1976 300D Glow plug replacement-now issues. - PeachParts Mercedes-Benz Forum

cjlipps · #1 11-10-2010, 04:13 PM

Changed the loop-style for the pencil-type and made jumper wires using info found on this forum. Disconnected what I believe was the ground wire from the #1 plug so the new ones are fed from the wire at the rear of the engine and beginning at #5 plug.
Questions:
1) What is the large, hex-topped block (appx 1.5-2 inches across) screwed into the head that the ground (?) wire went into? It also has what appears to be an instrumentation wire leading from it that plugs onto a male prong on the item. Is this a temp gauge sender that also opens the ground for the glow plugs above a certain engine temp?
2) The new glow plugs aren't heating up. When I check for voltage at the supply wire (with a test light) I get a very brief indication of voltage when the key is placed in the glow position and then the relay clicks and there is no power to the plugs. Then, if the key is turned so that the engine is cranked, it shows power to the plugs. Is this a bad relay? Do I need a different relay with the pencil plugs? The system worked ok before the plug swap because the jumper wires between the glow plugs got red hot, I just wanted to upgrade the system since I am making this a DD.

Any suggestions appreciated.

Thanks in advance!
Chuck.

kerry · #2 11-10-2010, 04:22 PM

Sounds like you didn't disconnect the ground wire. You disconnected the temp sensor wire. The ground wire is braided wire going from the front plug directly to the block, or at least it was on my 77 300d.
Some relays do need to be replaced in order for the pencil plugs to function. I'm not sure if the 76 relay is one of those. There is a thread on here somewhere in which that topic is addressed and now that I think about it, that poster's symptoms may have mimicked yours.
Try running a jumper wire directly from the battery to the plug nearest the firewall (disconnect the hot wire) and see if the plugs work then.

cjlipps · #3 11-10-2010, 04:29 PM

There was no braided wire like I've seen in other pix on the forum. Just this wire from the #1 plug to this object in the head. And I did disconnect it.
I've searched and can't find my symptoms anywhere although I have seen where people replaced the relay along with the glow plugs. I've also seen folks that did not need to and their setup worked fine.

Thanks Kerry!

kerry · #4 11-10-2010, 04:32 PM

here's one thread on the topic. I'm not sure this is the one I had in mind but it seems to describe a similar problem to yours.

http://www.peachparts.com/shopforum/showthread.php?t=181503&highlight=pencil+plug+relay+ground+wire+115

79Mercy · #5 11-10-2010, 06:59 PM

yes, you need a different relay in that W115 for the pencil plugs to work.

I went through the same thing with my 76 300D. I just decided to leave the old loop plugs in the motor, they work fine they just take 40 sec in the morning to get hot.

cjlipps · #6 11-11-2010, 10:43 AM

Thanks guys. Any idea which relay I need? I'm not totally committed to this changeover but I'm far enough into it to see it through anyway.

kerry · #7 11-11-2010, 12:23 PM

If it were me, I'd use an inexpensive ford starter relay and a push button.

79Mercy · #8 11-11-2010, 06:20 PM

You need the fast glow, glow plug relay for those plugs. You should see it forsale here in fastlane.

A ford starter relay with push button is a little tacky, come on now lets not rig this up.

kerry · #9 11-11-2010, 07:38 PM

Not tacky. A superior system. Rather than relying on a timer, the length of glow is directly under the control of the driver. Pre-glow, post-glow, length of glow, all are controlled by the driver's mind instead of a mechanical robot.

79Mercy · #10 11-11-2010, 08:09 PM

A ford starter relay, excess wires, a rigged push button on the dash, in a Mercedes-Benz is tacky, sorry but it is.. Thats just how i look at it...

Now back to the original topic

you'll need the fast glow relay for those plugs. I looked on fastlane but didn't see it, however they are not hard to find.

Diesel911 · #11 11-11-2010, 08:11 PM

Quote:

Originally Posted by kerry

Not tacky. A superior system. Rather than relying on a timer, the length of glow is directly under the control of the driver. Pre-glow, post-glow, length of glow, all are controlled by the driver's mind instead of a mechanical robot.

