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Quick Glow Plug Relay/Controller Question

Discussion in '6.9L IH & 7.3L IDI Diesels' started by thevanbulance, Mar 22, 2020.

  1. thevanbulance

    thevanbulance Registered User

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    Hi guys,

    I'm having some cold start issues on my ambulance (1990 e350 7.3 idi na) and in the process of tracking down the issue I've come across a question I haven't been able to find the answer to in any threads here/Google searches.

    When the ignition is turned to starting position and my "wait to start" lamp is illuminated:

    How much voltage should I see coming out of the base of the bendy metal connection point that goes out to the glow plugs from the glow plug controller? This is the connection point with two brown wires going into the wiring harness and powers the glow plugs (red circle in the pic for reference). I'm reading between 0.5 and 1.0 volts and it just doesn't seem right.

    To verify I'm testing correctly: with the ignition switch to on and while my "wait to start" light is active I'm putting the black terminal of my voltage meter to the ground (white circle in photo) and the red terminal on the point mentioned above (red circle in photo).

    I've checked everything else on the list from this site and it all is working appropriately: https://www.dieselhub.com/maintenance/idi-glow-plug-troubleshooting.html

    IMG_20200322_142201.jpg
     
  2. MtnHaul

    MtnHaul Supporting Member Supporting Member

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    I don't know the answer to your specific question but I did find some info that says voltage at glow plugs should be 9-10 volts.
    https://www.thedieselstop.com/threads/voltage-at-glow-plugs.155128/

    If voltage is supposed to be ~9volts at GPs then I would assume the Z-bar metal thingy(?) should also read ~9volts.

    Are you hearing the Gp relay click on and off rapidly when you turn the key?
     
  3. thevanbulance

    thevanbulance Registered User

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    Thanks for the reply and helpful link! I'm not hearing the relay cycle on and off but it does audibly click on when I first turn the key. I have a new one on order to see if this is the issue or not. Pretty the unit I've got in mine is original so no harm in installing a new one is my way of thinking :)
     
  4. Slicknik

    Slicknik Full Access Member

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    @thevanbulance


    Just Incase you wanted to investigate a little before installing a new relay

    1. find the “hot side “ of your glow plug relay, should have 12 volts (yellow two wires in photo)

    2. after turning the key to the wait to start position , does the side opposite of the hot lead (stud connected to the metal squiggly band ) get 12 volts ?

    3. if you Don’t get 12 volts then your relay is failing , if you do get 12 volts then you need to test your glow plugs individually and the leads at the end of the GP harness and see if your getting 12 volts were the harness attaches to the glow plugs

    4. Could also have a bad ground , so also ohm out your relay as well to rule that out , you could do that to the glow plugs as well

    DB0D1AEC-F370-405F-A061-6D6EDEA89151.jpeg
     
  5. Exhumis

    Exhumis Full Access Member

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    @thevanbulance :
    The link you posted to the diesel hub, check the paragraph below the diagram of the relay, if I recall correctly it tells you what voltages you should be seeing at the relay. I generally get 12ish at the constant hot post and 11.7ish at the bus bar /switched post. I get 10ish volts from each gp connector.
     
  6. aggiediesel01

    aggiediesel01 Full Access Member

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    You are correct, it's not right; you should have as much voltage coming out at that point as going into the solenoid on the heavy wires. The bendy metal connection is called a Shunt. It is used by the controller to measure the current being delivered to the glowplugs. The resistance/length is very specific and the length is very specific and from those known values a fairly accurate current can be calculated for the controller to use. Unfortunately this information isn't able to be viewed outside of the controller unless one were to connect a calibrated (to the shunt value) ammeter to the top and bottom of the shunt just like the controller uses. Now to your question: The voltage at the bottom of the shunt should be identical to the voltage coming into the system as it's under load. If you can measure 12V at the incoming wire (heavy dual yellow) then you should be able to measure 12V at the output terminal where the two brown wires are. HOWEVER; several parts of the overall system can and do affect how much voltage you will measure at the incoming terminal:

    1. The condition of the voltage supply (batteries). When the glowplugs are energized in the system they are a large load and the battery voltage will drop depending on how good the batteries are, good batteries are able to deliver more current to the load resulting in less of a voltage drop. Bad batteries could measure 12V+ with no load, and when a load is applied can squat below 9V.

    2 Condition of the multiple connections between the battery and the glowplug controller. Dirty/corroded/overheated terminals/connections add resistance to power flow path and increase the voltage drop to the GP system. The more corrosion and overheated connections are between the battery and the GP controller the lower the voltage to start with at the GP Controller. This leads to problems at and after the GP controller. If you know/remember V=IR (ohm's law) and resistance of your R (load, i.e. glowplugs) stays the same and V starts dropping (b/c of bad connections upstream) then I (current) has to go up to keep the equation balanced. B/C the current goes up, as V drops, you see increased heating and damage of wiring feeding the controller and downstream of the controller (this is why the nuts on top of your solenoid are rusty, they've been over heated and cooked off the cad plating leading to corrosion and more overheating).

    As I recall, there is a heavy wire coming from battery + to the starter solenoid, then a heavy two wires (on one ring terminal) headed to the engine harness connector ('87-'91)('92-'94 have a heavy red wire going directly to the GP Controller), then through the engine harness connector on to the GP Controller. All those connections need to be super clean and tight and protected with anti corrosion grease or spray to minimize issues at and beyond the controller. I would also suggest that for '91 and older trucks that the engine harness connector be bypassed like the newer trucks. If you haven't had the connector melt yet, it's coming your way after all these years; head it off sooner than having to rebuild the connector and deal with damage to other circuits in the connector.

    3. Resistance through the GP solenoid at the controller. Ideally there is no resistance through the solenoid but over time and use and overheating (due to above) the resistance of the connections through the solenoid goes up. Eventually the solenoid only passes almost nothing and has failed. This is likely your condition but the overheating and failure is likely a result of bad connections upstream. Since you are measuring .5V at the output of the terminal, you need to compare that to the voltage of the input terminal. If it's more than .2-.5V difference I would suspect that the solenoid is on it's way out. I believe the solenoid can be replaced separate from the controller if you think the controller is still good.

    I can't really see what the black wire is connected to but if you can take a resistance reading from that point back to the neg terminal of the battery you should see less than a half an ohm. If you see that or more, then you need to check the ground cables/connections from your block to the chassis and batteries to ensure that the return path can handle the current of the GP system as well.
     
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  7. thevanbulance

    thevanbulance Registered User

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    Want to close this off and say after installing a new relay everything is peachy! I've not had her starting this good since the day I brought her home :) I really appreciate your additional responses and help aggiediesel01, exhumis and slicknik.

    Now it's on to sorting out a sticky RABS valve but we'll save that for another thread haha
     
    IDIBRONCO, DaveBen and Scotty4 like this.

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