Overdrive Relay Buzz

[MKTR]

Hi Forum,

My TR4 overdrive has been working fine for many years until last week. Overdrive worked fine for part of my drive out with healthy quick engagements and now I just get buzzing from the relay with no overdrive engagement.

I have changed the 6RA unit and tried a modern type relay today and both just buzz when overdrive is engaged.  There are no obvious faults with the wiring connections to the relay and the power supplies are good 11ish volts. My reading of possible problems from previous posts and internet is that the earth grounding of the solenoid or gearbox switch might be the problem.

Is there a simple way to confirm the earthing of each - maybe using a multi meter resistance setting?

Can I bypass the relay to just send power to test the solenoid engagement when in the garage?

Is there anything else I should be checking before I have to trace each wire and switch?

Many thanks

Mark

[RobH]

First thing Mark is that 11 volts does not sound "good-ish" and may represent the problem, but that depends on what you are measuring it with and under what circumstances. With the engine off and a charged battery you should see around 12.5 volts.    It may be that rapid switching of the relay is upsetting the meter you are using so the reading is spurious.

The buzzing may result from a bad connection in the power to the relay coil or a fault in the earthy side connections or it might be the solenoid.   There are some simple tests you can do to try and isolate the cause.

Connect a jumper wire direct to earth from the relay W2 terminal - there is no need to disconnect the yellow/green wire.  

Turn on the ignition (no need to start the engine)  -  the relay should pull in without buzzing.   If there is no buzz  there is a problem in the wiring to the OD switch or interlocks, or the OD switch is faulty. 

If the buzz is still there remove the yellow/purple  wire from the relay C2 terminal and repeat the first test.  That will remove the high current solenoid.

If the buzz has gone there might be a problem with the internal switch in the solenoid which is supposed to switch from a pull-in 20Amps to a hold-in 2Amps. 

If the buzz still remains there is probably a problem in the white supply wire to W1 on the relay. 

 

 

 

 

[MKTR]

Thank you Rob - I will give it a go later.

[MKTR]

Update:

The overdrive is working but I do not know what the problem was.

I followed Rob's checks and was getting buzzing, even though the power supply was 12v, direct power to the solenoid worked and there were no obvious wiring errors. Removed the steering column cover to check the switch connections, and removed the gearbox cover to confirm continuity across the inhibitor and function when in 3rd/4th. No obvious issues.  I tested again and all worked as it should. Put all interior back together and tested overdrive at each stage and it still operates.

Maybe a glitch or maybe an intermittent fault like a failing inhibitor but time will tell.

[RobH]

Sounds as though you might have a dodgy connection in the wiring somewhere Mark.  You may have disturbed it when removing the covers or doing the tests, so that contact has been restored.   Fingers crossed that it lasts.