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Service - Cold Start System, Cold Start Valve


Cold Start System

The cold-start system is somewhat inappropriately named. In reality, this system helps the engine start more easily regardless of its temperature.

Unlike a carbureted engine, a fuel-injected engine has no choke to cause additional gasoline to be admitted to the engine to aid the starting process. As is the case in all fuel injected Mercedes engines of this period, there are two separate, but closely related, devices that perform the same function as a choke.

One of these two devices is an electric solenoid attached to the rear of the injection pump which, when actuated, causes the fuel control rack in the pump to be moved to its "maximum delivery" position. The other device is an electric solenoid attached to the Cold-Start Valve assembly which, when actuated, causes a valve to be opened allowing pressurized raw gasoline to be squirted directly into the intake manifold.

When the system is working properly (and the engine is in good condition and well tuned), the engine will start easily under almost any operating conditions. If the system is not working properly, even a well tuned engine can be difficult to start.

The electrical part of the system consists of the two solenoids, one or two relays, and as many as three different switches. The 300SE cars had three different versions of the electrical part of the system. The types of switches used, and the wiring itself, differ depending on which of the three versions the car has. The names of the switches are all very similar and it is therefore easy to get a bit confused about how each works, so be forewarned.

The three switches are the "thermo" switch, the "thermo-time" switch, and the "time" switch. The names do indicate what controls each switch's state. The "thermo" switch is controlled only by temperature, the "time" switch is controlled only by a time period, and the "thermo-time" switch is controlled by a combination of temperature and time.

In the case of the switches controlled by temperature, the switches are located on top of the water manifold and sense the temperature of the engine's coolant. The time switch is mounted on the firewall beside the relay(s).

It is important to remember that the Cold-Start System only aids in starting the engine. It is completely disabled, and has no affect, when the engine is already running. More accurately, the system has control power applied to it only when the ignition key is in the "start" position. When the key is released from the start position and moves back to the run position, control power is no longer applied to the system.

When operating properly, the system will energize the solenoid(s) for a variable period of time while attempting to start the engine. This longer period of time applies when the engine is stone cold. The system also helps the engine start easier when it is already fully warmed up by actuating the solenoid(s) for a fixed period of 1 second.

Version 1 wiring diagram and description of operation. (Attached)

Version 2 wiring diagram and description of operation. (Attached)

Version 3 wiring diagram and description of operation. (Attached)

Troubleshooting the system

Problems with the system on a 40 year old car are not uncommon. The key to fixing it is to understand how it is supposed to work, and to realize that the system may have been "modified" at some point in the past.

Power to energize the solenoids is available any time the ignition key is in the "run" or "start" positions, but power for the "control" part of the circuit is available only when the ignition key is in the "start" position. Assuming that everything is working properly, there is no possibility of either solenoid being energized when the engine is running because there will be no power available to energize the relay(s) which control power to the solenoids.

The relays are simple, straight-forward devices. If the operation of relays (in general) is not understood, you should probably enlist the assistance of a helper that has some electrical/electronic knowledge. The operation of the various thermo/time switches are also fairly straight-forward given a simple description.

The thermo switch is the most simple of the switches. Above +35 C it is open, and below +35 C it is closed. A quick test is to do a continuity check with a meter. At room temperature the switch should read closed. Using some appropriate method (I use heated water) raise the switch's temperature to above +35 C and check it's continuity again. It should read open.

The time switch is a bit more sophisticated. This switch uses an internal heater to cause a tensioned contact to open. Note that there are a total of three electrical connections on the switch. One of the contacts (terminal 85) is the switched contact and one of the contacts (terminal 86) is the heater contact. The third contact is the metal case of the unit. This third contact is the "common" of the switch and the heater and must be connected to the car's "ground" point (the car's body) to operate correctly. Testing this unit is fairly simple.

With the device removed from the car, check for continuity between terminal 85 and the case of the unit. If there isn't any, the unit is defective. While continuing to monitor continuity, apply 12 volts between the case of the unit and terminal 86. This will cause the internal heater to heat up and you should see a loss of continuity between the case of the unit and terminal 85 after about 1 second. If not (and you properly connected to the unit!) , the unit is defective. Note that this device only assists the starting process when the coolant temperature is high enough that neither the thermo switch or the thermo-time switch are closed, in other words, the engine is already warmed up. If the engine is having problems starting when it is "cold", the time switch is not the problem.