About 1993 the Glow Plug Relay on my Diesel Volvo (I am still driving the car) went bad. I replace it with a Starter Relay and in my case a Rocker type Switch and in all of that time it has been 100% reliable.

I have had no more than 3 Glow Plug failures (Champion).

It is a very elegant system because it works well and on the Volvo the switch does not look like an add on.

The issue on a Mercedes is where to mount the Switch.

I do find the stock Mercedes setup nice in the fact that you can turn the key to the Preglow position and just let it glow while you wipe your Windows or something till the Relay clicks off and start.

But, if you live in a cold area and have to Preglow several times having a manual Switch would be more convenient.

kerry · #12 11-11-2010, 09:11 PM

I agree that the aesthetics of the switch is the biggest issue in a Benz. Spring loaded rocker switch near the antenna switch might not be too bad. Or a spring loaded rocker down near the window switches might work too. Even a switch hidden under the seat might be possible. I used to have an anti-theft switch under the seat of my Camaro and I can imagine a glow switch in the same location working well. Just because someone has put a tacky chrome push button switch next to the key doesn't mean that someone couldn't install a tasteful switch.
Where did ForcedInduction have his switch?

cjlipps · #13 11-12-2010, 10:16 AM

I have looked for this replacement relay and have found a fast glow kit with the new plugs, wires, harness and relay but all I really want is a plug-n-play relay to drop in place of the original relay.
Does such a thing exist?
Thanks for the suggestion and I'm sure it would work fine but I think I'll stay away from the alternate relay-outboard push button thing.

kerry · #14 11-12-2010, 10:21 AM

I'm under the impression that there isn't a plug and play relay. The new relay requires some wiring changes I believe. It's why I didn't replace the relay when I installed pencil plugs in my 77300d and why I suggest the ford starter relay alternative.

Alastair · #15 11-12-2010, 11:04 AM

I have sorta the reverse issue...(Which turned out to be a non-issue)

I'm converting a w123 280TE to fit OM.617. The engine has the older loop-type, and I have the later, pencil-type glow-timer/relay and wiring etc....

With the injectors out, I timed how long the loops take to attain an orange-glow in all cylinders.

It takes 12-14 seconds for them to get glowing orange, when connected directly to the batt via a thick lead and 70A fuse.

This is with all the original 'resistors' (The shaped wire links between the glow plugs) I have no idea if the 'proper' loop-type system for a 5 pot motor has any additional 'resistors' in the circuit, each plug is rated for 0.9V (Havent yet checked the actual voltage present across a single plug yet)

I had to install the glow-timer and run all the additional wiring to the glow-lamp on dash, the ign switched supply, ground leads and the purple 'start' signal lead.
--Adding the full wiring and timer/additional lamp to the dash took only a few hours and was pretty easy. I made up a 'loom' and ran the wires from the timer on inner wing to the point under the air-filteron opposite inner wing where the starter wiring is joined to chassis and connected it there, the main heavy current cable, the start purple lead. Ign supply was already present near the timer so this was connected there. The black cable to the dash lamp was tapped out of the loom near the large cable-grommet and passed through into the cabin.
--I used the correct instrument cluster from a Diesel and modded the 'gells' at the bottom to include the 'ABS' as well as the 'Glow' symbol.

The original 5 leads from the timer to the pencil-plugs were all joined to the single lead to the 70A fuse inline with the loop-plug supply at the fire-wall end of the chain.
(I adapted a spare plug and leads for this, as sometime I would like to replace the loops with pencil, but couldnt at this time justify the costs, and have retained the 'official' pencil-plug wiring loom for this--each plug is connected by its own lead back to the timer using this loom and 6-way plug)

Working with what I had available, the system operates just as it would with the correct pencil plugs. Wait for light to go out, then fire it up. Starts fine....

Maybe they wont last long being used with 'post-glow' timer, But I have Loads of the things accumulated over the years and the current set were brand-new--dirt cheap off fleabay!