Over the past couple of months, I have been working to improve the hot start of a 250SL. After sitting between 15 minutes and two hours, it took a 10+ second crank to get it to fire followed by several seconds of stumbling until finally all cylinders were firing.
The first step was a rebuilt starter, which easily doubled the cranking speed and improved the delivery of the injection pump. This helped a lot, cutting the average crank time down to 3-5 seconds.
Before going further, I investigated whether the cold start valve was actually firing, and found that it was not. It was only receiving 9 volts while cranking. To get some more power to it, and the solenoid on the pump, I wired terminal 15 of the two relays directly to the battery through a fuse box. That resulted in the CSV firing and improved cold start.
Next step was a magic button to ground the thermo time switch manually as a test. We found that depressing the button for a second while cranking resulted in an instant start with no stumbling.
The final step was to make this automatic using a time interval relay from Allied Electronics (TDRSOXP-12V). As I wired it, when the starter is engaged, it supplies ground to the TTS for one second. It is adjustable with a range of .1 second to ten hours. I ran the wires through the firewall and stuck it up under the dash with some Velcro, which allows it to be detached if adjustment is needed. Cost was about $85 for the relay and the wiring base.
So far so good with the hot starts.
