1991 M998 starting issues that I just cannot figure out Please help!!!

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MarkM

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The story and sorry for the length. Water got into my control box as the glow would flash now and then driving along and sometime the glow light would be dim while driving. So I pull the control box and open it to find a little water in it. I replaced the control box, Installed new glow plugs most were dead (open).

When trying to start the truck put the key switch in run and the starter would crank (switch in run not start) and no glow light. New switch is wired correctly. Swapped out the key switch for a new M998 reg switch and same issue. The engine harness was not in good shape so picked up a new harness and today it was installed.

Same issue the starter engages on run mode. Disconnected the starter trigger wire just to test and when the key switch is put to run the glow light does not come on. I even replaced the glow plug unit that screws into the coolant port. I am at my wits end on this. Any help would be greatly appreciated. Very frustrating.

Regards

Mark
 
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NDT

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Sounds like the wires to the ignition switch are not in the right place.
 

MarkM

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Thanks for the fast response but I did check that and they are correct. I replace the cheap key switch for the real lever switch hoping that was the problem. Power is being back fed somewhere it shouldn't. I am wondering if I may have a problem up at the alternator as a lot of wires head up to it. Just at a loss as to where I go next.

Mark
 

NDT

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Where did you disconnect the starter trigger wire, at the starter or at the ignition switch? When you did this, did the instruments come on in “run” position?
 

MarkM

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I disconnected the wire at the starter and no the gauges do not work on run. No movement at all on the amp meter even when starter was cranking.

Mark
 

MarkM

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With the stater wire disconnected and switched moved to start you can hear a solenoid clicking under the dash but thats all. No gauge movement. With the starter wire hooked up and switch moved to run the starter cranks and no movement on gauges and no wait light. When switch movered to start the starter cranks but no movement of gauges and wait light flashes. Strange huh?

Mark
 

NDT

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Need you to triple check the connection of the wires to the ignition switch.
 

Retiredwarhorses

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You said you replaced your start box?
With exactly what brand and type?
New or used box?
Battery disconnected during swap yes or No?
 

MarkM

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Yes the batteries were disconnected each time anything was done. The start box was replaced with same model number (used take out) and I had a backup start box that I installed just in case I had a bad one and same issue. Thank you for the help.

Mark

Below is a pic on the boxes. Top pic is the one that has been in the truck for years and the one that got wet. Bottom on is the take out replacement unit and I have a spare one of those.

20190801_194108.jpg
20190801_172137.jpg
 

911joeblow

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So we have seen the connector (under the hood to the control box) have poor continuity and cause these issues you describe. Last truck we had in the owner had used dielectric grease inside the connector (worried about water getting in when he was fording) and that caused nearly identical issues as you describe. We used electronics contact cleaner and slightly bent the pins to make a better connection and problem solved. I would check that.
 

MarkM

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I installed a new glow plug controller that came off the same take out truck so they are a matched set. Original glow plug controller when installed had the same results so everything matches.

Mark
 

MarkM

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When we installed the new harness it had the new connector for the box. Same issue still there. Thank you for the help all good stuff. The relays under the dash are working Two of them. I will try to clean the contacts for the **** of it but have had two different known good boxes in there with the same result.

Mark



So we have seen the connector (under the hood to the control box) have poor continuity and cause these issues you describe. Last truck we had in the owner had used dielectric grease inside the connector (worried about water getting in when he was fording) and that caused nearly identical issues as you describe. We used electronics contact cleaner and slightly bent the pins to make a better connection and problem solved. I would check that.
 
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papakb

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Run power comes from the Master Start Switch on 29A and splits into 29B and 29C. 29B goes thru circuit breaker CB2 and supplies power directly to the instrument cluster, the wiper motor, and the Wait to Start LED so if your never seeing any of these things the circuit breaker (which should auto reset) is probably bad. The Wait to Start LED gets it's ground thru body connector pin G on circuit 571A.

Run power into the control box comes in on body connector pin B so if there's no power to the control box nothings going to work.

The Start signal from the Master Start Switch comes into the control box on body connector pin A and moves thru the logic in the box to tell the starter to turn with a signal to the starter motor on circuit 74A from pin I on the engine connector. You can check pin S on the STE/ICE connector to see if the start signal is getting to the starter motor.

If your starter motor starts turning as soon as you put the key/switch into the run position you either have a short in a harness or bad control boxes.

Keep in mind that the glow plugs don't cycle if the engine or the outside air temp is warm. I realize this isn't a problem your having in Ma right now!
 
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MarkM

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Great info thank you very much.

Mark

Run power comes from the Master Start Switch on 29A and splits into 29B and 29C. 29B goes thru circuit breaker CB2 and supplies power directly to the instrument cluster, the wiper motor, and the Wait to Start LED so if your never seeing any of these things the circuit breaker (which should auto reset) is probably bad. The Wait to Start LED gets it's ground thru body connector pin G on circuit 571A.

Run power into the control box comes in on body connector pin B so if there's no power to the control box nothings going to work.

The Start signal from the Master Start Switch comes into the control box on body connector pin A and moves thru the logic in the box to tell the starter to turn with a signal to the starter motor on circuit 74A from pin I on the engine connector. You can check pin S on the STE/ICE connector to see if the start signal is getting to the starter motor.

If your starter motor starts turning as soon as you put the key/switch into the run position you either have a short in a harness or bad control boxes.

Keep in mind that the glow plugs don't cycle if the engine or the outside air temp is warm. I realize this isn't a problem your having in Ma right now!
 
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