1975 350SL D-Jet Cold Start Injector Diagnostic - Mercedes-Benz Forum
 
LinkBack Thread Tools Display Modes
post #1 of 7 (permalink) Old 04-01-2015, 11:50 AM Thread Starter
BenzWorld Member
 
Date registered: Jan 2015
Vehicle: 1975 350 SL
Location: Illinois
Posts: 142
Mentioned: 0 Post(s)
Quoted: 30 Post(s)
(Thread Starter)
1975 350SL D-Jet Cold Start Injector Diagnostic

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 seemed good. I ran power to the cold start injector and it sprayed fine. See fifth photo. Due to the severe cold I was experiencing, 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 was 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 the 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 replaced it with a copper one. I don't like those aluminum seal rings. Every one I tried to reuse has leaked and I can't buy them at auto parts stores.

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.

I solved two possible reasons why the cold start injector didn't work, but I still didnít have THE answer why the Cold Start Injector valve wasnít 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 itís 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!

Hereís 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. Iím certainly glad I had the resources of the EPC and the contributions of members on this forum available.

Once I had a good understanding of the D-Jetronic injector system, I found it relatively easy to work on.

GD
Attached Thumbnails
Click image for larger version

Name:	1975350EuroRelays_2.jpg
Views:	393
Size:	92.4 KB
ID:	1382314   Click image for larger version

Name:	1975350SLEuroRelaySockets_2.jpg
Views:	399
Size:	116.7 KB
ID:	1382322   Click image for larger version

Name:	ThermoTimeSwitchWithConnector_1.jpg
Views:	418
Size:	50.6 KB
ID:	1382330   Click image for larger version

Name:	75-350SL-ColdStartInjectCir_1.jpg
Views:	386
Size:	106.2 KB
ID:	1382338   Click image for larger version

Name:	75350SLColdStartInjector.jpg
Views:	432
Size:	56.0 KB
ID:	1382354  

Click image for larger version

Name:	NewThermoTimeSwitch_1.jpg
Views:	1509
Size:	60.0 KB
ID:	1382362   Click image for larger version

Name:	ThermoTimeSwitches_1.jpg
Views:	727
Size:	40.4 KB
ID:	1382370  
GoldDragon is offline  
Sponsored Links
Advertisement
 
post #2 of 7 (permalink) Old 04-01-2015, 12:33 PM
Moderator
 
rowdie's Avatar
 
Date registered: Jun 2008
Vehicle: 1986 560SL: '84 500SL: '84 280SL 5 speed: other 107s
Location: OklaMoKan
Posts: 31,232
Mentioned: 51 Post(s)
Quoted: 111 Post(s)
Lifetime Premium Member
004 545 91 24 is for an M110 engine. 280SL. 004 545 92 24 for an M117.962 500SL. 003 545 90 24 for your engine. Easy to look up in the EPC.
rowdie is offline  
post #3 of 7 (permalink) Old 04-02-2015, 03:35 PM
BenzWorld Veteran
 
Date registered: Jan 2007
Vehicle: 1998 SL500, 1959 220S, 1970 280SL
Location: Falls Church, VA, USA
Posts: 777
Mentioned: 5 Post(s)
Quoted: 94 Post(s)
Good detective work, and a nice write up.

One minor point of clarification. The thermo-time switch doesn't power the cold star valve. It provides a ground that activates the cold start relay, and the relay powers the valve.

Cheers,

CT

Chuck Taylor
Falls Church VA
1998 SL500
1959 220S
1970 280SL
ctaylor738 is offline  
Sponsored Links
Advertisement
 
post #4 of 7 (permalink) Old 04-02-2015, 05:50 PM
BenzWorld Elite
 
Date registered: Jun 2011
Vehicle: 75, 280Sl /5speed
Location: Northern California.
Posts: 1,277
Mentioned: 4 Post(s)
Quoted: 279 Post(s)
Quote:
Originally Posted by ctaylor738 View Post
Good detective work, and a nice write up.

One minor point of clarification. The thermo-time switch doesn't power the cold star valve. It provides a ground that activates the cold start relay, and the relay powers the valve.

Cheers,

CT
Unless you have a 75' 280, no cold start relay, wired direct to the thermotime switch.
107merc is offline  
post #5 of 7 (permalink) Old 04-02-2015, 08:43 PM Thread Starter
BenzWorld Member
 
Date registered: Jan 2015
Vehicle: 1975 350 SL
Location: Illinois
Posts: 142
Mentioned: 0 Post(s)
Quoted: 30 Post(s)
(Thread Starter)
Quote:
Originally Posted by ctaylor738 View Post
Good detective work, and a nice write up.

One minor point of clarification. The thermo-time switch doesn't power the cold star valve. It provides a ground that activates the cold start relay, and the relay powers the valve.

Cheers,

CT
Thanks CT! Yes, very good point about the cold start relay. If the thermo time switch didn't break the points within the cold start relay, the cold start injector would keep spraying fuel into the intake manifold. The bi-metal strip within the thermo time switch should break the circuit to the relay at the correct engine coolant temperature if the heating coil failed to heat it sufficiently. I have to wonder, though, what within the thermo time switch prevents any kind of short to ground.

If the thermo time switch was shorted, the cold start injector would continue to spray until the ignition was switched off. I have read posts where the engine was flooded somehow to the point of hydro lock. I measured the cold start injector's spray at about a quarter teaspoon per second. It would take a lot of spray time to cause hydro lock, but if someone's engine was running way too rich, the thermo time switch might be a good place to check for a dead short to ground. Just a thought...

GD
GoldDragon is offline  
post #6 of 7 (permalink) Old 04-04-2015, 07:31 AM
BenzWorld Veteran
 
Date registered: Jan 2007
Vehicle: 1998 SL500, 1959 220S, 1970 280SL
Location: Falls Church, VA, USA
Posts: 777
Mentioned: 5 Post(s)
Quoted: 94 Post(s)
Are you sure that there's no relay in 74-75 280? Even the 63-71 113s used a relay for the starting aids.

I don't think that the TTS can have a short to ground, because it is a ground. The positive side of the circuit runs from the relay. If the TTS malfunctioned and provided a constant ground, the CSV would squirt the whole time the starter was engaged, which might flood the engine if it didn't catch quickly.

Cheers,

CT

Chuck Taylor
Falls Church VA
1998 SL500
1959 220S
1970 280SL
ctaylor738 is offline  
post #7 of 7 (permalink) Old 04-04-2015, 03:33 PM Thread Starter
BenzWorld Member
 
Date registered: Jan 2015
Vehicle: 1975 350 SL
Location: Illinois
Posts: 142
Mentioned: 0 Post(s)
Quoted: 30 Post(s)
(Thread Starter)
Quote:
Originally Posted by ctaylor738 View Post
I don't think that the TTS can have a short to ground, because it is a ground. The positive side of the circuit runs from the relay. If the TTS malfunctioned and provided a constant ground, the CSV would squirt the whole time the starter was engaged, which might flood the engine if it didn't catch quickly.

Cheers,

CT
CT,
Yes, you're right. The Cold Start Injector would only spray when the starter was cranking. How much it would take to flood the engine is hard for me to gauge. I don't have enough experience with my car yet to know.

GD
GoldDragon is offline  
Sponsored Links
Advertisement
 
Reply

  Mercedes-Benz Forum > Mercedes-Benz Roadster Forums > R/C107 SL/SLC Class

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Mercedes-Benz Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 

Title goes here

close
video goes here
description goes here. Read Full Story
For the best viewing experience please update your browser to Google Chrome