Glow plug fuse/wiring question. - PeachParts Mercedes-Benz Forum

r.e.wing_fc3s · #1 06-09-2010, 01:06 PM

tried searching. my 78 300d keeps blowing the glowplug fuse. i replaced the glow plugs. i checked the resistance of the wiring from the fuse to the ground strap and it was ok. i also checked the wiring from the input to the ignition switch, to the glowplug relay and to the fuse terminal. the only problem i found was some suspect solder joints on the pcb inside the relay box.

this glowplug wiring of this system is different then the later models that i am more familiar with. i normally think i have a decent grasp of electrical theory. in this case i don't understand why the glowplugs would work at all because of the ground at the end of the glowplug wire. instead of individual wires to each plug they are all fed off a common line which then grounds at the end. why doesnt all the power just flow to ground? and draw lots of current blowing the fuse? what "motivation" is there for the power to flow through the glowplugs? and why does the damn fuse keep blowing? the system seems so simple. the harness is not shorted, it has new plugs good ground at the end.

r.e.wing_fc3s · #2 06-10-2010, 06:49 PM

please? someone must be able to point me in the right direction

Jeremy5848 · #3 06-10-2010, 07:13 PM

The "old" glowplugs are wired in series like Christmas tree lights used to be in ancient times (the 1950s). The first glowplug gets (more or less) 12 volts and the voltage drops along the string until the last glowplug is grounded, as you have said.

The fuse-blowing indicates a short to ground at some point. Since you have replaced the glowplugs and checked all the wiring, I suggest one more test: "unground" the last glowplug and see if the fuse still blows. If it still blows, you probably have a defect in the pre-glow relay.

Another possibility is that you have gotten the wrong strip fuse. They are available in various amperages. Pre-glow systems generally use an 80A fuse but others are available, including 50A and 30A versions.

BTW, it's possible to "upgrade" your system to a new pre-glow relay and fast-acting, parallel-wired glowplugs.

Jeremy

r.e.wing_fc3s · #4 06-11-2010, 01:54 PM

ive read that when using new glowplugs bosch duraterm that i need to dissconect the ground strap at the last glow plug?

Jeremy5848 · #5 06-11-2010, 05:08 PM

Quote:

Originally Posted by r.e.wing_fc3s

ive read that when using new glowplugs bosch duraterm that i need to dissconect the ground strap at the last glow plug?

Not only that, you need to replace all the wiring as you are changing from series to parallel wiring of the new glow plugs. A factory-new wiring harness will be rather expensive but you could get one from a junkyard for very little.

r.e.wing_fc3s · #6 06-12-2010, 02:24 PM

this problem started when when my friend replaced the starter in his car. after that the car would crank but as soon as the glow plugs start to cycle the fuse blows. he replaced the g.p with large base bosch duraterm plugs. the old style in series type that uses wire with connectors and a ground strap on the last plug. i checked for shorts after the 80 a fuse and the resistance between the fuse the relay, ignition switch and b+ all seem fine. the wiring to the starter seems fine. i opened the relay and the solder connections seem ok and the relays inside click when you turn the key on and when you try starting it. i disconnected the ground from the last plug as suggested above and the fuse does not blow. the g.p. light comes on for about 45 seconds and then goes off. the car still won't run.

i thought the only thing that could cause the 80a fuse to blow was a current draw on the g.p. side of the fuse?

does the relay limit the current? is there a way to test other then simple voltage check?
the circuit seems so simple i just don't see what im missing #!@%&^*

micalk · #7 06-12-2010, 03:07 PM

First off, I don't have a schematic of your system, so I can only reply based on what's been stated above. The problem here is that there are alot of variables to deal with and they need to be isolated. New starter, new glow plugs, different glow plugs than original, etc. It would probably help to install all the old glow plugs first and try troubleshooting that.

First thing to do is to make sure you know which kind of plug you are dealing with. Remove the glow plug wire from one of the plugs. Check the resistance from the top of the plug to the engine block. If you see very little resistance (~0.7 ohms), then you have the plugs you are familiar with.

Five glow plugs in series means that you're dropping about 2.4 volts across each glow plug. So instead of grounding to the motor as in the later models, each plug has two connections, a positive and a negative and no connection to ground through the base of the plug. If you disconnect the ground strap at the end of the series, then you will have no current flow through the plugs at all. If the new plugs have a grounded base and only one connection at the top, then they need to be wired in parallel with the ground strap removed. In the newer system this is done internal to the GP relay and a new wiring harness would be required with a wire to each individual plug.

It may work with the new plugs, but you'll definitely have to remove the ground strap. They new plugs may impose a different current requirement on the glow plug relay which may not be designed to handle the load. But this would result in burnt relay contacts, not blown GP fuse. If the plugs ground through the base to the engine, but are wired in series through the connector at the top, AND the ground strap is installed after the last plug, you will definitely blow the fuse. You'll have a direct connection from the glow plug relay to ground. Hearing a click doesn't necessarily mean that a relay connection has been made. If the relay contacts are crispy, you won't get any appreciable current flow. If you've been blowing fuses, then there's a good chance this may be what is keeping the car from starting when the ground strap was removed.

