voltage drop across resistor?

[rodman68]

I replaced all eight glow plugs from the wellmans to ac 60g four of the wellmans were open. Definately helped it starting but when it didnt start very good the other day. Checked all 60g all ok during glow plug cycle had 9.0 volts on the output of the relay. Next day I checked the input to the relay 9.0 volts also so the relay seems ok. Next day checked the bus bar terminal that goes to the resistor and had 23.6 volts. So I am dropping 14.6vdc across the resistor that seems excessive, what I understand is I should have 10-13vdc at the glow plugs. Tomorrow I will check on the glow plug also to see what voltage I have there. So is this voltage drop a little on the high side?

 

[rodman68]

I am doing this testing every day to make sure the truck is cold.

 

[richingalveston]

check your batteries and the connections to them and to the buss bar. your input voltage should be higher than 24 volts. I think around 24.8 should be normal. If you don't have the right amount going into the resistor you wont have the right amount coming out.
make sure your batteries are load tested and fully charged then perform test. your resistor may be going bad also but you need to start with the batteries and make sure you have propper input voltage to the resistor.

 

[rodman68]

Should have mentioned all voltages were taken with glow plugs cycling, So I had 24.9vdc then went to 23.6vdc with the glow plugs on so only a 1.3vdc drop from the batteries under the glow plug load. Next time I will check it at the 24vdc bus bar with glow plugs on and also when the starter is turning. I dont have a battery load tester at home so trying use the VOM at this time.

 

[alburms311]

This doesn't answer your question, but have you considered bypassing the resistor all together? This will save your GPs from burning out due to over-voltage should one fail. If you do this mod, I would also suggest replacing the GP relay while your at it. Just my

 

[Mark R]

If your resistors are going bad won't they let too much voltage thru? Is there a good source for new resistors? I had 24.5 volts at my relay and I think I fried my controller board. The fuse block took some heat too. Maybe I should convert to 12 volts.

 

[alburms311]

While I can't answer that question with certainty, I would think it could go bad either way, too much or too little. Its also possible the relay itself is bad. The best bet in my opinion is to not bother with replacing the resistor, they can be had with a quick search, but rather just by pass it and update the relay while your there. Unless you plan on being jumped through your front slave receptacle, from what I gather, you don't need it. It's a simple mod and insurance for down the road.

 

[richingalveston]

typical from what i have seen the resistor stops passing power when it goes bad because the internals burn in half and stop passing current. I supose it could go bad and not cut the current like it is supposed to. however, when no load is present, you will get the full 24 volts to the relay. In order for your controler to get over voltage something else had to go wrong. The resistor only cuts power to 12volt when it is fully loaded so if you have 4 of the 8 glowplugs not working then you will get a voltage half way between 12 and 24 volts. The fewer the working glow plugs, the higher the voltage that passes through the resistor and your relay. The only thing connected to the relay should be the glow plugs and the over voltage should not affect the controler card.

 

[richingalveston]

i have two resistors that were good when removed, PM me if you want one.

 

[rodman68]

Yes I am thinking of removing the resistor pack that will need to upsize the wire to the 12 volt terminal.. I am thinking at least a 6 gauge but a 4 would be better not sure how I am going to fuse it though. Larger the wire the less voltage drop across it in turn hotter plug temps. So does the controller know the difference between say 9 volts and say 11 volts in the duration it cycles the glow plugs?

 

[richingalveston]

the controler card does not know the difference, it is connected to the 12 volt side of the relay. It only cycles the relay just like the ignition switch does. It konws the difference in timing by the glow plug temp sensor. The card will cycle the relay until it reaches the propper temp. When it is very cold, the contoler card will perform an afterglow cycle after the engine has started. This is why you sometimes hear the relay cycle after the engine has started. no real need to upsize the wire, it is big enough already. you can use the same fusable link that should be located between the resistor and the relay. just take it off of the resistor and put a new terminal on it and connect it to the 12v terminals next to the glow plug relay. there is a sticky in the cucv forum for this procedure.

 

[MarcusOReallyus]

While I can't answer that question with certainty, I would think it could go bad either way, too much or too little.

typical from what i have seen the resistor stops passing power when it goes bad because the internals burn in half and stop passing current. I supose it could go bad and not cut the current like it is supposed to.

Yep, you're both right. The most common failure mode is for a resistor to open, thus passing NO current. However, once in a while, they will short.

 

[rodman68]

So in this whole process of troubleshooting it looks like I dont have after glow anymore also. So I need to see if my glow plug temp switch is working actually its not really a switch is it more of a variable resistor. It use to work so not sure if the only thing wrong is the resistor.

 

[rodman68]

If I was the only driver I would remove controller install push button with a analog volt meter that is hooked to the output of the glow plug relay. This way I can monitor the voltage going to the glow plugs.

 

[rodman68]

Also when the engine is warm it will cycle the glow plugs it has done that since i got it thought it was suppose to do that but I found out its not.

 

[richingalveston]

yes, if it does not recognize the temp sensor it will suposedly have a fail safe in it to auto cycle the plugs. the sensor suposedly has contacts that close or opens not sure which one once it reaches 130 degrees. It is not a variable resistance sensor
it should only after glow if it is really cold. under normal conditions it should not come back on once the engine is warm

 

[rodman68]

I will try and find more info in the tm but sometimes IMO its a little difficult. Thought it was a variable resistor due to the tm says check for 600 ohms cold if its open check wiring and the glow plug switch. Unless its 600 ohms during cold and when hot it closes the switch, the tms dont tell what it should be when hot.