Cold Start Valve Voltage - PeachParts Mercedes-Benz Forum

350SL4spd · #1 06-26-2006, 05:36 PM

Car in Question: '75 450SL

Question: What should the voltage reading be @ the cold start valve for a cold start? I'm having a rough idle and stall attempts when cold starting. Condition smooths out in 10-15 seconds. Should I have stright 12V?

Where is the CS relay on this car?

Thanks!

Tomguy · #2 06-26-2006, 06:57 PM

You should have ~12v at the cold start valve when trying to start cold - defined as engine temperature below 35°C. This may be higher or lower depending on current draw from the battery at cranking, or alternator running. The thermo-time switch is the main trigger for the cold start valve. It works by sensing water temp, and if it's too cold, its bi-metallic spring (like a thermostat's) snaps closed, engaging the CSV. Hence the "thermo" function. Once closed, the current passing through it heats up windings around the switch. This heat causes the switch to shut off at the right time - hence the "time" function. The colder your coolant is, the longer it will take for these windings to heat up and close the CSV. According to the manual it should be engaged for about 12 seconds at -20°C (and 0 at 35°C and higher). The relay is a cylindrical-shaped relay - not quite sure where it is on the 107 chassis. On the 108/109, I believe it's right behind the D-Jet computer.

These crappy starting conditions can arise from a vacuum leak, misadjusted air speed screw, misadjusted idle mixture, misadjusted throttle plate, or misadjusted throttle position sensor as well as a malfunctioning startup circuit. Also, your EFI trigger points can be crudded up (especially if the vehicle has been sitting a while) and may need a cleaning (DO NOT USE SANDPAPER!)

If you provide me with your email address (PM me if you like) I can send the D-Jet manual. It's meant for 108 and 109 chassis with the specific locations of some relays and whatnot, but you can use it to tune your system to factory specs. Adjusting my linkage, for example, exactly as defined in this manual made my throttle response INCREDIBLE.

mbdoc · #3 06-27-2006, 08:16 AM

First thing...cold start injector has little OR nothing to do with HOW the engine runs when COLD in the "summer"..even at 70F that injector MIGHT spray for 1-2 seconds ONLY with the key to the start position..once the starter is DIS-engaged the cold start does nothing!

IF the engine doesn't run properly when cold there is another problem..LIKE valve adjustment..OR excessive carbon on the valves..OR fuel pressure bleed-off.

350SL4spd · #4 06-27-2006, 09:44 AM

My main reason for suspecting the cold start was the incredibly short period of time before it smooths out & runs perfectly; really just a few seconds. I'm going to test it today for spray pattern & voltage if I don't have spray.
Does this engine have solid lifters or hydrualic?

Tomguy · #5 06-27-2006, 11:01 AM

Neither. It has mechanical ball studs (overhead cams). These require adustment every 15k or so.

350SL4spd · #6 06-27-2006, 11:31 AM

Cold Start is fine. I feel better for checking it though.

At this point I think I'll just run her in a bit and see if that helps. I need to pull the valve covers off to check the chain guide rails, so I'll check the valves then. In the meantime I'll start reading the D-Jet book and see what I can learn...

She's only got 94k mi, but as the original plugs/wires show, not that much service. I know she's only had 6-700mi put on her in the last 5yrs or so... I aim to change that.