DavidTR4A Posted April 16, 2018 Report Share Posted April 16, 2018 Hi All, I have a dicky overdrive. From the plethora of gearbox and OD articles online, I think I know the problem but thought it would be worth posting...very grateful for any input. The set-up is a TR4A gearbox with Type-A OD from a Vanguard. The oil is Hyp-90 and the level is good (fill to spill). The OD drops out, but only in after a period of time running (20 mins+) and only on braking or downhill... and this has led me to a conclusion around the Hyp-90 and scavenging/pumping when warmed-up. I'm looking to change to SAE40 and the article in the latest TRAction (304) has given me the confidence to press ahead. However, the problem is only apparent in 4th gear. No drop-out in 2nd or 3rd when braking or going downhill. That doesn't seem to fit with my pump/oil type theory. Perhaps I am off the mark with my diagnosis? As I say, grateful for any thoughts. Best David Quote Link to post Share on other sites
Lebro Posted April 16, 2018 Report Share Posted April 16, 2018 I would suspect the 3 / 4 isolator switch is only just making contact when in top, & on the overrun it looses it. Bob. Quote Link to post Share on other sites
ChrisR-4A Posted April 16, 2018 Report Share Posted April 16, 2018 (edited) Hi Bob, do you still run a dynamo, there could be a problem with not enough current/voltage getting to the solenoid to hold it in as the revs drop in 4th gear on overun. Are all terminals clean especially those on the back of the ammeter. I use 80 grade gl4 oil and it's fine just a bit slow to engage for first 15 mins until warm then instant. Chris Edited April 16, 2018 by potts4a Quote Link to post Share on other sites
Drewmotty Posted April 16, 2018 Report Share Posted April 16, 2018 Does the gear stick get moved by the tunnel /gaiter on the overrun knocking oout the isolator switch? Does it still happen if you hold it in gear? Quote Link to post Share on other sites
peter clarke Posted April 16, 2018 Report Share Posted April 16, 2018 I think it could be oil flow/pressure problem as the oil pump is driven by the gearbox output shaft which is pumping slower in top than it is in 3rd at a given road speed. Quote Link to post Share on other sites
Bill Bourne Posted April 17, 2018 Report Share Posted April 17, 2018 I would suspect the 3 / 4 isolator switch is only just making contact when in top, & on the overrun it looses it. Bob. I would agree with Bob. I had this on my 6 a while ago. I'm not 100% on what I did now, but I think it was a case of taking the switch off and replacing the fibre washer with a thinner one. All works fine now. Hope that might help Best Bill Quote Link to post Share on other sites
Fireman049 Posted April 17, 2018 Report Share Posted April 17, 2018 I would also recommend that you check the isolator switch. Tom. Quote Link to post Share on other sites
DRD Posted April 17, 2018 Report Share Posted April 17, 2018 I would agree with Bob. I had this on my 6 a while ago. I'm not 100% on what I did now, but I think it was a case of taking the switch off and replacing the fibre washer with a thinner one. All works fine now. Hope that might help Best Bill +1 Quote Link to post Share on other sites
DavidTR4A Posted April 17, 2018 Author Report Share Posted April 17, 2018 Thanks very much for the advice everyone. I will take a look at that isolator switch. cheers David Quote Link to post Share on other sites
DavidTR4A Posted April 18, 2018 Author Report Share Posted April 18, 2018 Quick test drive tonight and the OD stays in if I hold it in gear. Thanks Drewmotty. Think I'll have to adjust spacing on the isolator? cheers Quote Link to post Share on other sites
peejay4A Posted April 18, 2018 Report Share Posted April 18, 2018 It might just need nipping down a bit. The threads are quite coarse and Ive had one undo itself a bit. Quote Link to post Share on other sites
Waldi Posted April 19, 2018 Report Share Posted April 19, 2018 I found my switches had poor electrical contact. I sprayed them with contact spray and now they are fine. Did not open them, just sprayed along the pin. Maybe this is not entirely the right way, as i may have dissolved the grease. Worth checking while you are there. Waldi Quote Link to post Share on other sites
stuart Posted April 19, 2018 Report Share Posted April 19, 2018 The good thing with the original switches is you can open them up by carefully prising the peened over edge up and then strip and clean the contacts and a little grease to lubricate and they will go back together and work for another 50yrs. Stuart. Quote Link to post Share on other sites
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