I think that going backwards is the way to go when troubleshooting a glow plug system, meaning starting from the grounds. I need to look at the wiring diagram (am still not a home) on the CD, as I think there's five different systems for the series/loop style plug and at least two for the parallel/pin style plug. Your car may have a transmission interlock and probably has a coolant temperature sensor tied into the system.

By the way, most multimeters can't resolve less than an ohm, regardless of what the display says. The trick is to put a resistor between the glow plug and one of the ohmeter leads. 100 ohm is plenty. Subtract what you measure for the resistor alone from the resistor/glow plug combination and you get a realistic value.

If you want to see if a parallel style glow plug heats then find a nut to screw the glow plug into, clamp the nut with glowplug in a vise, and apply battery power to the glow plug and vise. They get hot! This assumes the vise isn't electrically connected to something, like a metal workbench top.

If you're still having problems Sunday morning (my time) then I'll give it a shot. I've spent some time figuring out the electronics on these things but I need those diagrams to figure out a troubleshooting method for your car.






Young people think nothing of this, but to me it's astounding that people can help fix a car half way around the world in nearly real time.

I am not too handy with electricals. Just enough to get by. So I will let someone else steer you in right direction. Dabenz seems to be guiding you good.

good luck

MVK

Thanks dabenz , I think it is pretty amazing that we can get together and do this trouble shooting from this distance too .
The GPs all glow when direct across the battery .
I wondered if there was some possible intermittent fault in the ign switch , that might be causing me some grief.
The business with the on again / off again glow plug indicator is confusing .

Dusty-NZ,

I’m home and let’s hope you have a heated garage to work from. This is going to take some patience. Think of your glow plug system as two systems: the power system that provides high current to the glow plugs, and a control system that replaces your very capable brain. Either side could be goofy, or a part of the interaction between control and power (the relay box) could be bad.

First of all, you want to make some toys to go with your “reasonable” multimeter and make your life a bit easier.
1. Make another set of leads of 12AWG wire, about 3ft long, with alligator clips on both ends.
2. Make a 100ohm jumper for those small resistance tests. Two alligator clips, two 4in lengths of 14AWG wire, solder, heat shrink and you know the assembly drill.
3. A 6-12V test light isn’t part of the multimeter setup, but they sit together in my toolbox with the other stuff.

Please keep in mind that I’m going from what I see from my W114-115 CD, which (inadvertently?) gave me the glow plug schematic for the W123.133 engine 617.952. This system has the parallel/pin style plugs, uses the ignition key to run the starter (No. 3 position), has a dash mounted light bulb, and a built in 20-30sec timing system. The glow plug relay box has one 80A fuse and two electrical connectors (one is four pin and the other is either six or eight). No coolant sensor (measures temperature inside the relay box instead!). Does have a transmission interlock. The CD wasn’t nice enough to have a chassis electrical diagram so I don’t know if there’s a seat belt interlock. If your system is not like this then somebody’s going to need to send me the wiring diagrams for your car. The differences should be in how the starter interlock is connected and possibly the control power. I’ve studied these systems enough to be able to figure it out.

On to the fun! This should take 30min to absorb and no more than an hour to test.

First the power side. Disconnect the car battery.
1. Clean all the connections at the starter, and especially the ground strap. Then clean the connections for the wire between the starter and the glow plug relay box. Put the wires back on at the starter, then put your ohm-meter probes at the wire end where it goes into the relay box and at the starter. Should be zero. This is the supply to the power side of the glow plug system. Then put it all back together.
2. Now install the glow plugs and attach the wires. Unplug the big connector at the glow plug relay box. Hook one end of your ohm-meter to the engine block, and put the 100ohm jumper between the other end of your ohm-meter and one of the connector pins. The number on the connector should correspond to the cylinder. Write down the ohms as you go along. The resistances should be within ½ ohm of each other, and when you subtract the resistance of your jumper resistor, the resistances should all be less than one ohm. This is pretty important, as an out of balance situation tricks the fault detection, which is a magnetic field actuated reed relay inside the box. The result of an imbalance should be a glow lamp on the dash that turns on then immediately off and stays off when you turn the key to No.2 position. Switch glow plugs around if you need to balance your system. (In a normal world this situation indicates at least one glow plug or connecting wire that has failed open, but the timer should activate the system and the good plugs should glow without the dash lamp being lit.)
This is about all a normal person can do on the power side, and the procedure should be the same for any parallel system.

