17 degree C thermo switch and cold idle

[jeffc280sl]

I have been slowly sorting through cold idle issues. Bad cold start valve relay. Recently I discovered a failed FIP thermostat. These fixes have helped to greatly improve the cold start idle. Now I have found a failed low temperature switch.  This could explain why many others have had cold start problems.  I can't remember any discussion about this particular issue.  I could be wrong however.  Anyway my 17 degree thermostat switch has failed in the open mode.  My garage is at 0 degree C right now and the switch is open.  It's easy to check with an ohm meter.  At 17 degree C and below this switch should be closed.  When the 17 degree switch is closed the two way valve is switched on and the distributor is charged with atmospheric pressure.  The effect is the same as removing the vacuum on the distributor. Timing is changed and idle is elevated. Until now I did not understand the relationship between the the thermotime switch, 17 degree C switch and cold engine idle.  This situation may not fit all year cars.  I know it includes my 1970 model.  That's one more issue to check off my list.  I still have many more items to work on.

Jeff C.
1970 280SL 4-speed

[ja17]

Hello Jeff,
Certain elements of these systems vary from year to year. The ignition changeover systems are mosty unique to USA delivery cars after 1968, I believe. Here is some information which may help you. I also have the information for the previous year if this does not seem to match your car.

Download Attachment: ignition change over.jpg
63.51 KB

Download Attachment: ignition change over 2.jpg
58.97 KB

You will have to click on the enlarge button to make the scans readable.

Joe Alexander
Blacklick, Ohio

[jeffc280sl]

Thanks Joe,

I've not been able to find this part listed by any online parts sellers.  Is this the kind of part that is seldom replaced?

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

Thought I would update the group with some data from my 1970 280SL.  Still having a low idle upon cold startup.

During cold start to warm up the following conditions are present.

1) CSV is not leaking (at anytime)
2) While still cold no change in idle rpm as FIP air filter is removed
3) Cold idle CO is 7%, rpm 700
4) New FIP thermostat appears to work correctly.  As the engine warms the air inlet to the FIP closes.
5) Warm CO is 4.5% with idle at 800 rpms

Other current observations.
The car starts almost imediately when cold.  The car will re-start immediately if started again after a short off cycle.  If the car has been driven for a while and turned off for an hour or so it is hard to start not unlike many other SL cars I have read about on this site. Once started the car runs poorly with some small misfiring until fully warm after which it runs great.  I'm going to move forward and purchase the one-second time switch will help with the luke warm starts.  I expect this change will have little impact on the cold start idle.

Back to the cold start idle issue.  As you may have read earlier I have a defective low coolant temperature switch which is part of the emissions system. I intend to buy a new one as soon as I can find a source.  This switch removes retard on the distributor if the collant temperature is below 62.5 degrees F.  This will increase cold idle rpms.  

I have read quite a bit on this subject and I think this will work out.

It seems to me that the car is running too rich upon cold start evidenced by the high CO level.  Air seems to be provided from the right places at the right time via the FIP thermostat air valve.  

What say you?  Is the missing piece here the low temp switch?

Thank you for any inputs.

Jeff C.
1970 280SL 4-speed

[hands_aus]

Hey JeffC,
Have you checked your points and timing etc?
In my 250SL (with no emission controls) I had the slow cold idle speed symptoms. It also was running poorly.
I checked the points to find the fibre rubbing block worn and the contact surface pitted.
After installing new points the car nearly jumpped off the floor of the garage when I started it.
I adjusted the timing to BBB specs, adjusted the idle speed because it was too high, and the air fuel mixture also.
The engine was more responsive and very willing to get up and go.

Since then I have replaced the points with a Pertronix unit with great benefit.


Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[jeffc280sl]

Bob,

Thanks for your comments.  Points are not the problem as I have the Crane optical trigger ignition system.  Anyone know the part number for the low temp coolant switch.  I might have to call the MB dealer as a last resort.

Jeff C.
1970 280SL 4-speed