Glow Plug Testing

[jimbo1364]

1976 300D, 115/114 body

First attempt at trying to start my MB engine. It appeared to take forever to see the glowplug light go out so I could attempt the start!

I have all 5 plugs (Bosch, from MB dealer, NOT the pencil style) connected as they would be in the car. They are setting on my work bench with nothing touching metal and the glow plugs orientated facing up. Can I hook up the battery (using jumber cables) to see if all 5 of the plugs glow, prior to installing in the car? This would ensure that the plugs are good and eliminated 1/2 of the diagnosis testing if they don't glow. If they do glow and I still don't get the engine running I could assume that the engine block switch, the timer relay, or wiring are at fault. Any thoughts as to this procedural test and my diagnosis. Lastly, where is the glow plug relay module and what is its physical shape?

[BrierS]

Follow this link. It may help guide you. If not, do a search and you will find a number of threads dealing with easy testing procedures.

http://www.peachparts.com/shopforum/showthread.php?t=133267

Steve

[Lycoming-8]

If these are the series style GP's, as I believe they are, you should probably consider that in the car the glow indicator on the dash is also drawing a lot of current. Voltage drop from that item needs to be compensated for, or the bench test will put too much voltage across the five real plugs. Does this car have the large knob that is pulled to activate the glow sequence and the starter? If so, I believe you are the glow plug relay.

[jimbo1364]

My car (1976 300D) has just the key that operates the starting. There is no knob to pull/turn to start the car. There is a, I do believe, a rotating knob that pulls the IP off of its idle stop for cold operation.

I did hook up all 5 glow plugs outside of the car. After applying power to the plugs, all five glowed within about 5 seconds, so I stopped the test. I am thinking that I might not have had a very good connection between the plugs and the connector wires/nuts!?
Now I am going to test the in-block thermal switch for continuity. My last test is to find the location of the timer relay and take a look at it also. Would you now where this timer relay is located and what it looks like?

[Lycoming-8]

The relays are usually on the drivers side inner fender. Often looks like a rectangular box with a flip up top. There is a thin metal strip fuse in the relay box. Sometimes it is part of the problem with glow plug operation. Fuse often looks ok, but is cracked and won't conduct 12v.

[t walgamuth]

i have always tested them one at a time, holding the glow plug with a vise grip and using a jumper cable at the battery. careful, they get red hot...if the're good.

tom w

[Brian Carlton]

Originally Posted by jimbo1364

I did hook up all 5 glow plugs outside of the car. After applying power to the plugs, all five glowed within about 5 seconds, so I stopped the test. I am thinking that I might not have had a very good connection between the plugs and the connector wires/nuts!?

This appears to be a perfectly good test. If all five glowed within five seconds, you should be good to go. What's the problem with the connectors?

Also, do not test the plugs individually as recommended above. They would be getting five times the recommended voltage and might not survive the test, unless you were really quick with the off switch.

[t walgamuth]

strokes for different folks. it worked for me. i just like keeping things simple. they either glow or they don't. no need to overheat them. you will know in a few seconds whether they are good or not.

tom w

[Brian Carlton]

Originally Posted by t walgamuth

strokes for different folks. it worked for me. i just like keeping things simple. they either glow or they don't. no need to overheat them. you will know in a few seconds whether they are good or not.

tom w

Can the series plug rated at about 2.5V survive a 12V encounter?

How fast do you need to remove the 12V to save the plug?

[t walgamuth]

you see it glowing you remove the power.

it is a matter of seconds.

tom w

[t walgamuth]

instantaniously. takes a few seconds to get hot.

i didnt know that it only gets 2.5 volts in service application. assumed it was 12v. so i guess it heats up faster with 12. still not fast enough to be so quick you cant get the power away before damage.

i have done it this way many times and had no problems.

but still, now, i have more money than i used to, if i find one bad, i prob will just replace all of them. though in recent years they have gotten a lot more expensive... with the exchange rate being weak.

i only use bosch.

tom w

[Brian Carlton]

Originally Posted by t walgamuth

i didnt know that it only gets 2.5 volts in service application. assumed it was 12v. so i guess it heats up faster with 12. still not fast enough to be so quick you cant get the power away before damage.

i have done it this way many times and had no problems.

OK, I stand corrected. I was not aware that you had sufficient time to pull the current off the plug prior to burning it out.

In series, you get a proportional voltage drop across each plug. 12/5 = 2.4V.

Just for confirmation, we are talking about the older series plugs (vehicles from the late '70's and earlier). The newer parallel plugs all get the full 12V.

Your tests were with the series plugs, correct?

[t walgamuth]

if i have tested the early plugs this way or not. definately the newer style.

tom w

[Brian Carlton]

Originally Posted by t walgamuth

if i have tested the early plugs this way or not. definately the newer style.

tom w

Well, the newer style is 12V, so, my original statement expressing concern about applying 12V to a 2.4V glow plug may still be applicable.

Be careful of suggesting such a test, if you have not personally done it.

[t walgamuth]

you were talking of the newer style... but i would not hesitate to try the older style as well.

tom w