RPM’s rise significantly after turning headlights on (1 Viewer)

[Chachi254]

Hi everyone,

Today, while on the interstate, I was cruising along at 65-70mph and my RPMs were hovering around 2500-2600 or so. It started to get darker so I turned on my headlights and my RPMs immediately jumped up 200-300. I then turned off my headlights to see if my RPM’s would drop, and they did immediately. Turning them back on jumped the RPM’s up again. Is this normal? I can’t say I’ve noticed it ever before today.

This may or may not be a related symptom, but I’ve been having some weird electrical issues as of late where my cruise control doesn’t work when my headlights are on, pressing the brake turns on my dash lights, and shifting from P to D turns off the radio backlight. I’ve been told these are all pretty much related to an electrical grounding issue, which I have yet to resolve or know where to look for a loose/faulty ground connection.

Could this RPM jump be a symptom of the grounding issue as well? Or am I looking at something different?

Thank you all in advanced! I’m constantly amazed at the family-like subculture of the Land Cruiser community. Y’all are great.

 

[Mighty690]

I'm not going to be a whole lot of help here, but I have experienced the same thing on one of my rigs on a long trip last summer (I spent a lot of time looking at that dash.) But it wasn't related to the lights. Could have been related to the cruise control now that I think of it. It's very intermittent and I haven't seen it since. I'm pretty sure it's electrical too. I have thought of having my alternator tested but haven't gotten around to it. I look forward to others' comments.

 

[blkprj80]

A good way to find a bad ground is to tie your multimeter black wire to the battery minus. Use an extension wire if you need it. Then, everything you touch with the red lead that should be ground should be zero volts. Start touching stuff that should be ground, like body panels, engine block, etc., with the headlights on and off. Check your setup by touching the battery positive. It should read 12.x Volts.

The cruise control is grounded behind the left kick panel at point "ID". Your combination meter (instrument panel) is also grounded at point "ID". Ground point "ID" is behind your left kick panel. Start there.

There should be a bunch of wires grounded to the body with a screw, it will be white wires with a black stripe, I think. My diagram shows brown wires as well.

I think you're looking for a bad corroded connection, possibly due to a windshield leak, behind that kick panel. Pull the kick panel and look around, and take some pictures to share with us.

Good Luck!

@Malleus always has good diagrams to post. Hopefully he'll chime in.

 

[CJF]

Probably all related.

Your rpm's are jumping because something is telling the torque converter to unlock. (As it should when, for example, you apply the brakes.)

 

[gummycarbs]

as it should when, for example, you apply the brakes.)

That immediately made me think of the trailer lights module...

 

[Mighty690]

Probably all related.

Your rpm's are jumping because something is telling the torque converter to unlock. (As it should when, for example, you apply the brakes.)

That's the interesting part, the actual rpm and tranny gear do not change. It's a tach related swing in my case.

 

[CJF]

That's the interesting part, the actual rpm and tranny gear do not change.

I wasn't talking to you.



Okay OP, better clarify: Can you hear/feel the actual rpm's jumping by a couple hundred, or is it just a false reading on the gauge?

 

[Chachi254]

Can definitely feel the jump in RPM’s in addition to the tach showing. The tranny is in the highest gear (4th?).

I wasn't talking to you.



Okay OP, better clarify: Can you hear/feel the actual rpm's jumping by a couple hundred, or is it just a false reading on the gauge?

 

[Chachi254]

That immediately made me think of the trailer lights module...

I have loose trailer wires wrapped around my trailer hitch assembly but they’ve always been loose and uncapped and I’ve never had a problem until recently.

 

[gummycarbs]

I have loose trailer wires wrapped around my trailer hitch assembly but they’ve always been loose and uncapped and I’ve never had a problem until recently.

If I were you, I'd disconnect the module and see if it magically fixes your problems. I had some truly weird behavior that went away when I removed mine.

 

[Chachi254]

If I were you, I'd disconnect the module and see if it magically fixes your problems. I had some truly weird behavior that went away when I removed mine.

Forgive me if this is a dumb question, but where would I find the connection point to disconnect the module?

 

[bigredmachine]

your just going to have to follow the wires back to where their spliced in, theyl either be t-tapped or soldered and spliced

Forgive me if this is a dumb question, but where would I find the connection point to disconnect the module?

 

[Malleus]

This sounds like a mess. Let's take your first "oh, and by the way" faults: Cruise control fails when the headlights are on.
Here's the headlight ground point (these are all from the 96 LX450 EWD):

So, the ground for both headlights is on the PS inner fender. Easy to get to and remove, clean, coat liberally with dielectric grease (I like https://www.permatex.com/products/l...ricants/permatex-dielectric-tune-up-grease-4/) and reinstall.

 

[Malleus]

Here's the cruise control's ground:

Physically, 1D is behind the fuse panel on the inner fender, behind the kick panel trim. Harder to get to, but possible. Treat as above.

IMHO, neither of these is going to solve your problem, but you'll feel better afterwards.

 

[Chachi254]

Here's the cruise control's ground:
View attachment 1625517

View attachment 1625518

Physically, 1D is behind the fuse panel on the inner fender, behind the kick panel trim. Harder to get to, but possible. Treat as above.

IMHO, neither of these is going to solve you problem, but you'll feel better afterwards.

Thank you for taking the time to research and post both of these suggestions! I’m going to try those tonight and I’ll keep you all posted.

Can’t thank you enough!

 

[Malleus]

Again, IMHO, you have multiple short circuits in your cowl wiring harness, behind the instrument panel. Even if you got it out, the harness is triple wrapped and impossible to trace inside the loom.

You may have either a). a sunroof leak or b). a windshield leak. Either should be corrected before you try to fix the electrical problems. You can see rust on the passenger side of the firewall, behind the instrument panel, if the windshield is leaking (although you have to pull the panel to see the firewall and it's no small job). Every one I've seen leak did so in the corners.

The sunroof is easier to identify. Remove the DS and PS kickpanel trim pieces and run it through a carwash. If it's leaking you'll know it. Mine leaks, but only in heavy rain and carwashes. No matter how much water I sprayed on it in my driveway, I couldn't see a leak. It drowned me in the carwash, after I'd removed the headliner and could watch it.

Without knowing more about your situation, I'd suggest you get a copy of the LX450 EWD and troubleshooting sections for every combination of problems you have and go after them, one by one, in the order you remember them occurring. Anything else is just shotgun repair.

 

[Malleus]

@blkprj80 has the best advice for you. It's not going to be quick, but it is possible. Just don't quit after you fail to identify the problem after the first few tries. I always wait until after I've solved the problem to quit. I feel better about it then.

 

[Malleus]

After a little more digging, it seems the combination headlight s switch (on the column) is grounded at 1D too:

So, a ground fault may not be out of the question. Still researching the rpm fault. More to follow...

 

[Malleus]

So, from inspection of the ECM EWD:

Point 4 leads us here (the blue box at the bottom is the ECU):

(TACO is tachometer)

C and D are:

the 12 and 22 pin dark gray connectors in the ECU.

 

[Malleus]

I don't see that the fault is apparently in the combo meter (at least I wouldn't look there first):