The cold start system on the '66 250SE isn't working; the valve works, the relay clicks, the system has fuel. I'm down to believing it's the thermo-time switch mounted on the firewall, but a new one is $250.00 and I don't want to buy on a guess.

How can I ensure it's the thermo-time switch?

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- Brian


1989 500SEL Euro
1966 250SE Cabriolet
1958 BMW Isetta 600

The TTS is in the coolant jacket.

If the relay is clicking, then the TTS has to be working. Be aware that there are two relays, one for CSV and one for Injection Pump rack solinoid..they are the same type relay, so don't mistake one for the other in your diagnosis that the relay is working]

You want to bring 12v bat + V. over to the CSV while you have the test plug out..[ use a rag there to catch any petro....this test port has a 7mm hex plug and is on the CSV.]. The key has to be ON b/c the electric pump has to be running for the CSV to have fuel feed.
You are looking for fuel to come out the test port when CSV has power.
If YES, then you have verified CSV and fuel feed, so you then want to test for 12v at the CSV when cranking a cold engine ..you should have 12v at same time as starter activation..if NO, then post your finding..
Many guys use a cabin switch directly to the CSV and operate it at will from the cabin. Kinda like having a manual choke, but electrical ......

Here is a simple TTS schematic/pictoral of mine.
I have some used ones laying around that I have Temp/Time tested, if need be.

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A Dalton