Must be connected to start connections?

Mountainman · **Posted:** Sun Dec 08, 2019 10:33 pm

Trying to figure out the minimum sensors and such that need to be connected under the hood. Most are obvious like cam and crank sensors. But, the engine won't start if the temp probe on the thermostat isn't connected, and the same goes for the little plug on the alternator. Anything else that might cause a no start condition?
Won't start without:
-coolant temp sensor
-alternator plug

I've got one I just put back together after PO took almost all of the wires out from under the hood, and naturally something isnt happy, and it does barely fire (#4 injector wire not swapped) then the ecu stops the starter. I'm missing something here, only cranks like two seconds

WWDiesel · **Posted:** Mon Dec 09, 2019 3:59 am

Mountainman wrote:

Trying to figure out the minimum sensors and such that need to be connected under the hood. Most are obvious like cam and crank sensors. But, the engine won't start if the temp probe on the thermostat isn't connected, and the same goes for the little plug on the alternator. Anything else that might cause a no start condition?
Won't start without:
-coolant temp sensor
-alternator plug

I've got one I just put back together after PO took almost all of the wires out from under the hood, and naturally something isnt happy, and it does barely fire (#4 injector wire not swapped) then the ecu stops the starter. I'm missing something here, only cranks like two seconds

Just off the cuff:
Map sensor
two sensors on the fuel rail
two sensors on the injection pump
air box sensor

My list:

Fuel Pressure Solenoid - at end of fuel rail
Fuel Rail Pressure Sensor - on side of fuel rail
Fuel Quality Solenoid - on back of CP3
Cascade/Overflow Valve - on side of CP3

Crankshaft Sensor
Cam Position Sensor
IAP sensor (Mercedes sensor) side of air box
Map Sensor
Coolant temperature sensor

flash7210 · **Posted:** Mon Dec 09, 2019 9:21 am

Here is my experience diagnosing various sensor and electrical faults.

Engine cranks but wont start:
Crank sensor
Cam sensor
Fuel rail pressure solenoid
MAP sensor
Any short to ground on the 5 volt buss. This includes pretty much all the engine sensors. (discovered
this when my MAF sensor harness was just flopping around causing intermittent sputtering and stalling)

Engine cranks, starts, but runs poorly:
Fuel rail pressure sensor
Fuel quantity solenoid
Air box pressure sensor (a.k.a Mercedes sensor)

Others:
The small connector on the alternator has NO EFFECT on engine starting, running, or performance. In fact,
in most cases the alternator will still continue to charge and operate normally.
The coolant temperature sensor- AFAIK the engine will start and run but I'm not sure how it will effect
overall performance.

Quote:

I'm missing something here, only cranks like two seconds

First thing I would check is the battery.
Next would be the starter itself. You can use a jumper wire to bypass the starter relay and the starter
should just crank and crank and crank.
Next would be the SKIM. Could be a bad key or bad module/key sensor ring.
Could also be a bad ignition switch on the column.

Mountainman · **Posted:** Mon Dec 09, 2019 12:54 pm

Thanks Flash.
A couple more details I shouldn't have left out.
It was cranking 8 seconds and slightly firing before I plugged in the pressure sensor.
And, it doesn't turn the oil pressure light even after a few 8 second cranks.
I'm thinking its the oil pressure circuit. Not worried about the oil pressure actually being low as I checked the pump when I had it out.
At least in my GDE tuned jeep the oil pressure turns off while just cranking a few seconds.
Not getting any useful fault codes yet.

Edit: Oil pressure isnt the cause of the 2 second crank, as it had that even with the 8 second crank

Mountainman · **Posted:** Mon Dec 09, 2019 12:55 pm

Going to unplug the Mercedes and see if it goes back to cranking normally.

Mountainman · **Posted:** Mon Dec 09, 2019 1:00 pm

The slight fire makes me think the fuel filter might be clogged. Any thoughts on barely firing? I so have all CAC hoses off and the turbo vacuum unhooked. Maybe its mad that the MAF isnt seeing any flow?
I usually dont leave this much disconnected by I'm doing a quick "no knocks " test before I add coolant. I usually run them 10 seconds before I complete assembly...

2 wires to the fuel heater might have been shorted as they were ripped out by PO. I'll have to find which fuse might have popped

Mountainman · **Posted:** Mon Dec 09, 2019 1:12 pm

2 codes might be clues.
P0180, fuel temp. I dont normally have this one unplugged, need to rewire. But I'm fairly certain that it will start with this unplugged as I pull the relay all of the time...

P1186. This is the weird one that isn't on the fault code list...

I think the oil pressure is causing the "barely fire".

flash7210 · **Posted:** Mon Dec 09, 2019 1:16 pm

Mountainman wrote:

The slight fire makes me think the fuel filter might be clogged. Any thoughts on barely firing? Maybe its mad that the MAF isnt seeing any flow?

A disconnected MAF wont prevent the engine from starting and running.

Absolutely gotta make sure its getting fuel and a new fuel filter is a good place to start.
Crack a injector line, crank engine, fuel should come out.

Quote:

P0180, fuel temp.

P1186. This is the weird one that isn't on the fault code list

Both those fault codes will NOT prevent the engine from running.
P1186 is the air box pressure sensor.

