GP controller question - 84 300CD
 

My 300cd started getting a little hard to fire up in the morning a few days ago. It would start, but run really rough and puff white diesel-smelling smoke. Seemed like it was mis-firing on a couple cylinders, but after for 15-30 seconds, it would smooth out and be just fine. On warm restarts there were no symptoms or issues.

I figured it was a couple of GPs gone bad, not a big deal, they're 2-1/2 years old and probably due for replacement anyway. THEN suddenly my GP "wait" light on the dash quit lighting up and the car won't start. Gets really close to starting, just won't quite get there.

I know how to check the GPs, that's easy, but here are my questions:

1) If ALL the GPs went bad, would the dash light quit coming on? Or will it still light up normally?
2) Where is the GP controller located?
3) I presume there is a fuse for the controller in the regular fuse panel?

W haven't had a chance to start troubleshooting it yet, so any up-front advice,and help that will speed up the process will be appreciated...

Depends on the nature of the failures.

Black box on inside left fender, behind headlight.

No. There is an 80 amp strip fuse in the controller box. Very apparent when the lid is removed.

Probably a couple of bad glow plugs. I think your controller will extinguish the light on the dash with that senario.

If you own a meter or test light just read for voltage on the plugs during the initial glow plug warm up sequence. Or watch your interior light for initial dimming showing the glow circuit controller is drawing quite a bit of current to service the remaining good glow plugs.

The GP light is designed not to light if one or more of the GP's is bad. If the light does not come on and you crank the car for a long time and no start, examine the 80 amp fusible link. Mine went one summer and I could still start the car with a long cranking session- until the car sat overnight.
If the gp light does not come on and the car starts as normal, it might be a loose connection in the wiring- right now mine works sometimes and sometimes it does not.

Turn the key to start the GP's in a quiet spot. You should har the GP circut turn off after 25-35 seconds (it depends on how warm it is) If you do not hear a click, then there is an issue.

That is not quite correct. What the fault detection circuit is "designed" to do is measure the current balance between the #1 plug and the remaining 3 or 4 plugs. If there is no significant imbalance, the light will illuminate. Depending on the nature of the failure(s), the light may or may not "tell the truth." The light is definitely not guaranteed to work properly with a single plug failure, unless the failed plug is #1.

Good info guys. Just from what you are all saying and the hard starting I was experienIcing before the light stopped coming on (it had never failed or been intermittent before) have me convinced that I have a couple of dead GPs. I'd about bet I had 1 or 2 failed before the light stopped coming on, and now at least 1 more went - causing the light to stop working.

What did the last couple of days to overcome the no-start condition is unplug the GP harness at the connector on the inner DS fender and leave it unplugged. To cold start it I turn the key to the run position and open the hood. Next I flip open the little black connector cover on the PS fender (to get access to the terminals) and use a pair of plier tips to jump from one of the big "hot" terminal screws to the little terminal (which is connected to the starter solenoid). This starts the engine cranking and once its cranking over then I give it a little "whiff" of ether (about 1/4 of a second blast from the spray can is more than enough) and she fires right up with hardly a clatter.

Fortunately it starts great if I have to shut if off again - even if its only been running a few minutes. All it takes is for it even begin to warm up and it can then be shut off and will restart with no problem. Obviously I don't want to do the ether thing any more than I have to and thankfully I don't need to unless it sits more than 6-8 hours - long enough for the engine to be dead cold.

I'll put the Ohm meter on the GPs to see which ones are toast, and report back. Are there any GPs out there that are better and last longer than the Bosch units. These are only 2-1/2 years old and I'd like to see them last a little longer than that.

Thanks again...

The Strip Fuse can sometimes crack; take a good look at it. Also 2 members have had the Strip Fuse screws become loose.

My guess would be the strip fuse, especially with the sudden onset of the problem. The crack may not be apparent until you try to remove the fuse.

The above is actually what happend to me. I did a quick check of the Fuse on a bright Sunlit day. I opened the hood which shaded everthing. The Fuse was not burned so I so I put the Relay Cover on and Ohmed out the Glow Plugs that turned out to be OK.

The symptoms persisted and the I decided to recheck. But, this time it was it was just starting to get a little dark. I had the Key on the Glow Cycle and was polking around with the Volt Meter and a I put the probe on the Fuse and got a spark in the middle of the Fuse.
I went and got my strongest Flash Light and finally saw the crack.

It took me several days to find out where I put my only spar Strip Fuse.
In the below pic is how I fixed the cracked Fuse so that I could use the Glow Plugs.

Just thought I'd post up what I found. Put the Ohm meter on all 5 plugs - open circuit on every one of them.

So, anybody got any recommendations on a better plug than the standard Bosch units?

Bosch and Beru are good, Champion(s) are not.

I also just changed out all my GP's to bosch less than 3 months ago, and I am getting no light, and very minor issues starting... I'm guessing #1 went out. kinda disappointing from Bosch...

The best place for the spare strip fuse is in the controller. Just leave one terminal not connected.

I must say that is unusual. I'd start looking at the controller operation. Put a test light on a glow plug run it out the hood and watch it. Does the light stay on after the engine start, does it come on while you drive? Also check the alternator charge voltage. Do you have a lot of carbon in the glow plug holes?

Sometimes the detection circuit for the light is flakey. If you have a 10A or greater battery charger with a meter, an easy way to test glow plugs is disconnect the plug to the controller and apply power to each glow plug (not necessary to remove from head). If you get 8-10 A or so the plug is good.