What is this in a 616 head? - PeachParts Mercedes-Benz Forum

JimmyL · #1 07-26-2007, 12:56 AM

The old seized engine from China has two "probes" in the head.

The "new" engine in China only has one.

I'm guessing it has something to do with the glow plug system, as it goes to the relay.

I know one usually checks temp during glowing (right?). What would the other be? Where is the temperature guage probe, btw?
Thanks.

#2 07-26-2007, 01:06 AM

I believe the early style used cylinder head temp to control how long to glow and the later used a sensor built into the GP relay.

The second should be for the temperature display in the instrument cluster.

JimmyL · #3 07-26-2007, 01:33 AM

Quote:

Originally Posted by ForcedInduction

I believe the early style used cylinder head temp to control how long to glow and the later used a sensor built into the GP relay.

So, the original GP relay is going to want a temp reading from a probe that isn't there. Should I swap back to the other harness provided I can find where I put the other relay from Scar? Seems like other wiring is different too, at the relay.....
Uffffff.....

Mustang_man298 · #4 07-26-2007, 11:58 AM

This one's an easy fix. Lance is right about their functions. I ran into this same thing when I put the 617 in my '80, my fist thought had been to take out one of those coolant plugs and replace it with the bushing and sensor from my old engine, only the plug was frozen in, yours will probably be too. I did try to glow it without the sensor, the GP's did work from what I remember, but they didn't work correctly.

Here's the fix:
First, did either of the car's have an EGR setup that used the plastic v-shaped vacuum switch on the t-stat housing? Use that housing, remove the switch, put a plug in the smaller hole if you have 2 holes. The sensor will now fit right into the housing, will not interfere with the t-stat, and since it's above the t-stat it will still read the temp in the head. Then you can either solder an extension to the existing wire for the sensor, or do what I did. I didn't want to cut up the wiring, so I went to the wrecker and found a newer model (I think 82 up has these) look at the GP relay, there will be a single round connector with a single wire on it. I don't know what it was for (it only apeared on the later models) you want the male half of that connector, and you also want one of the sensor connectors from the wrecker too. I then carefully popped the connectors open, and soldered in an extension wire, which now plugs to the factory wire and the sensor and looks clean and factory-ish. I even took some of the plastic sleeving fron a GP wire too to put on my new wire for the look (& protection). I then ran it along the upper A/C hose in front of the engine with a pair of zipties. My only dead giveaway that it was added was that I only had white zipties on hand, factory would've been black,lol. (you could even snag a couple of the factory reuseable ones while you're at the wrecker.

mplafleur · #5 07-26-2007, 12:28 PM

Yep, it's a temp switch for the glow plug relay. I had one go out on my '75 300D.

JimmyL · #6 07-26-2007, 09:28 PM

Chris, you are a stinkin' genius!! I do believe that I have a plug on the t-stat housing in that location. I'm headed outside to see if that sender will budge from the seized engine.
I'm optimistic for now......

Quote:

Originally Posted by Mustang_man298

This one's an easy fix. Lance is right about their functions. I ran into this same thing when I put the 617 in my '80, my fist thought had been to take out one of those coolant plugs and replace it with the bushing and sensor from my old engine, only the plug was frozen in, yours will probably be too. I did try to glow it without the sensor, the GP's did work from what I remember, but they didn't work correctly.

Here's the fix:
First, did either of the car's have an EGR setup that used the plastic v-shaped vacuum switch on the t-stat housing? Use that housing, remove the switch, put a plug in the smaller hole if you have 2 holes. The sensor will now fit right into the housing, will not interfere with the t-stat, and since it's above the t-stat it will still read the temp in the head. Then you can either solder an extension to the existing wire for the sensor, or do what I did. I didn't want to cut up the wiring, so I went to the wrecker and found a newer model (I think 82 up has these) look at the GP relay, there will be a single round connector with a single wire on it. I don't know what it was for (it only apeared on the later models) you want the male half of that connector, and you also want one of the sensor connectors from the wrecker too. I then carefully popped the connectors open, and soldered in an extension wire, which now plugs to the factory wire and the sensor and looks clean and factory-ish. I even took some of the plastic sleeving fron a GP wire too to put on my new wire for the look (& protection). I then ran it along the upper A/C hose in front of the engine with a pair of zipties. My only dead giveaway that it was added was that I only had white zipties on hand, factory would've been black,lol. (you could even snag a couple of the factory reuseable ones while you're at the wrecker.