Mountainman · **Posted:** Mon Dec 09, 2019 1:33 pm

WWDiesel wrote:

Mountainman wrote:

Trying to figure out the minimum sensors and such that need to be connected under the hood. Most are obvious like cam and crank sensors. But, the engine won't start if the temp probe on the thermostat isn't connected, and the same goes for the little plug on the alternator. Anything else that might cause a no start condition?
Won't start without:
-coolant temp sensor
-alternator plug

I've got one I just put back together after PO took almost all of the wires out from under the hood, and naturally something isnt happy, and it does barely fire (#4 injector wire not swapped) then the ecu stops the starter. I'm missing something here, only cranks like two seconds

Just off the cuff:
Map sensor
two sensors on the fuel rail
two sensors on the injection pump
air box sensor

My list:

Fuel Pressure Solenoid - at end of fuel rail
Fuel Rail Pressure Sensor - on side of fuel rail
Fuel Quality Solenoid - on back of CP3
Cascade/Overflow Valve - on side of CP3

Crankshaft Sensor
Cam Position Sensor
IAP sensor (Mercedes sensor) side of air box
Map Sensor
Coolant temperature sensor

Duh! Thanks WW. I missed one of the connectors on the CP3, I'm getting rusty...
Now to worry about oil pressure!

flash7210 · **Posted:** Mon Dec 09, 2019 1:41 pm

There is only one connector on the CP3 and its for the fuel quantity solenoid.

The cascade overflow valve is just a mechanical valve. No electrical.

Mountainman · **Posted:** Mon Dec 09, 2019 2:12 pm

flash7210 wrote:

There is only one connector on the CP3 and its for the fuel quantity solenoid.

The cascade overflow valve is just a mechanical valve. No electrical.

Dang, oh well, back to the drawing board.

Mountainman · **Posted:** Mon Dec 09, 2019 2:34 pm

Fuel is good.
Probably popped a fuse with fuel heater wires touch. Hopefully I didn't smoke the ecu!

Mountainman · **Posted:** Mon Dec 09, 2019 3:37 pm

Crazy thing cranks normally with the mercedes disconnected! Swapped the sensor, same condition.

All fuses good.

Fires briefly. I'll try a different battery. Maybe it's failing under load

Mountainman · **Posted:** Mon Dec 09, 2019 8:56 pm

Not battery.

Ok, so this weird cranking situation needs further explanation. So, with the key on, the starter cranks what sounds like 3 revolutions (1 second) and then stops cranking, and does another 1 second, and repeats this until I release the key. See below...

- I pride myself with not killing starters, but this strange cycling of the starter motor for 5 seconds, with long rest time seems to have smoked the solenoid. Swapped starter and I'm back to the same old state...

The only good news is I have more than a dozen of every CRD part to choose from for quick replacement or swapping. I know, I've LOST my mind, but why else would I be posting here? :lol:

Mountainman · **Posted:** Mon Dec 09, 2019 9:07 pm

Cranks normally with starter relay jumped. Swapping relay next, I'll be back!

Mountainman · **Posted:** Mon Dec 09, 2019 11:45 pm

Not the relay. Can anyone verify the control voltage for the starter relay? The page in the manual would be much appreciated as well. I'm a commercial electrical guy finally forcing myself to use automotive diagrams where they rarely list the voltages.
I think I traced it back to the battery (12v) then to the PCM, then starter relay, but I'm only getting 3.4V at the relay coil.

WolverineFW · **Posted:** Mon Dec 09, 2019 11:56 pm

It is unlikely that the control voltage for the starter relay is anything other than 12 VDC.....pretty sure all those relays are the same. Now a big truck might be 24 VDC, but not this.

Sent from my SM-G965W using Tapatalk

Mountainman · **Posted:** Tue Dec 10, 2019 12:55 am

WolverineFW wrote:

It is unlikely that the control voltage for the starter relay is anything other than 12 VDC.....pretty sure all those relays are the same. Now a big truck might be 24 VDC, but not this.

Sent from my SM-G965W using Tapatalk

Yeah, I was thinking that everything is 12v, 5v, or PWM?
But, the jeep cranks, so it has me thinking that 3.4 is supposed to be 5v, and I'm experiencing relay chatter. I don't think that 3.4v would close a 12v contact at all.
Then again, I dont think that chatter would be so perfectly rhythmic. Sure seems like the controller (pcm?) Would be commanding such behavior.

Billybob · **Posted:** Tue Dec 10, 2019 1:46 am

The voltage across the starter relay coil should definitely be +12 volts.

You need to check the wiring diagram....8W-21-2 and 8W-30-31...Transfer Case Position Sensor shown there as well which may be relevant!

Are you measuring this voltage across the coil itself or with reference to a ground somewhere...ground could be bad.

Check Female contacts in the relay socket are clean and firmly clamped together...they can open up and make bad contact!

Connector C111 pin 10 has the Dark Green/Orange wire from the relay pin 85 going to the ECM connector C2 pin 58...could be dirty connections at any of the these connectors.

Pull out the Starter Relay and disconnect connector C2 on the ECM with battery disconnected.

The Dark Green/Orange wire should now be hanging in mid air..measure the resistance of this wire to ground, there should be no reading at all to ground while from one end of the wire to the other end should be perfect continuity.

Been through this type of procedure many times after a front end collision caused many shorts to ground or to some other random wire! :roll:

Mountainman · **Posted:** Tue Dec 10, 2019 12:39 pm

Thanks man. I finally read the reference designator page, and now these diagrams make sense.

Must be connected to start connections?

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