaming a module. That's the branch I'd chase this week. When does it actually happen? This is the version I'd tell a friend.
HybridShopperCommunity StarterCommunity StarterMay 19, 2026permalink
Start with the test that can prove you wrong. Write temp, speed, and in-gear vs coast when the 1968 Ford Mustang does it — then decide on parts. If the cheap test changes it, stop and reassess. Happy to compare notes if your setup is different. I keep a notes app thread for this now.
MarketGal186Community StarterCommunity StarterMay 20, 2026permalink
Start with the test that can prove you wrong. Get a real battery/charging test on the 1968 Ford Mustang, not just resting voltage, before blaming a module. If the cheap test changes it, stop and reassess. Keep sale price, fees, and rate as three different conversations. I've seen both outcomes, which is why I'd test instead of assume. Might be overthinking it, but I'd rather overthink than overspend.
ProjectTraveler753Community StarterCommunity StarterMay 20, 2026permalink
Ask what changed right before it started. Move a coil or plug on the 1968 Ford Mustang if it's cylinder-specific and see whether the code follows. You'll know quickly if you're on the wrong tree. I'd want a dollar figure or a measured spec, not 'should be fine.' Only after sitting overnight so far.
NoMarkupGarage716Community StarterCommunity StarterMay 20, 2026permalink
If it were in my driveway, a pressure test beats staring at a dry driveway if the 1968 Ford Mustang is losing coolant with no puddle. Save the big job until that test fails. When does it actually happen? I refuse to clear codes until someone else sees them.
ProjectTraveler753Community StarterCommunity StarterMay 20, 2026permalink
Intermittent stuff lies if you only catch it once. Capture voltage while the 1968 Ford Mustang is actually failing — after it restarts you've lost the clue. Save the big job until that test fails. On “service lifetime transmission fluid or leave it,” that was the fork that mattered for me. If it doesn't change the out-the-door or the repair estimate, it's noise. If the story changes I'll post the new data.
TorqueMechanicishCommunity StarterCommunity StarterMay 20, 2026permalink
Ask what changed right before it started. Capture voltage while the 1968 Ford Mustang is actually failing — after it restarts you've lost the clue. That's the branch I'd chase this week. Run the total, not the monthly, before you commit. That's my plan before I call anyone.
RustBeltMom362Community StarterCommunity StarterMay 20, 2026permalink
I wouldn't throw parts yet. Get a real battery/charging test on the 1968 Ford Mustang, not just resting voltage, before blaming a module. You'll know quickly if you're on the wrong tree. If it doesn't change the out-the-door or the repair estimate, it's noise. Only after sitting overnight so far.
BatteryDIYerCommunity StarterCommunity StarterMay 21, 2026permalink
Same neighborhood, different next step: Swap the cheap/easy piece first on the 1968 Ford Mustang and see if the fault moves. Save the big job until that test fails. Verbal 'we'll take care of it' is not a plan. Parking it somewhere I can hear it better next time.
BatteryDIYerCommunity StarterCommunity StarterMay 22, 2026permalink
Quick add on that: Write temp, speed, and in-gear vs coast when the 1968 Ford Mustang does it — then decide on parts. Don't erase codes until someone who can help has seen them. On “service lifetime transmission fluid or leave it,” that was the fork that mattered for me. If they can't show the line item, I assume it's hiding. If your situation is the opposite, ignore me. I'd rather look cautious in this thread than stuck later.
NoMarkupGarage716Community StarterCommunity StarterMay 22, 2026permalink
Yes, with one caveat: Capture voltage while the 1968 Ford Mustang is actually failing — after it restarts you've lost the clue. Save the big job until that test fails. On “service lifetime transmission fluid or leave it,” that was the fork that mattered for me. Did the worksheet show tax yet? This is the version I'd tell a friend.
CanyonSeller690Community StarterCommunity StarterMay 20, 2026permalink
Intermittent stuff lies if you only catch it once. If a shop can't duplicate it on the 1968 Ford Mustang, bring a video plus freeze-frame data, not just a story. Save the big job until that test fails. On “service lifetime transmission fluid or leave it,” that was the fork that mattered for me. Do that first. Climate here is different, so treat that as a local data point. If this is nothing, great. I want that in data.
MarketGal186Community StarterCommunity StarterMay 21, 2026permalink
I'd want a measurement, not a hunch. Get a real battery/charging test on the 1968 Ford Mustang, not just resting voltage, before blaming a module. That's the branch I'd chase this week. Keep sale price, fees, and rate as three different conversations. I'd rather be boring and correct. I keep a notes app thread for this now.
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