loooooooong glow - PeachParts Mercedes-Benz Forum

zeke · #1 12-05-2013, 04:16 PM

I have read several threads about the glow timer, but I am still a little foggy on how the glow circuit works. This is in my 1979 with the loop style plugs. Over the past several months, I started to noticesome starter issues. As always, I glow until the light goes off, then engage the starter. Usually when the car was cold (sitting, it is rarely cold in San Antonio), when I moved the key to start,nothing would happen. I would hear a click sometimes. I could always turn the key fully to off, then straight back to start and it would start right up.

Recently, that has been less the case. Now I have to wait to start. It seems like the glow timer is remaining on for a longer time and sucking up all the amps which prevents the starter from engaging.

This morning I let it glow, then went back in the house for something...came out about 30-sec to a min later and it started right up.

My alternator is strong (~14V running) and the battery floats at about 13.1V when the car is off.

Where should I start?

jay_bob · #2 12-05-2013, 04:56 PM

The first thing I would suggest is upgrading to the parallel style plugs. Unless you are trying to keep everything original, then I would suggest new glow plugs.

Remember the series circuit is truly only as strong as the weakest link.

Heat = current squared times resistance.
Series plugs all have the same current flow through them per Kirchhoff's current law
Current = voltage divided by resistance

So if you have one weak plug, they all suffer, since if the resistance of one plug goes up, it reduces the current flow through that plug, which in turn limits the current to all the plugs, and so none of them can make their optimal amount of heat.

zeke · #3 12-05-2013, 05:09 PM

I am trying to keep it original, but would consider the upgrade if I can't sort it out easily.

So are you saying you suspect that a weak GP is slowing down the heating and increasing the length of the glow? Like i said I still don't really understand how the glow timer works... Some say it is temperature dependent, others say it is a relay/timer.

uberwasser · #4 12-05-2013, 05:28 PM

My understanding is that it's a two part thing. One is that indeed there is temperature measurement (either with a temp sensor or by measuring resistance, not sure off the top of my head) that informs the light when to turn off. This is to indicate when the system has measured that the glow system has heated enough to properly aid a start. This is why when you cold start the car the glow light stays on much longer than when you are starting it when already warm.

However, the system also has a timer built in. After the glow plug light goes out, your glow plugs are still lit. They will stay lit until you start the car, or until the timer goes off. The timer is to prevent burning up the plugs or other components by leaving constant power flowing.

So it could definitely be that a poorly operating plug is causing the whole system to work less effectively and so your light is staying on.

It's a good time to mention that, for reasons outside of my current understanding, the conversion to parallel style plugs (which I've done) retains the temperature feature and so the light turns off appropriately, but the timer in the relay stops working. Some people swap out relays; I just make sure not to leave the key on.

Before I realized this feature was disabled, I did leave the key on while testing window regulators and ended up burning up my fuse box. So I either need to disconnect the first glow plug every time I need to leave the car's 12v system going to test stuff, or maybe I'll wire a switch in line to cut off the circuit at will. Anyway, we'll see.

funola · #5 12-05-2013, 05:35 PM

Easiest way to observe what your glow relay is doing is hook a light bulb across the glow plugs and watch. On my 85 the bulb (glow plugs glow) lights when I turn the key to on, stays lit while I crank the starter, light goes off as I release the starter.

zeke · #6 12-05-2013, 05:44 PM

Quote:

Originally Posted by funola

Easiest way to observe what your glow relay is doing is hook a light bulb across the glow plugs and watch. On my 85 the bulb (glow plugs glow) lights when I turn the key to on, stays lit while I crank the starter, light goes off as I release the starter.

So it seems likely that one of my plugs is sucking excessive amps and preventing the starter from engaging (since the starter is supposed to spin with power to the GP). Will check the resistance of each plug this weekend. brrr.

edit - should say AT LEAST one of my plugs.....

funola · #7 12-05-2013, 05:56 PM

Quote:

Originally Posted by zeke

So it seems likely that one of my plugs is sucking excessive amps and preventing the starter from engaging (since the starter is supposed to spin with power to the GP). Will check the resistance of each plug this weekend. brrr.

edit - should say AT LEAST one of my plugs.....

No. Your plugs are in series. The plugs resistance are added together. One plug cannot pull the system down.

zeke · #8 12-05-2013, 06:11 PM

funola, so why is the system drawing excessive current? I thought the reason people upgrade is that one plug in the loop style can take the system down?

yvairguy · #9 12-05-2013, 06:18 PM

because a problems with one plug or one connection can take down the whole glow plug circuit.

funola · #10 12-05-2013, 07:35 PM

Quote:

Originally Posted by zeke

funola, so why is the system drawing excessive current? I thought the reason people upgrade is that one plug in the loop style can take the system down?

