Needs some ideas on 603 odd voltage drop
 

I posted about this yesterday, didn't get much help and did some more testing last night.

Basically the problem is when I turn the key to "On" initially I notice a substantial voltage drop (battery goes from 12.4 to 11.4), turn the key off and back on again and the voltage does not drop as low (high 11s). The drop is substantial enough on the first key turn that it seems to cause the GP relay to NOT give power to the glow plugs, hence no glow plug light.

Tested battery at Autozone and Napa Auto, both said it was fine, as was the alternator.

Tested the relay for power, tested that all the GPs are getting power (they are brand new), tested the GP harness for continuity. I really don't know what to do and am looking for somebody who understands the 603 GP system and can point me in the right direction.

Have you got a battery charger?

Don't "good" batteries usually have a little more voltage than that on them most of the time? My glowplugs on my 603 just pull it down to low 12.x usually. I'll check later.

This is how it went down this morning, engine cold after sitting overnight, outside temp was in the low 60's...

Key not on, battery voltage at 12.4
Turn key on, battery voltage dropped to 11.4 and no GP light
Turn key off, battery voltage goes back up to 12.4
Turn key on, batter voltage dropped to 11.6 and GP light comes on, car starts perfectly (better than it did before I replaced the GPs).

No I don't have a battery charger, maybe I should get one.

Normally, a charged up battery that's in good shape should read 12.6, at least, at rest. Of course as they get a little older that might drop - but a new battery, fully charged, should read AT LEAST 12.6... but IDK how much of a difference that 0.2v makes, I wouldn't think too much but I'm not an electrical person.

12.4 is a normal battery voltage without a load.

11.4 is a reasonable battery voltage with an 80 amp load from the plugs.

11.6 is also a reasonable battery voltage. Since the plugs are already warm, the resistance is higher and the current draw is less..........hence the higher battery voltage.

Your only issue is the light. If you are positive that all plugs are getting 12V (or 11.6 or so) then the light is not illuminating for an unknown reason. The plugs get voltage but the light does not illuminate. This normally signals a bad glow plug or plugs, but, with all replaced........it's not likely.

What other electricals do you have coming on line after turning the key? A killer stereo with a big amp, or the HVAC system, or if you are using WVO, with an electic heater/fuel pump, etc?

After some thought there is nothing wrong with your car than probably a defective glow relay. When you first turn the key even though the glow light does not activate the plugs are on.
The reason the voltage drop is a little greater is the colder the plug the more initial current surge is occuring. When you turn the key the second time the light comes on and the current draw is a little lower because the plugs have warmed a little from the first partial glow cycle.
You had me going for a minute or two with your description. Although logic said other than a shorted alternator this could not happen. The alternator would not function normally a minute later after starting. . All other circuits are moderatly fused and I kind of eliminated the starter as well.
Had there been something funny in the key switch area you would have smelt the wires burning at those large current draw levels. I have seen glow relays act exactly as yours is right now on older 123s. It is almost as if the glow plug bulb latching circuit will not activate with the first key try. Yet it does the second try. Some take three or four tries.
I would grab another relay from an auto salvage place or borrow one for a test to verify it first as they are not too cheap new. This should stop all your concern at present as well. If you are still uncertain just monitor the plug voltage when you turn the key the first time. I believe it will be there. Where else really could a current of this magnitude be going with out any other symptoms? It's just elementary Watson. :D If it is not as I sugest just do not pay my bill.:D Yet still post the answer so other people can draw and quarter me if wrong. :eek: Too cool to be fooling around out there anyways in most north american locations right now. :)

Thanks for the thought and ideas, I'll grab a relay and see how it works out.

Well sure enough, the GPs are getting power even when my light isn't on, just like you said. Looking into a relay, the local wrecker is asking $75 for a used one, trying to find a better price...

Thanks again.

I would still try to borrow one for a test. I am only about 90 percent sure it is the problem. You can also live with it for a short time while you locate a cheaper one like off ebay . Or perhaps another auto wrecker. Also if you take note certain members always seem to be sourcing used parts from pick and pulls reasonably for members. I think our members are good guys to deal with.
I see your problem was actually defined twice by the way. Brian had thought about it as well earlier and posted. I missed his post while skipping down the thread earlier but not a few minutes ago.