When you move the key to the "glow" position, does the fuse blow then? Or does it only blow when you engage the starter? If the fuse blows when engaging the starter then you have to isolate the two circuits and troubleshoot from there.

r.e.wing_fc3s · #8 06-12-2010, 03:31 PM

ok that clarifies a lot of things. what i don't get is how the glow plugs were originally wired. ive seen the style with the copper bus bars, where one of the bars is closer to the end and one is closer to the cylinder head i.e. positive and negative. im confused because this car doesn't have those large buss bars. nor does it seem to have glow plugs that have the inner and outer connections. the ends look just like the later pencil type with one small brass nut. which would explain why the fuse keeps blowing, since the current isn't actually passing through the plugs. it has wires with connectors that look similar to the later style parallel wired glow plugs. except wired in series. was this the wiring used after the copper buss bars but before the parallel wired pencil plugs?

i think i need to replace the bosch duraterms with some of the old style plugs, the ones that work with the buss bars. will these also work with the wiring that ive mentioned above?

micalk · #9 06-12-2010, 03:54 PM

Not likely. Either you need two connections to the plugs with a ground strap at the end (old style, serial), one wire from the relay with a connection spanning from one plug to the next and no ground strap (essentially parallel but fed with a single wire), or all parallel wires from the glow plug relay (modern system you are familiar with).

If you have the newer style plugs and the ground strap is removed (second version as described above) engage the glow plug circuit (turn key to run, not start) and immediately check the voltage at the top of one of the plugs. Place the ground connection for your meter in several places - engine block, negative battery post, chassis - and see if that makes a difference in your voltage reading. If it does then you need to check the ground strap from the engine to the chassis. This would also affect starter operation. If the voltage is the same to all ground points, and it is any lower than 10 volts, then your problem is further upstream. 10 volts may be enough, but I'd guess if it isn't starting then you will probably have much less than that. This is to say you have resistance somewhere in the circuit before the plugs that is dropping the voltage. And it doesn't have to be much resistance at these current loads to drop alot of voltage. Ideally you should have between 11 and 12 volts at the top of the plugs. You can try cleaning connections, but the first thing to check would be the gp relay contacts.

r.e.wing_fc3s · #10 06-12-2010, 04:25 PM

so is the "second version" actually factory? thats the one i have but with a ground strap at the end. the only way i can make sense of it is if someone replaced the glowplugs with the wrong ones that ground through the head. the only problem with this theory is that the resistance "bus bar" that i thought needed to be used with the old style plugs isn't on the car. it just has the wire that goes from 80a fuse on firewall to plug to plug to ground that you mention in the second version

so if i have the ground wire at the end then i automatically need to use the old style series wired plugs?

i understand about testing the g.p. voltage above for the second version, is that just to verify power getting to the glowplugs? i did check the resistance of the wiring, ill recheck the relay resistance with the relay energized.

should the second version work if the ground strap is left removed?

thanks for the patience

micalk · #11 06-12-2010, 05:10 PM

I'm an electrical engineer, and I was a tech before that so I can troubleshoot pretty good. I've only owned two MBs, an 84 123 and an 84 300SD. I have an 82 300SD, but its a parts car.

Is this your car or a friend's car? Just wondering how much of the history is personal and how much is hearsay from your friend.

First off, second version that I described had no ground strap. No idea if the second version is factory. I'm just thinking the possibilities through based on what you've described. With everything connected including the ground strap and the key to 'off', check the resistance from the first glow plug to the ground strap. If it's a dead short then disconnect the ground strap. I don't know what the resistance of the old style plugs should be but I gotta believe they are on the order of 0.05 to 0.15 ohms each (or .25 to .75 ohm for the series circuit) which would draw from 16 to 48 amps, depending.

If you have the new style plugs, remove the ground strap at the end. I haven't done any research on this, but my guess is that it should work. With the new style glow plugs there may be an addition current load that the old relay wasn't designed to handle. Would probably work fine for a while, but the contacts would fail much earlier. And the wiring may not be up to the additional load either. As wire heats up its resistance will increase which will drop more voltage meaning less voltage at the plugs. If the current is spread across several wires (modern design), there's less resistance in the circuit overall.

Checking the voltage at the glow plugs is to ensure that _sufficient_ power is getting to the glow plugs, not just that there's a connection there. I'd have to see the circuit to tell you if you can check the resistance of the relay contacts when energized. I'm sure it could be done on a bench, but maybe not so easily in the car. The best thing to do is check for voltage drops when the relay circuit is energized. Use the voltmeter to check for voltage drop across the relay contacts. If the voltage drop is less than 0.3 or 0.4 volts it's a good connection, and if it's less than 0.1 volts it's a great connection. Remember, V = I * R . If you have a 0.1 ohm resistance, but you have 20 amps running through it, you're dropping 2 volts right there. And those new style plugs will draw alot more current than 20 amps total.

raslaje · #12 06-12-2010, 05:16 PM

You can download this pdf from zSHARE.
Checking Pre-Glow System.
15-710.pdf - 0.82MB

Also the Mercedes EPC (might still be free, but they will want your credit card info) where you should be able to find the part number for your car's glow plug relay.
http://epc.startekinfo.com/epc/welcome.jsp

micalk · #13 06-12-2010, 06:03 PM

Quote:

Originally Posted by raslaje

Also the Mercedes EPC (might still be free, but they will want your credit card info) where you should be able to find the part number for your car's glow plug relay.
http://epc.startekinfo.com/epc/welcome.jsp

still doesn't work on a Mac.

Dolts.

wonder if it will work from my iPhone?

.....

.....

DOH!

micalk · #14 06-12-2010, 06:09 PM

That pdf does not describe the system that the wingman has been describing.

raslaje · #15 06-12-2010, 06:43 PM

Quote:

Originally Posted by micalk

That pdf does not describe the system that the wingman has been describing.

Yeah looks like your Mac won't work. The EPC site says this:
This site is optimized for Microsoft Internet Explorer 6 or above. Other browsers may not function properly. We do not officially support Apple Computers.

It seems like he has a modified glow plug system so I thought there might be some value from the pdf since the first schematic says up to 1980. The EPC should give him the original part number for the glow plug relay which might help him determine if his system has been modified by comparing part numbers.