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1975 350 SL
142 Posts
Discussion Starter #61
Cold Start Injector Valve Problem Solved!

Solving the Cold Start Injector problem proved a bit tricky.

I used the warmer temps to pull the kick panel off and look at the relays. There are only 4 out of the 8 I've seen pictured on the forum. I'm certain that some of that is because of not having A/C, automatic transmission, power windows and emissions controls. Also, the relays are mirrored of the US arrangement. So, the Cold Start relay #2 is on the top left. See first photo at end of text.

The grounds, cold start injector and relays were good. I ran power to the cold start injector and it sprayed fine. Due to the severe cold it took some time to test all the circuits. The schematic scan is a very poor reproduction so I've had difficulty tracing down the circuits. Figuring out the color code in German helped! See second photo.

As best I could tell, the Thermo Time Switch looked like the problem area. I had yet to determine if there is a bad connection to it because I had not been able to remove the wire connection cap to the switch. I didn't want to push it too far in the cold. The age of the plastic and cold easily causes breaks. By the time I solved the cold starting problem I knew the electrical and D-Jet fuel system well. See third photo.

I spent a lot of time studying the 350SL schematic. Unfortunately, the part I need takes a bit of work remapping the missing lines. The color codes have been accurate. I cross checked everything with the information I've down loaded from the forum. It took a while to get through. See fourth photo.

Two times I thought I'd found THE answer to the cold start injector not working. I found and fixed some problems. The Thermo Time Switch connectors were corroded in the connector. Typical turquoise green before I cleaned them up. I may fill the holes with bulb grease to stop it from happening again. Since the Thermo Time Switch is the type that grounds to the engine, I removed it and cleaned the threads and brass switch body. Even though I cleaned and sanded the aluminum seal ring, it still leaked, so I took it out and put a copper one. I don't like those aluminum seal rings. Everyone I tried to reuse has leaked and I can't buy them at auto parts places.

The other problem I fixed was the cold start injector relay. When I tested the relays earlier, I just powered them up to see if they clicked. Since they are all the same relay, I swapped them around to see if they all worked the same. The cold start injector relay in the warning buzzer socked didn't work. I took the cover off the relay and cleaned the points. It worked fine after that. See fifth photo.

I solved two possible reasons why the cold start injector didn't work, but I still didn’t have THE answer why the Cold Start Injector valve wasn’t working. I examined the G and W terminals on the Thermo Time Switch and used a test light from positive of the battery to ground. Neither had a good ground. I had determined that the G circuit was a resistance coil that heated a bi-metal strip within the switch. The W circuit made contact on the bi-metal strip completing the circuit to the Cold Start Injector relay, which in turn completed the circuit to the Cold Start Injector. What I was afraid of was that the ECU was also creating problems. The only way to know was to replace the Thermo Time Switch and see if the Cold Start Injector worked. See sixth photo.

I searched EBay and found some possibilities, but none of the listings go back to 1975. It's possible that most of the years used the same Switch all manufactured by VDO. They are all in the $100+ range so I want to make sure the one I have is bad before buying a new one.

I rooted out what information was available on EPC for the correct part number for the Thermo Time Switch on my 116982-10-005728 3.5L Euro engine and came with my best guess of 003 545 90 24. From earlier searches on EBay, I had found a listings for that number plus 004 545 91 24 (from somewhere in the country of Turkey.) I ordered the first one (US address). It was the last one and was about half the cost at $123.50 with shipping of the price I found on several Mercedes "discount" online stores. The 004 545 91 24 part supposedly fits the '84-'85 models of 550SL. Actually, it looked like the same part to me, had the same specifications (-20 9.5 Sec and was half the price, but I didn't want to take the chance of it being wrong, so I bit the bullet and paid the $123.50. Better than $230 like the online "discount" places. I'll work on other problems while I'm waiting for the part.

I received the new Thermo Time Switch in an MB package, so I assume it’s stock. I have to wonder why MB can't include a $0.05 washer or seal to go with it? In preparation for installing this new switch, I found a nice set of copper washers at Harbor Freight. Not metric, of course, but after grinding out the hole in one and smoothing out the burrs, it sealed nicely.

After installing it, I tested the poles with my test light. The G pole with the heating coil registered with a dim light. That was encouraging, but the W wasn't making a circuit. I connected everything and tried the engine. It started! I think the weather had warmed up enough to not need the cold start injector. The next morning it was below freezing so I tried the test again. This time the W pole was making a good contact. I then tested the connector to the cold start injector and my test light lit up. So, I knew that all the circuits were working properly. The engine started right up! That was a relief!

Here’s a photo showing the old and new Thermo Time Switches. From the research I did, I believe that one from a '84-'85 would work as well and would be half the price of this one. The only difference I can see is that the duration of how long the cold start injector sprays. 9.5 seconds as opposed to 12 seconds. Even this 9.5 second rating goes between 8 and 12 seconds. See seventh photo.

I was surprised how many things had gone wrong to cause this cold start problem. First, the fuel pump check valve was plugged, the Thermo Time Switch connector was badly corroded, the Cold Start Injector relay points needed cleaning to make contact and the Thermo Time Switch was defective both in both the thermo coil G circuit and the W injector circuit. Any one of these issues causes the Cold Start Injector valve not to function. The only way I found them all was to understand the schematic and trace down the circuits. I’m certainly glad I had the resources of the EPC and the contributions of members on this forum available.

I’m moving on to other problems now. Un-pimping the car is going well. The left over wiring from various radio installations is a mess and I need to rebuild the center console. The blower runs full speed even without the control switch connected. Something is incorrectly powering a circuit somewhere. I’m working on a replacement ashtray for the one missing. One surprising find is the kill switch mounted inside the left heater vent that leads to the door. Here’s a photo. When I found it, I wondered if it was causing my cold start problem, but it’s connected directly to the ignition circuit, not the fuel system. See eighth and ninth photos.

That’s all for now folks!



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