LMTV M1078 Wont start.

[Issati]

No, you just had a bad NS relay. Two of the relays(NS and SF-03) are supposed to be 12V, so you need to get a pair of 12V relays from your local auto parts store and replace them. The reason why this is important is that the dropout voltage on a 24V relay is not very far below the normal 12V voltage.

A 24V relay in the NS position may not reliably stay energized while you are trying to crank the engine...

View attachment 823028

Got it, Thank you!

 

[Issati]

Hi guys.
So here is a problem I haven't been able to fix. The oil pressure reads very high.

When I Unplug the engine oil pressure sensor, the gauge reads zero.
when i disconnect any of the wires for the sensor next to the oil pressure gauge (white wires), the truck would not start.
see the picture and the video bellow

I have a 0.6 Volts in the white wires. Also I don't think my batteries are being charged which I think is related.
any help will be appreciate it.

 

[Ronmar]

Hi guys.
So here is a problem I haven't been able to fix. The oil pressure reads very high.

When I Unplug the engine oil pressure sensor, the gauge reads zero.
when i disconnect any of the wires for the sensor next to the oil pressure gauge (white wires), the truck would not start.
see the picture and the video bellow
View attachment 830863

I have a 0.6 Volts in the white wires. Also I don't think my batteries are being charged which I think is related.
any help will be appreciate it.

What was the engine temp when you took that video?

The black and white wire go to the oil pressure switch. It opens when the pressure goes above 15PSI. When you turn on the main switch, it powers the oil and stop light in the dash, the starter inhibit relay K19 and the alternator excite relay K11. When you start the engine, and the oil pressure goes over 15PSI and the pressure switch opens, the idiot lights go out, K19 and K11 de-energize. K19 contacts(30-87) opening disconnect part of the starter pushbutton circuit to prevent you from cranking a running engine. The de-energized contacts of K11(30-87A) provide 24V down to the alternator regulator to turn on the alternator. So the truck not starting is perfectly normal if the oil pressure sw wires are disconnected Or the sw is bad. The alternator not coming online would be normal if you shorted that pair and they didn’t open like they should after the engine starts...

that pressure doesn’t look abnormal for cold engine oil. Whats it read when the engine is at 180 degrees...

 

[Issati]

What was the engine temp when you took that video?

The black and white wire go to the oil pressure switch. It opens when the pressure goes above 15PSI. When you turn on the main switch, it powers the oil and stop light in the dash, the starter inhibit relay K19 and the alternator excite relay K11. When you start the engine, and the oil pressure goes over 15PSI and the pressure switch opens, the idiot lights go out, K19 and K11 de-energize. K19 contacts(30-87) opening disconnect part of the starter pushbutton circuit to prevent you from cranking a running engine. The de-energized contacts of K11(30-87A) provide 24V down to the alternator regulator to turn on the alternator. So the truck not starting is perfectly normal if the oil pressure sw wires are disconnected Or the sw is bad. The alternator not coming online would be normal if you shorted that pair and they didn’t open like they should after the engine starts...

that pressure doesn’t look abnormal for cold engine oil. Whats it read when the engine is at 180 degrees...

The outside temp was about 80F.
I have never ran the car for more than 10-15 min so it has never gotten to higher temperatures.
I have always been worried about the red STOP light and didn't to ruin the engine.

 

[Ronmar]

Ok, well you have oil pressure, put a jumper wire on that black to white wire and the truck should start normally, then remove the jumper and the oil and stop lights should go out and the alternator should come online. You are looking for 14.1 and 28.2V at the batteries...

Then take it out and drive it and get that temp gauge up to 180 and I think you will see the oil pressure drop down in the 40-60 range as the oil gets warmed up. It is probably fairly new oil also, so will behave a little more stiffly when it is cold...

A 15PSI normally closed pressure switch with a 1/8NPT port should fix the issue... probably about $10 at any autoparts store. Most all will have only one connection and they provide a path to ground thru the switch body. Of those 2 wires, white and black, once disconnected from the switch, one comes down from the dash and should have voltage on it when the main switch is on, and the other is a connected to ground, so you will have to determine which, and connect the one that is powered, to the new switch terminal...

the original spec switches are out there, somewhere, the ones I have seen must be gold plated or something

 

[Issati]

Ok, well you have oil pressure, put a jumper wire on that black to white wire and the truck should start normally, then remove the jumper and the oil and stop lights should go out and the alternator should come online. You are looking for 14.1 and 28.2V at the batteries...

Then take it out and drive it and get that temp gauge up to 180 and I think you will see the oil pressure drop down in the 40-60 range as the oil gets warmed up. It is probably fairly new oil also, so will behave a little more stiffly when it is cold...

