MORE heated seats information.

[CDTi]

For those who are fed up with my meanderings on heated seats (or who may already know what I am about to impart), then my most humble apologies for wasting your time. . So here goes...

I thought the wheels had come off the cart for me today WRT retro-fitting the heated seats. I decided to do a little advance preparation having already found the hidden loom for the switches (http://www.the75andztclub.co.uk/foru...ad.php?t=18556), so I plugged my multimeter to pins 1 and 2 of the large black plug under the seat.

No voltage was present without the HS(heated seat) switch connected - as expected. When I connected the switch, I was getting 5.3V, irrespective of whether the switch was on or off. The green LED on the switch was lighting OK, but no change with the voltage on the under seat plug.

So I spent a good hour in the car with the RAVE circuit diagram and my multi meter. Everything looked OK - 11.8 volts to the switch, ground OK etc. I have 3 HS switches and all gave the same result. The chances of having 3 faulty switches is about the same as that of winning the Lottery, so I discounted that.

I put one of the switches on a bench supply and it worked fine i.e. grounded pin 1 when on. WTF ???. I went back to the car again - still wouldn't work. This was a real head scratcher.

I'm sure those of you who have heated seats are now screaming the solution at me

It finally dawned on me what the difference between my bench supply and car supply was i.e. I had the bench supply set at 13V, whereas the car battery was less than 12V (from my playing with the FBH).

So I lowered the voltage on the bench supply - SUCCESS . It seems that the switch has a built in voltage sensor that will not switch on the seats without the car engine being on i.e. the alternator charging giving the battery about 13.8 volts.

I estimate that the cut-off voltage is about 13 volts. Below that and the seats will not heat, even if the switch does appear to be heating.

As we say in my job, "forwarded for your information"

[Phil]

I did something similar once with a hrw on a Rover 200 Coupe, used car check, drove car in, switched engine off, ign on, test all electrics, no HRW, spent ages going through the wiring, gave up and went to park it outside, suddenly it started working. How embarrassed did I feel!

[JohnDotCom]

I only knew the Answer because of a Couple of Posts re non working Heated Seats before Alternator failure.
Another helpful way to realise its Dying.

[Duotone]

I have completed the upgrade to heated seats that you are planning.
I had the black plug under each seat.
My car was fitted with cloth, manual seats.
I bought a set of leather, heated, electric adjust and memory seats from Les at SMC.

I fitted the heated seat switches to the dashboard, then installed the seats.
I connected them up to the black plugs and also the airbag plugs.
Switched on the ignition and checked the electric adjust, this worked fine on both seats.
Checked the seat heaters, they worked fine.
Finally checked the memory function, this worked as well, although I was surprised that the memory function worked after reading on various forums how difficult it was to get the memory function working.

As your is a facelift 75, I believe that your installation of heated seats will work ok.

Dave T

[MGOracle]

Curious! My heated seats come on in position two without starting the engine.

So do my rear heated seats

[cjmillsnun]

Originally Posted by MGOracle

Curious! My heated seats come on in position two without starting the engine.

So do my rear heated seats

If your battery is fully charged, they will. However if you've had the radio on for a few hours, or used had the FBH on test then the voltage in the battery would drop below the threshold, hence the heated seats won't work, preserving the power left in the battery to enable the car to be started.

A very clever piece of design!