Cat 272d2 XHP preheat issue

[aw1]

I am having issues with a Cat 272d2 XHP with 3.8.

Pre Heat not coming on. Code 523544-4 Engine start aid voltage below normal.

I have replaced the fuse, relay, switch on side of frame where power wire comes to pre heat.
Pre heat has been tested for shorts. I have hooked the pre heat up to the battery and the system works fine once i direct the circuit to the 12 volt battery (reads 11.7 volts). so that tells me the pre heat works and the wiring to relay, switch, fuse, to the pre heat.
my issue is the eco is only putting out 9.3 volts to the pre heat switches/relay so it does not have enough volts to open the switch or relay.
i have installe a resistor at the temp sensor to trick it into thinking it was super cold with n luck. i am leaning toward a bad ecu.

i have charged battery as well.

 

[Nige]

Here's the troublshooting procedure that might assist you in determining what's wrong.
I'm assuming your machine is S/N prefix BL2.?

 

[aw1]

i have had cat here and they have ran through those tips. any other ideas?

 

[Nige]

Sorry, no.
Having read the procedure I find it hard to believe that the Cat tech would happen to have a dummy resistor kicking around in his toolbox. Did you stand over him while he did it..?

 

[aw1]

we had to order one. its in my drawer now,.

 

[Nige]

So he couldn't have done the test 100% as required by the procedure.
I would suggest repeating the test when all the necessary gear is available to do it - and I don't see why the dealer should charge you service time if they rock up to your machine without all the gear to do the job. Especially if you told them what Code it was showing beforehand.

 

[Patrioteq]

what did you end up finding with this? I have a 299 doing the same thing. Found a dead short on the signal for the solenoid. Fixed that but now only getting 9.3v out of the ecm to the signal side of the coil.

 

[Nige]

I have a 299 doing the same thing.

OK, so please start with a Serial Number because it's a different model to the one that started the thread.
I'm not saying that the diagnostic procedure will be any different but it's best just to make sure based on your S/N.

Found a dead short on the signal for the solenoid.

You mean a short in the power supply to the solenoid (or the ground) I assume.?

 

[Patrioteq]

OK, so please start with a Serial Number because it's a different model to the one that started the thread.
I'm not saying that the diagnostic procedure will be any different but it's best just to make sure based on your S/N.

You mean a short in the power supply to the solenoid (or the ground) I assume.?

Correct power supply to signal side of solenoid was shorted to ground.

Sn-dx200402

 

[Nige]

What Diagnostic Code do you have for the air intake heater driver circuit, 523544-3 (+B short) or -4 (ground short)
So the test procedure is the same. Here is an updated version. Per the discussions above you will need a dummy resistor of 15-25k ohms to do this.

I'll send you the electrical schematic. TBH the supply from the ECM only has to pull in the start relay coil. You might make more sense of it from that.

 

[Patrioteq]

What Diagnostic Code do you have for the air intake heater driver circuit, 523544-3 (+B short) or -4 (ground short)
So the test procedure is the same. Here is an updated version. Per the discussions above you will need a dummy resistor of 15-25k ohms to do this.

I'll send you the electrical schematic. TBH the supply from the ECM only has to pull in the start relay coil. You might make more sense of it from that.

I am getting both -3 and -4. I have the schematic. I pinned it directly from the ecm plug and with the plug to the solenoid unplugged have 9.3v. With the plug plugged in 0v.

 

[Nige]

I pinned it directly from the ecm plug and with the plug to the solenoid unplugged have 9.3v. With the plug plugged in 0v.

Which wire numbers are these.? I also have the schematic, I'm trying to figure exactly out what you are doing so that I can duplicate it.

All that the ECM signal does is pull in a relay. You keep referring to a solenoid and I don't see one. Are we using two different terms for the same thing.?

 

[Patrioteq]

Which wire numbers are these.? I also have the schematic, I'm trying to figure exactly out what you are doing so that I can duplicate it.

All that the ECM signal does is pull in a relay. You keep referring to a solenoid and I don't see one. Are we using two different terms for the same thing.?