A 15PSI normally closed pressure switch with a 1/8NPT port should fix the issue... probably about $10 at any autoparts store. Most all will have only one connection and they provide a path to ground thru the switch body. Of those 2 wires, white and black, once disconnected from the switch, one comes down from the dash and should have voltage on it when the main switch is on, and the other is a connected to ground, so you will have to determine which, and connect the one that is powered, to the new switch terminal...

the original spec switches are out there, somewhere, the ones I have seen must be gold plated or something

I have disconnected the white wires after the truck started and that removed both the oil light and the STOP light however that also removed all the other lights on the dash as far as I remember,
So you want me to try and start the truck by connecting the white wires together? Did I understand this correctly?

 

[Ronmar]

Ok, wait, lets backtrack a little. With the white wires connected to the pressure switch, you are getting oil and stop lights when you turn on the main switch? If you do, then the oil pressure switch is completing a circuit to ground like it should with an engine that is not running/below 15PSI oil pressure.

but when you start the engine the oil and stop lights stay lit, but the pressure gauge increases?

If so, sounds like the switch is shorted...

Tell you what, when you can get to the truck with a voltmeter, give me a call at 360 477-5106. I can walk you thru the starter circuit troubleshooting in just a few minutes...

 

[Issati]

f so, sounds like the switch is shorted...

Ok, wait, lets backtrack a little. With the white wires connected to the pressure switch, you are getting oil and stop lights when you turn on the main switch? If you do, then the oil pressure switch is completing a circuit to ground like it should with an engine that is not running/below 15PSI oil pressure.

but when you start the engine the oil and stop lights stay lit, but the pressure gauge increases?

If so, sounds like the switch is shorted...

To answer your questions:

With the white wires connected to the pressure switch, you are getting oil and stop lights when you turn on the main switch? Yes, the truck starts normally however the STOP and oil pressure lights stay on. (I have a 0.65 volts across the white wires)

When you start the engine the oil and stop lights stay lit, but the pressure gauge increases? The pressure gauge doesn't increase, its stays at around 50PSI on idle and when you rev the engine, it increases to about 60-70 but no more than that.


I live in a different city than where the truck is for another 2-3 months so I only get to spend a bout a day or 2 with it every 2-3 weeks. I will give you a call in about 2 weeks so maybe we can troubleshoot it then.

 

[Ronmar]

Ok, I think I understand now. I thought you still had a starting problem.

your oil pressure switch is shorted, it should open to turn off the oil and stop lights Once the engine starts. That would also keep your alternator from charging... should be able to get one at about any autoparts store, or google “15PSI oil switch” and I am sure you can find one online.. you want 15PSI normally closed...

watching your video again, your oil pressure gauge went to near 0 when you turned on power and jumped up to 50 after start, and increased with RPM. Thats about normal, especially for new oil on a cold engine. Your gauge is moving in jumps though so either the gauge or sender mechanism is sticky. You would need a properly sized variable resistor or a substitute part to tell which exactly is the issue... mine moves a little jumpy also...

 

[Issati]

Ok, I think I understand now. I thought you still had a starting problem.

your oil pressure switch is shorted, it should open to turn off the oil and stop lights Once the engine starts. That would also keep your alternator from charging... should be able to get one at about any autoparts store, or google “15PSI oil switch” and I am sure you can find one online.. you want 15PSI normally closed...

watching your video again, your oil pressure gauge went to near 0 when you turned on power and jumped up to 50 after start, and increased with RPM. Thats about normal, especially for new oil on a cold engine. Your gauge is moving in jumps though so either the gauge or sender mechanism is sticky. You would need a properly sized variable resistor or a substitute part to tell which exactly is the issue... mine moves a little jumpy also...

So in theory, if I start the truck normally and then after it starts running I disconnect one of the wires from the oil pressure switch, the truck should run normally and the oil pressure light and STOP light should turn off, the alternator should also start working until I turn off the engine. Then, it wont start until I reconnect the wires. Is that correct?

 

[Ronmar]

So in theory, if I start the truck normally and then after it starts running I disconnect one of the wires from the oil pressure switch, the truck should run normally and the oil pressure light and STOP light should turn off, the alternator should also start working until I turn off the engine. Then, it wont start until I reconnect the wires. Is that correct?

Correct, you also won’t get a oil or stop light if you should loose oil pressure

 

[frank8003]

electric/electronic trucks
read/understand/work work work

 

[Issati]

Correct, you also won’t get a oil or stop light if you should loose oil pressure

Okay, so today, with the truck running, I disconnected one of the wires of the oil pressure switch and as you said, the alternator started charging the batteries. It jumped up to 28V for the first time. The STOP and oil lights were also gone.
So I need to buy a new Oil pressure switch


Thank you so much again for you help Ronmar.

There are 3 more issues to fix

1. Fix all the air leaks
2. Fix the heater as it doesn't run at all
3. Fix the CTIS



The end is near.

 

[Ronmar]

Thank you so much again for you help Ronmar.

The end is near.

Your Welcome

And no it’s not...

 

[Issati]

So today I finally got my oil pressure switch and as predicted it solved all the problems. The Oil pressure and STOP light are working correctly now.

The next project is fixing the heater.