1986 500 SEC raft of minor problems - PeachParts Mercedes-Benz Forum

jjrodger · #1 04-17-2008, 04:18 AM

I have just bought a 1986 500 SEC (series 2) and it has the following niggles with which I urgently need a bit of help. I have done some forum searching but have not got very far.

1. The instrument and console illumination does not work. Given that the fuses are in order and this is a blanket failure, I assume the N40 Instrument Illumination Control Unit has failed? Is this right or could it be something else?

2. The heater is inoperative. The fan works, the air distribution controls work and I even think the a/c works. However, there is no heat coming out at all. I assume this is a broken duo valve. Or could it be something else?

3. The cruise control is inoperative. I assume this is the cruise control amplifier?

4. The driver's side seat memory and fore/aft adjustment does not work. All of the other adjustments work (i.e. up and down and the backrest angle and the headrest). I have not got very far with this. I have squirted cleaner into the switch, but that has not worked. Odd that the memory function doesn't work, too.

5. There is a strange buzzing noise from the passenger side (left on my rhd car). I suspect this may be related to the heater.

Any help is most gratefully appreciated.

mbdoc · #2 04-17-2008, 08:58 AM

If you un-plug the heater valve you should get some heat, if not the heater valve is the most likely problem. Try a new mono valve insert.

On the interior lighting; does the gearshift illuminate? It doesn't go thru the N40 control unit.

jjrodger · #3 04-17-2008, 09:21 AM

Thank you, as always.

Will check the valve.

Gearshift does not illuminate: does that point towards the rheostat?

jjrodger · #4 04-17-2008, 11:06 AM

Dodgy rheostat to blame for the instrument lights.

jjrodger · #5 04-17-2008, 01:27 PM

Further update.

1. Instrument lighting "fixed". The rheostat is worn and just needs careful adjustment.

2. I have ordered a new duovalve as it did not respond to testing (it is getting 12volts and does not appear to work, no pulsing). This was expensive (GBP227.96 incl tax) but available soon, whereas the repair kits were expensive (GBP180 plus tax for the pair) but not available for a while. Also, the duovalve can be returned if not required.

3. The cruise control is not the switch. I note there are a number of modules, any ideas which it might be?

4. The seat is "fixed" in the sense that the problem was traced to the control unit, which has been by-passed with a pair of relays. So the seat works, but not through the control unit. No doubt there is some reason why this might be a bad idea, but I will be careful. I suppose it means I have not got the memory function. One for the future, perhaps.

5. I suspect this is the blower aspirator pump for the cabin temperature sensor. I do not think it is related to the lack-of-heat problem (for the reasons stated above and because there is never any heat rather than merely slow changes in temperature) but it will have to be fixed. I note it is not horrendously expensive. I wonder how easily accessed it is.

Further comments and assistance welcome.