On to the control side. Keep the battery disconnected and pull the small connector on the glow relay box.
1. #1 pin should go to #15 terminal at key switch. Clean all contacts, then put back together.
2. #2 pin should go to “plug connection starter lockout backup lamp switch term 50,” according to the glow diagram. I don’t have the chassis electrical diagram so you’ll have to chase the wire. The 1976 240D chassis diagram shows a violet wire going directly to the transmission lockout switch on the transmission, and the switch is closed when in park or neutral. If your car is like this then put in park or neutral, and check resistance between the relay side pin and either the transmission case or the brown wire coming out of the transmission lockout switch (you’ll need a long jumper wire). If a brown wire then also check resistance between it and engine ground (now you know why you cleaned those starter connections). Both resistances should be zero. If that wire actually goes to the key switch then we need a chassis diagram for your car.
3. #3 pin should go to the indicator lamp in the dash. Chase the lamp downstream to ground, then clean all connections. Put it all back together, then measure resistance between the pin and engine ground. Anything is OK. Zero or infinity is a bad bulb socket or bulb itself.
4. #4 pin should go directly to ground. Do the drill you just did for #3 pin.
If your car is different, then the difference is in this control system. If necessary, I’d wait until I can look at your diagrams as the wiring interactions here can get more complicated, especially if pins #1 and/or #2 are different. If you’re OK to here, then let’s move on.

Reconnect the battery. Make sure the engine is cold.
1. Unplug the little connector and connect one end of your voltmeter to #4 on the wire side. Have your helper turn the key switch to No.2, then you measure voltage at #1 pin on the wire side. Then have your helper move key to No. 3 position (start) and you measure voltage at #2 pin on the wire side. Keep your helper busy and have him write down the voltages. Turn the key switch off and compare the voltages. If either is zero at the wrong time then the problem may be in your key switch or upstream. If both are at battery voltage then you’re OK.
2. Put a jumper wire between #1 and #3 pins. Turn the key switch to No. 2 and the glow lamp should light until you turn the key off.
3. Remove the jumper and plug in the little connector. Unplug the big connector. Connect one end of your volt-meter to engine ground with the alligator clip. Have a helper move the key to No. 2 and you then quickly measure the voltages at pins 1 through 5, one at a time, at the relay box. You need to beat the timer circuit! Keep your helper busy and have him write the voltages down. Turn off the key and look at the voltages, which should be around 11V. Pins 2 through 5 should be the same or really close, and Pin 1 should be close to pins 2 through 5. Think of these as two sets of voltages. An imbalance between the sets tricks that fault detection circuit. No volts is either a blown fuse, a sticking relay, or a bad semiconductor, all in the box. High volts for either set means a shorted resistor upstream of the connector (#1 has it’s own, #2 through #5 are in parallel downstream of the second resistor). No volts at just one pin means a bad solder joint, and/or an opened resistor if it’s just #1 pin that has no volts. At any rate, you need a relay box unless you’re good at finding electronic parts and soldering them together.
4. Now the hard part: checking that fault detector reed relay. If everything above this checks out, but that glow lamp lights and immediately goes out when the key is moved to the No. 2 position, then either the reed relay or the timing circuit is bad. Without an expensive Hall effect clamp meter you’ll need to trick the system on purpose, which is a dangerous thing to do. Find nuts for the glow plugs to screw into, then solder heavy house wire (10 or 12 AWG) to the nuts. Bolt the other ends of the house wire together and run another heavy house wire to engine ground. You want to suspend the glow plugs to keep them electrically apart, and suspend the house wire as the plugs get hot and the insulation will start to burn off the house wire from the high current. Be safe! Have your helper turn the key to No. 2 and you watch the glow plugs. If they stay on for at least 20-35 sec then shut off by themselves, then the timing relay system is OK and the reed relay is bad. The cheap fix is to live with it and understand that you’ll get 20-35 sec of glowing without the light. Turn the key off and back to No. 2 to extend the time. If the plugs don’t heat then the timing relay system is bad, which means a relay box unless somebody knows what I don’t.

Good luck and keep us posted.


P.S. for those of you looking for my gorilla switch fix: you’ll have to wait a bit. A deer thought it was a hockey/rugby player and body checked the side of my commuter car. Instead of the expensive Hall effect meter I’ve always needed/wanted I get to replace a fender and a door this summer. One toy at a time at my house, and I need that meter to test my switching current.

Truely awesome instructions dabenz , I shall be able to get onto that in the weekend ,, nope no warm garage ,, drafty carport ,, LOL ,, :eek:
Many thanks for your time and effort , I shall report back my successes :D .

UPDATE
 

Many thanks to dabenz once again for your extensive guidance .
After working my way thru the instructions ..... phew ! ..... :D ..... I buried myself in the car for the duration , LOL .
Although I followed the list as best I could , I don't know that I found anything noteworthy , that I could definately say "this was it" .
Really , all I found was one GP cable not very tight.
Still all is clean and secure now , and I now know my way a lot more intimately under the dash , bump , ouch! , ahhh , .......:rolleyes: .
But hey , IT GOES , AND GOES , AND GOES .:D :D .
Thanks again for all your help guys .

I'm moving to New Zealand since weekends are a month long there.

You're most welcome ,,,,,, bring your CD :D LOL .

Gp
 

I have a question on my 79 300 CD : It was very hard starting after sitting only an hour or so, and worse overnight (only down to 30s or so here in Goergia), and my dash glow plug light would come on while driving with the engine already warm (150 to 175 deg. F). Relay or what?