How do you know the system is drawing excessive current? What tests have you done to reach that conclusion?

Left Coast · #11 12-05-2013, 07:36 PM

Quote:

Originally Posted by zeke

So it seems likely that one of my plugs is sucking excessive amps and preventing the starter from engaging (since the starter is supposed to spin with power to the GP). Will check the resistance of each plug this weekend. brrr.

edit - should say AT LEAST one of my plugs.....

No, this is incorrect. Re-read Jay Bob's post. One glow plug cannot suck excessive amps. If the resistance in a single glow plug increases, then the current through all of the glow plugs is reduced. If the resistance decreases, then you have a short in the plug, and therefore none of the glow plugs will heat-and you would likely blow the fuse.

You have two separate issues:

1- you perceive that your glow plugs are taking longer to get the prechambers up to starting temp. This could be the result of insufficient amperage conducted through the circuit. The quick cure for that is new glow plugs, but at least check the connections first to see if one is loose. If you value your time, then replace them all at once. The process for checking this type of glow plug system involves reading voltages across the various connections between the plugs with the circuit closed.

2- your starter will intermittently not turn. If you are certain that the battery is good, then the problem is either the cables, the connections, the starter switch, or the starter (or solenoid).

Keep these two issues separate, because they are not related.

cooljjay · #12 12-05-2013, 08:10 PM

Sounds like you need a new starter.....on this year car, Mercedes used a low power starter on cars shipped to warmed climates....this starter has since been discontinued....a starter on a 240 is easy to change, get a 300d starter....and you will love how fast it kicks over....

I see nothing wrong with your glows, except you would benefit from upgrading them....

Now I am judge my diagnosis on, all your ground connections being clean....battery to chassis and trans to chassis.....your battery cables have been replaced.....and a valve adjustment has been done....

A bad starter will also kill a battery....

Also an FYI....I have upgraded plugs in my car, the longest my system stays on is 30sec even if I don't turn the key and start it.....I verified this buy trying to measure the voltage at my plugs....the time I got the volt meter hooked up...they were off....finally I rigid it up on the windshield and was able to see it come on and go off....

vstech · #13 12-05-2013, 08:25 PM

if resistance goes up, VOLTAGE goes down... not necessarily current/amps. a high resistance in one plug, could result in increased current, but it's unlikely, most likely what's happening is you have a weak battery, and the glow plugs are operating as best as they can.
what's the voltage at the battery CABLES when you have the key off. you stated a float voltage of 13.1, but turn the lights on for 10 seconds, and retest. I bet it's around 12.5v, then turn on the GP's... I bet it drops to around 8v... nowhere near enough to spin over a cold diesel.
test again at the actual battery terminals... the center of the posts wile digging in the leads of the meter... see if the voltage stays around 12v with the key on.

Quote:

Originally Posted by Left Coast

No, this is incorrect. Re-read Jay Bob's post. One glow plug cannot suck excessive amps. If the resistance in a single glow plug increases, then the current through all of the glow plugs is reduced. If the resistance decreases, then you have a short in the plug, and therefore none of the glow plugs will heat-and you would likely blow the fuse.

You have two separate issues:

1- you perceive that your glow plugs are taking longer to get the prechambers up to starting temp. This could be the result of insufficient amperage conducted through the circuit. The quick cure for that is new glow plugs, but at least check the connections first to see if one is loose. If you value your time, then replace them all at once. The process for checking this type of glow plug system involves reading voltages across the various connections between the plugs with the circuit closed.

2- your starter will intermittently not turn. If you are certain that the battery is good, then the problem is either the cables, the connections, the starter switch, or the starter (or solenoid).

Keep these two issues separate, because they are not related.

funola · #14 12-05-2013, 08:33 PM

Quote:

Originally Posted by vstech

if resistance goes up, VOLTAGE goes down... not necessarily current/amps. a high resistance in one plug, could result in increased current........ .

This is incorrect or there are typos.

vstech · #15 12-05-2013, 08:39 PM

Hmm. I'm sure electrically, you're right, but a defective plug can cause a fuse to blow... or for it to draw excessive amps running down the battery if the battery is weak...
the series plug system is annoying. a plug can short dead, can fail internally, resulting in higher voltage to the rest of the plugs that are only supposed to see 2v, not 3 or 6 etc... this would draw MORE amps, and could cause a marginal battery to prevent a starter from engaging.