The thermo-time switch is the most sophisticated of the three switches. While it is sensitive only to temperature, it's effective temperature is controlled by two factors. Let's start by stating that the internal switch is closed below a certain "design" temperature, and open above that same temperature. Secondly, there is a heater inside the unit similar to that of the time switch. Like the time switch, the heater in the thermo-time switch  is energized anytime the ignition key is in the "start" position.

How long the switch contact in the thermo-time switch remains closed depends on how long it takes the switch to be heated to the "design" temperature. The thermo-time switch unit is mounted in the top of the water manifold in such a way that it is exposed to the engine's coolant. The lower the coolant's temperature, the longer it takes the internal heater to bring the unit up to the "design" temperature at which point the switch contacts will open. Thus, the "variable" nature of the switch.

Testing this switch is very similar to the testing of the time switch. Like the time switch, the thermo-time switch has three electrical connections. There are two screw terminals and the body of the device is ground. Terminal G is the heater connection and terminal W is the switch connection. Note that there are two different versions of the thermo-time switch: one version's "design" temperature is +5 C, and the other version's "design" temperature is +35 C. In both types, the "design" temperature is stamped into the body of the unit. Below the "design" temperature you should be able to read continuity between the body of the unit and terminal W. If not, the unit is defective.

If there is continuity, then heat the unit to a temperature above the "design" value (again, heated water is a good way to do this) and verify that the switch opens.

Assuming that everything checks out so far, allow the unit to cool to a temperature below the "design" value, and verify that the switch is again closed. While continuing to check for continuity through the switch, energize the heater by applying 12 volts between terminal G and the body of the unit. After some period of time you should see a loss of continuity through the switch portion of the unit. How long it takes for this to happen depends on how far below the "design" temperature the unit was when you energized the heater. I like to get the unit as cold as possible for this test, and will leave the unit in a freezer for a while to facilitate this. For the unit with the +5 C "design" temperature, it has to be this cold to even try the test.

In my experience, the thermo-time switch is usually the defective component in the system. Unfortunately, I have not been able to get the +5 C unit for a long time. The +5 C unit can be replaced with the +35 C unit without any real consequences though. In fact, doing so causes either of the first two versions of the Cold-Start System to operate much like the third version of the system. Note that this is not an inexpensive device and, even though it is frequently defective, it is still appropriate to test the unit before replacing it.

Cold Start Valve

The cold start valve is an important part of the engine's starting system. It's proper operation, along with the associated electrical controls, substantially shortens the period that the engine takes to start. Not starting quickly is only one of the problems that can occur when the cold start valve is not functioning properly.

The functional parts of the cold start valve are a fuel filter, fuel outlet port, fuel flow valve and a electrically operated valve actuator. The electrically operated valve actuator is nothing more than a solenoid that,  when energized, forces a pin out from the body of the solenoid. There is a spring that draws the pin back into the solenoid body when not energized.

The fuel flow valve is a simple device with a brass seat and "needle". The needle is held against the seat, preventing the flow of fuel, by a spring.

The fuel outlet port consists of a small block of aluminum with two brass "jets" in it. The shape of the jets cause fuel passing through the them to be partially atomized. The fuel is sprayed directly into the intake manifold.

When the solenoid is energized the pin in the solenoid is forced out and lifts the spring loaded valve needle off its seat. In this condition, the valve is opened and pressurized fuel flows through the valve and then through the two small holes in the outlet.

There are three connections to the cold start valve assembly. A fuel line, an air line, and an electrical wire. The wire provides the electrical signal to the solenoid and the fuel line supplies pressurized fuel to the fuel flow valve. The air line is connected to the injection pump's auxiliary air controller. Other than providing a convenient place to connect the air line to the intake manifold, the air line is in no way related to the operation of the cold start valve.

What it does

When the engine is being started, it needs a richer fuel/air mixture (more fuel) than when it is already running. This is especially true when the engine is cold. Carbureted engines have a choke in the carburetor to accommodate this requirement. Fuel injected engines need a means to accomplish the same task. The cold start valve is a component in one of the two systems that the engine has to perform this task.

The cold start valve does nothing (if it's working properly) unless the solenoid is energized. When the solenoid is energized pressurized fuel passes through the open valve, through the outlet port, and into the intake manifold. This effectively enriches the fuel/air mixture and helps the engine start faster.

The period of time that the solenoid needs to be energized is dictated by the engine's temperature. The lower the engine's temperature, the richer the fuel/air mixture needs to be, and therefore, the longer the solenoid needs to be energized.