Yeah. It looks just like a starter solenoid. But cat i believe refers to it as a relay.

Pin 24. Ecm connector 2
Intake heater relay coil +.

 

[Nige]

Yeah. It looks just like a starter solenoid. But cat i believe refers to it as a relay.

For the way it works I see it as a relay. When F793 OR is energized it pulls the relay in and allows Batt+ coming in on A113 RD to be supplied to the intake heater on K995 OR.

As you say F793 comes from the ECM connector J2, Pin 24. I would say that it wouldn't require a full 12v to pull the AIH relay in. What happens if you apply 12v to Pin 1 of connector RP-C9.?



EDIT: As per the troubleshooting procedure have you tried installing a 15-25k ohm resistor across the terminals of the engine coolant temp sensor to simulate the coolant temperature being below -30 DegC (-22 DegF).?

 

[Patrioteq]

For the way it works I see it as a relay. When F793 OR is energized it pulls the relay in and allows Batt+ coming in on A113 RD to be supplied to the intake heater on K995 OR.

As you say F793 comes from the ECM connector J2, Pin 24. I would say that it wouldn't require a full 12v to pull the AIH relay in. What happens if you apply 12v to Pin 1 of connector RP-C9.?

View attachment 300086

EDIT: As per the troubleshooting procedure have you tried installing a 15-25k ohm resistor across the terminals of the engine coolant temp sensor to simulate the coolant temperature being below -30 DegC (-22 DegF).?

View attachment 300087

If i apply 12v directly to the relay via rpc9 the relay clicks and sends 12v directly to the intake heater.

With a 20k ohm resistor in line of the temp sensor et reads -15*. Et shows start aid relay “on”. There is no voltage to start aid relay signal.

 

[Nige]

In that case unless there is another wiring issue somewhere between the ECM and the heater relay it appears as though you potentially have a failed ECM.

How are you testing continuity in the machine wiring harness.? Multimeter.? Sometimes they can give good continuity readings but the wire in question is breaking down under load. It might be better to use an external 12v test light with a decent amperage lamp in it to ensure that the wires in question are carrying at least a bit of load. IMO that would be the last throw of the dice before declaring the ECM as toast.

 

[Nige]

I was looking at the schematic and the only connector that wire F793 OR passes through between the ECM and the intake heater relay is CONN 22 which co-incidentally also carries wire H808 WH which is effectively a ground wire for the coil in the Start Relay.

Thinking out loud I wonder why these two particular wires are routed through their own specific 2-pin connector and not through one of the larger 12-pin harness connectors in the same area.? Something critical about them maybe.? I dunno.

It might be worth just a look at CONN 22 just in case there is something funny going on inside it that is tracking power from F793 via H808 to "Start Relay -ve"

 

[Patrioteq]

I was looking at the schematic and the only connector that wire F793 OR passes through between the ECM and the intake heater relay is CONN 22 which co-incidentally also carries wire H808 WH which is effectively a ground wire for the coil in the Start Relay.

Thinking out loud I wonder why these two particular wires are routed through their own specific 2-pin connector and not through one of the larger 12-pin harness connectors in the same area.? Something critical about them maybe.? I dunno.

It might be worth just a look at CONN 22 just in case there is something funny going on inside it that is tracking power from F793 via H808 to "Start Relay -ve"

View attachment 300123
View attachment 300124

When i was diagnosing the short that is how i found where it was. Between conn22 and conn-rpc9 the wire f793 was shorted to ground.

My theory is that wire shorted and there is no fail safe (fuse) between that and the ecm. So the short ended up damaging the ecm internally. Kind of silly the oem wouldnt at least have a fuse in line. Relays like that go bad all the time and usually just pop the fuse. Not fry the ecm.

 

[Nige]

What exactly caused the short.? Were you able to determine that?

 

[Patrioteq]

What exactly caused the short.? Were you able to determine that?

negative. customer did not care that the preheat was not working. asked to price out part for future reference and took the machine back as is. I never had the chance to cut the harness apart to look for a short.