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OD disengaging

Novamonte

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I took a three hour drive last week and the car behaved just fine. The day after I went back home and the OD started to disengage, then engage, then disengage etc until I switched back to Normal. Checked transmission oil level and it was fine, just under Max. I diagnosed the electrical circuit as per advice found on the Healey6.com site and everything seems to work. I hear the solenoid engage when I have ignition on, switch to Overdrive and put in fourth, it releases when I put the gear in neutral and stays engaged when I keep it in fourth gear until deactivated by the throttle switch. When I drive and it disengages it does so in a “crisp” way, not sluggish, which I understand suggests electric rather that hydraulic/mechanical problems. The transmission and OD were rebuilt by the PO two years ago so it should not be worn. Can the solenoid become “weak” so it engages but can not hold up against the pressure when the car runs? I measured the resistance between the white cable going down to the OD from the relay and earth and it was 2.5 ohms. I have not been able to measure the current because my multimeter can only handle 10 amps and I understand that the current can exceed that (although 2.5 ohms suggest a lower current, but I assume that there may be a stronger spike/transient upon engagement of the solenoid). Will get a better multimeter.

Any advice would be appreciated.
 

John Turney

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It sure sounds electrical, like a loose connection. The current has an initial spike, then drops significantly at steady state.
 

Healey Nut

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Dirty filter in the transmission . Low OD oil pressure .incorrect adjustment of solenoid etc .
 

Jack T

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Check the function of the switch on the side of the gearbox before you do anything else. See if keeping a little pressure on the gear lever to the right when in OD makes a difference. Very common, very easy to fix (after you remove the transmission tunnel!)
 

Bob_Spidell

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Change out the switch on the dash; very common problem, and cheapest and easiest thing to check. The relay on the scuttle would be my next guess; you can open them up carefully and file the points (I had same symptoms with both of these causes).
 

healeyblue

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I am going with Jack on this one. If the switch for 3rd and 4th is getting worn it will act like that. Try driving it and when it does it hold the lever over to the right side while driving and see if it will now stay engaged. I recently changed this on a BT7 that was acting similarly.
 
OP
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Novamonte

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20190527_063721.jpgI did some further testing this weekend. I connected a wire directly to the battery and routed it to the cockpit, and disconnected the wire from the OD relay to the gearbox switch and routed an extension to the cockpit as well. By joining the two wires while driving I could bypass the control circuit and check the function of the gearbox switch, the OD solenoid and the OD itself. They all functioned as intended and the OD did not disengage, even when I wiggled the stick while in 3rd and 4th gear, but did disengage when I put the gear in neutral (as it should). A renewed test of the control circuit showed that it also works as intended, so the problem seems to show only while driving.
As a next step I will connect a voltmeter between the W2 connector on the OD relay and earth. If that connector looses voltage when the OD disengages during driving the problem must lie with the dash switch or the cables running to and from it. Otherwise the problem must be the OD relay, the cable from W1 to earth or the cable to C1.

I purchased the clever (and cheap) little devise on the pic. It is intended to check electric circuits by being plugged in instead of the fuse and shows the current flowing through the instrument (up to 20 amps). Plugging it into a fuse holder with a female and a male spade connector enables it to be connected in line with the wires running to and from the OD relay (and other devices). I checked the current flowing through the OD solenoid when engaged and it shows around 0.9 amps, which seems right.
 
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