Mustang_man298 · #7 07-27-2007, 12:11 AM

Quote:

Originally Posted by JimmyL

Chris, you are a stinkin' genius!! I do believe that I have a plug on the t-stat housing in that location. I'm headed outside to see if that sender will budge from the seized engine.
I'm optimistic for now......

Genius? Lol, thanks, I dunno about genius, but I'm pretty resourceful and I've been tinkering for a long time.

FYI, if you ever decide to go with a 5 cyl, all you need to do is add one pin and wire to the gp relay harness going to the block in the hole labeled "G5" on the connector. The relay, even on a 240D, is already set up internally to power up to 5 plugs

I had the harness with the 5 cyl I used, but the covering of it was crispy and my 240 one wasn't, so I pulled out the 5th wire from that one and slid it into my 240's harness, so its essentially still all factory wiring,lol.

JimmyL · #8 07-27-2007, 12:37 AM

Quote:

Originally Posted by Mustang_man298

Genius? Lol, thanks, I dunno about genius, but I'm pretty resourceful and I've been tinkering for a long time.

FYI, if you ever decide to go with a 5 cyl, all you need to do is add one pin and wire to the gp relay harness going to the block in the hole labeled "G5" on the connector. The relay, even on a 240D, is already set up internally to power up to 5 plugs

I had the harness with the 5 cyl I used, but the covering of it was crispy and my 240 one wasn't, so I pulled out the 5th wire from that one and slid it into my 240's harness, so its essentially still all factory wiring,lol.

I've grown very fond of 4 cylinders! Heck, I installed a starter in 10 minutes the other day. From the top!!

Wow did I do a horrible job of focusing the camera on the desired object.

Guess I didn't have it centered.
Here is the sender removed from the nice and clean seized engine.

It is 19mm is size, and I had a wrench and my prying pvc for the end of the wrench, but it was darn near finger tight!
I wish I had taken a before pic, but my thermostat housing on the horribly dirty non-seized engine {circa 1981} had a 19mm bolt in a threaded hole. Out it came, and in went the sender. Looks like it was made for it. I guess it was made for something similar.

Now I just need to make a wire for it. Also, my AC line running across the engine has both it's securing brackets broken off, so right now it just lays on the upper radiator hose. Something I need to fix somehow.......
Chris, I appreciate the fix advice. Like I've said before, I need you for a neighbor!!

Jimmy

JimmyL · #9 07-27-2007, 01:12 AM

I now have developed a question.

With the sender in it's original location, which is very close to the glow plugs, seems like that part of the head would heat up and tell the sender to cut off the glow plugs.
With the sender in the other location, it will only read warmth if the engine has been up to operating temp recently. When engine cold, it would seem like it would never turn off the glow plugs, at least until a timing circuit cuts it off, if there is such a thing.
Am I overthinking??

Mustang_man298 · #10 07-27-2007, 10:47 AM

No biggie on the advice my friend,lol.

I think you're probably overthinking it a bit on the temp reading. The amount of energy and time it would take for the heat soak to get from those glow plugs, thru all of the head casting, into the coolant, and warm that much water up around the sensor would be tremendous. Really the plugs are on a timer, the length of that timer is controlled by relative engine temp read thru that sender, to glow according to ambient temp on a cold engine, or to provide a shorter glow on a hotter engine. It's just a close enough is good enough thing. I still have great starts every time with this setup.

It's really too bad that other engine got seized up, it looks about like a brand new engine, hardly any oxidation even......

Quote:

Originally Posted by JimmyL

I now have developed a question.

With the sender in it's original location, which is very close to the glow plugs, seems like that part of the head would heat up and tell the sender to cut off the glow plugs.
With the sender in the other location, it will only read warmth if the engine has been up to operating temp recently. When engine cold, it would seem like it would never turn off the glow plugs, at least until a timing circuit cuts it off, if there is such a thing.
Am I overthinking??

JimmyL · #11 08-05-2007, 11:08 PM

I finished up on my glow plug harness conversion this afternoon, and Chris' idea was simply perfect! It was a simple yet completely effective repair, and it looks "factory" also.
I suspect my glow plugs were working, but the light never came on. Well now it works just as it should, and I appreciate Chris for the repair idea. With pics!

Good folks here for sure, and they reinforce it daily. Yall make the rest of us look better than we are......

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