300SE engines have one of three different versions of control circuitry to energize the solenoid. The first two versions had both a temperature switch and a thermo-time switch. The third version eliminated the temperature switch and used a different thermo-time switch. All three versions provide the power to energize the solenoid for a period of 1 to 17 seconds, depending on the engine's coolant temperature. All three versions operate only when the ignition key is in the "start" position. When the key is moved from "start" to "run", the operation of the system is disabled whether the full "cold start" period has timed out or not. Likewise, the system is disabled after a maximum of 17 seconds, whether the engine started or not.

What goes wrong

In a nutshell, the cold start valve solenoid may not operate, or the assembly may leak either (or both!) fuel or air, and the filter may become clogged.

It is rare for the solenoid to mechanically or electrically fail, but it could. The quick check is to jumper from the battery's positive terminal to the screw terminal on the solenoid. If you can hear it click, it's probably working just fine. In the unlikely event that you have a bad one, be aware that is not available separately. You have to buy the entire assembly. If you do this test, first attach a jumper wire to the solenoid and another to the battery. You will generate a spark, so touch the two wires together away from anything flammable or explosive. Both the cold start valve assembly and the battery fall into the categories of flammable and explosive.

What does frequently happen is the two O-rings in the assembly fail, and the fuel control valve sticks open. The valve can become stuck when the car has spent a lot of time just sitting without being started. What happens is that the sitting fuel slowly becomes gummy. The next time that the cold start valve solenoid is energized, the valve needle is pushed off it's seat (it's a strong solenoid) and through the gum. However the spring that is supposed to close the valve when the solenoid is de-energized isn't strong enough to overcome the gum, and now the valve is stuck open. This is a BAD situation, and must be fixed immediately. At a minimum, the engine will run too rich. How rich depends on how far the valve is stuck open. If the valve is stuck all the way open, more fuel will be admitted than the engine can burn, and it will find it's way into the engine's oil pan, diluting the oil. BAD situation.

A valve that is only partially open will cause the cylinders physically closest to the cold start valve to run rich.

The O-ring between the solenoid and the body of the cold start valve assembly is almost always defective on older cars. This O-ring is supposed to keep air from leaking in, and fuel from leaking out. Because it is exposed to fuel, the rubber can deteriorate into a gooey mess. Even if it hasn't deteriorated to this point, it may be split. And, if it's been on the car for any period of time, the rubber has lost it's tension and isn't going to seal well regardless of anything else.

Worse than the minor vacuum leak that can occur when this O-ring fails is the fuel that leaks out during the starting period. I doubt anyone needs a lecture on what can happen with raw fuel dripping into the engine compartment. If this O-ring is failed and the fuel control valve is stuck open, amazing quantities of raw fuel get squirted all over the place. Not just during the starting period, but anytime the ignition key is on. Now you have a serious fire hazard.

I won't embarrass the other folks I know who have inadvertently ignited their cars, but I freely admit that I have done it, and more than once. I CAN'T EMPHASIZE THIS ENOUGH: REPLACE THE O-RING!

The second O-ring seals the cold start valve assembly against the intake manifold. Same problems, different part. Replace the O-ring.

The part number for the small O-ring is 001-997-75-40 and the part number for the large O-ring is 002-997-52-45.

The other two problems that can occur is that the fuel outlet port jets become plugged, and the fuel flow valve seat gets dirty and the valve can't completely shut off the flow of fuel. If the valve fails in this fashion, the fuel that leaks through is in much smaller quantities than when the valve gets stuck open. This can still have adverse effects on the engine's smooth running, especially at idle.

The factory service manual states that it may be necessary to lap the valve's needle onto it's seat to obtain a perfect seal. So far, I haven't run into one that couldn't be fixed with just a thorough cleaning.

To check for fuel leaks through the fuel control valve, the factory included a hole in the body of the cold start valve assembly. The hole is threaded and normally has a small bolt screwed into it to close the hole. The head of the bolt is 7 mm. Remove the bolt and turn the ignition switch to the "run" position (the fuel pump runs whenever the key is in this position, whether the engine is running or not). Keep a constant watch on the hole. If you see ANY fuel emerge from the hole, the fuel control valve is leaking. Watch the hole for at least a minute.

Fixing plugged fuel outlet port holes is tougher. The aluminum block is NOT intended to be removed from the housing, so don't try to do it. Aerosol carburetor cleaner helps, but it can be a slow process. The most effective method is to force pressurized cleaner through the ports, but BE CAREFUL. Most automotive solvents are flammable and atomizing them through the ports makes them even more so. If you can't be absolutely certain of the safety of the process, DON'T DO IT! If you do it anyway, you do so at your own risk.


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