Sticky vintage transmission (when cold)

Joined
Mar 16, 2023
Messages
3
Location
Ecully - FRANCE
Car is a vintage 1982 Renault R5 automatic. I've just swapped the transmission. It seems the 1st gear solenoid does not properly work unless the transmission is hot. On cold start, there is no way the car will start in 1st. It usually starts in 2nd but when really cold, no matter what, only the 3rd gear will work! After approx 2minutes, 2nd gear comes to life (this requires to activate one of the two solenoïds); and after 5~10minutes 1st gear works (this requires both solenoids to work).

Background information that's the same transmission internals as the DeLorean (different case to accommodate the specific differential). Friction parts are common to the 3L30. Needless to say this transmission is at home in the little Renault that weights 500kg less than the DeLorean and does not have even half the power and torque of the V6...

I've drained the oil twice since the initial refill (initial filled with Dexron 3, then Dexron 2 as recommended back then, and finally Dexron 3). I've clocked approximatively 200miles since the swap. The transmission I've used was 34.000km (20.000miles). Was partially open to check the clutch pack. It had no sign of wear. It was quite clean, although a few spiders had found their way through the dipstick. This being said I had bought this transmission in 2014 and stored it till now so it did not operate for quite some time. Also after installing it I realized the mosfet & capacitor of the transmission ECU were shot. So I suspect the car had been running in manual mode for quite some time prior to my purchase. So chances are that the 1st gear solenoid did not operate for close to 20 years...

Any way to perform a gentle clean-up of the gearbox? No idea if "cleaners" will do it any good?
Does the use of Dexron 3 instead of Dexron 2 has an impact on old transmission?

Any advice on how to revive this transmission would be greatly appreciated.
 
A couple of drain and fills of Dex 3, plus repairing the primitive Transmission ECU so it can control properly, should be all this little unit needs

Once it's got fresh fluid, and a controller that can shift it, then you can reevaluate

Paging @clinebarger , he may know about this orphan unit
 
Use a synthetic transmission fluid to aid cold operation? Can you check pump output pressure?

You said the unit was "quite clean" inside, so why are you thinking about cleaners?

Assuming the solenoids have O-rings: Are they intact?
 
A couple of drain and fills of Dex 3, plus repairing the primitive Transmission ECU so it can control properly, should be all this little unit needs

Once it's got fresh fluid, and a controller that can shift it, then you can reevaluate

Paging @clinebarger , he may know about this orphan unit

Never seen one much less worked on one, But Raemin did a good job explaining how it works.

This would be a job for a scope.....When Solenoids operate the way their suppose to (Mechanically), You'll see a "Pintle Bump" in the amperage waveform, Went to a class/seminar a while back that went in-depth on the subject & it works for diagnosing mechanically sticky shift solenoids, Pintle Bump IS caused by the Pintle physically moving.

This article does a decent job of explaining it.....HERE
 
Thanks for the suggestion, my trusty transmission mechanic passed away so I am quite on my own with my exotic transmissions...

0) I already repaired the ECU, thanks to the DeLorean forum. On the bench it works, would have however to check if it receives proper power supply from the old wiring harness.

1) I was considering gentle cleaning (lubgard Red or Auto-Rx?) just in case some varnish had found its way after such long period unused. I was also concerned by the dead spider I found in the tranny (god knows what this beast had brought inside the case). Also the filters have been long NLA (and no possibility to add an external filter), so I'd better keep it as clean as possible.

2) I did not check pressure, could be a good idea to match pressure against modern oil viscosity, so will buy a suitable gauge and make the tests as per the workshop manual. The issue here, is that there is no external adjustment (there used to be till 81!), so I have to remove the oil pan in order to adjust pressure. Will also have to buy a specific gauge, as the ports are odd and there is a large range of pressure tests.

3) As suggested, I will just try synthetic ATF. Here in France we do not have so much choice, so will give a try to Motul Multi ATF or Yacco ATF X. These are Dexron III with slightly more viscosity, so quite close to fresh Dexron II. Does work quite well on my TH400 Switch Pitch. Quite expensive unfortunately, so not the juice I usually use when I have to drop the oil pan a couple of times.

4) I do have the original Renault diagnostic tool that tests the solenoid, unfortunately, just like the pressure adjustment they removed the diagnostic connector which allowed to perform the test... So I will have to butcher the harness and find a way to properly tap the tool.

Finger crossed, I've just ordered synthetic ATF, lubgard Red (not sure I will use it), pressure gauge, connectors for the harness and will report once I've tested all these (one by one).
 
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Well it was the ECU!

I just removed all the conformal coating (royal pain), re-soldered all components (advice given by a helpfull DMC owner), replaced the ribbon that was interconnecting the two ECU boards, replaced two caps, upgraded 2 fets, added a small cap next to the comparator base voltage, and finally a new layer of conformal coating . Now the solenoids are locked with autority, regardless of the temperature.

As a bonus I've checked the oil pressure, which is a bit high (17bar instead of 14.5). I suspect that it was adjusted for the Renault Elfmatic D2 fluid, which back then was probably not as stable as nowadays fluids. This could explain why my weak ECU could not lock the solenoid(?).

I am not going to lower the pressure right now: I've added a shot of lubgard Red and plan to clock a few more hundreds mile. Once the fluid viscosity has settled I will just adjust the pressure to match the factory manual and switch to synthetic.

For now the sweet Renault is back on track. Thanks for your tips.
 
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