Lexus082

My tachometer is receiving the wrong tach signal. At cold idle it reads 750 rpms. At idle when hot it reads in between 250-500 rpms. What sensor is responsible for the tach signal?

PureDrifter

iTrader: (10)

thats not the signal, thats how its supposed to idle. High cold then settle down once warmed.

although your car is idling a bit low.

Lexus082

Not too sure about that response. When the car is first started after being completely cold it idles above 1000 rpms and then starts to lower down to 650 +/- 50 rpms. My engine is actually idling at 700 rpms when fully warmed up/hot, but my tach reads 250-500 rpms.

My tach is bad or is receiving the wrong input. I am trying to find out where the tach signal comes from. The tachometer shows the idle as being low, but my engine idle is fine..

trukn1

Welcome to CL. Um,, was looking at some of your other posts. Do you have an LS as well? Only reason I asked is because most of yor other post and even your profile page state that you have an SC400. It is no problem cross-posting but the internals of a LS and the internals os an SC are slightly different even though they share the same engine. Your question may be better answered in the SC forum. Here in the LS forum, PureDrifter is one of the better sources of tech info on the LS which is why he answered as he did.

PureDrifter

iTrader: (10)

how do you know what the actual RPM at idle is if your tach is mis-reading as you say?

reason for asking is because im not sure there is a way to calibrate the tach.


please post the year and model of your car in the thread title next time.

Lexus082

Both the Toyota tech and the guy from advanced auto said the engine was idling at around 700 rpms. Toyota tech hooked up a tach to the diagnostic port in the engine bay. The guy from advanced auto hooked up a machine to just the battery posts and through the electrical pulses it determines the engine speed. Which I am know starting to think the tach signal is fine, but it maybe the tachometer on the gauge cluster giving the improper reading. (Meaning a mechanical problem within the cluster itself). The toyota tech said the tachometer was receiving the wrong tach signal and that it may be a wiring problem. I guess when I have some free time I will swap out clusters to compare the tach. positions. I asked this in the Ls forum to get as much attention as possible. Both cars have the same engine and it should idle at the same rpm level at cold and hot in both cars. The car is a 94 Sc400. I'm just trying to figure out where the tach signal originates from. So I can follow it and find out whats interupting it (if anything). Thanks.

Lexus082

Is the tach signal created by and sent from the igniters?

Sanpete

If no one here knows, try calling a dealer service department and asking. Sometimes they know stuff ...

Lexus082

I talked to the UPS mechanic at work and he said usually the tach signal comes from the ignition coils. Anyone know for sure?

qha_vn

tach signal comes from igniter No.1, pls see wiring diagram attached

Lexus082

Just the answer and diagram I needed! So according to that diagram the same tach signal comes from the igniter and then goes to the diagnostic port in the engine compartment and then it goes to the guage cluster tachometer.

So if I have a correct tach reading at the diagnostic port (DLC), then it must be something messed up in the wiring to the cluster or the tachometer in the cluster is bad.

qha_vn

you are absolutely right

Lexus082

Yeah it was the tachometer in the cluster. One problem down another one to go. I still have a misfire going on also. I have pulled all the plug wires one by one and each time it got worse. So I know I have spark in all 8 cylinders. Could a coil possibly put out a weak spark that could cause a misfire? Or maybe an igniter is partially going bad? I am unfamiliar with these parts and how they can mess up if they are going bad. I know that when a coil completely fails the engine will only be running on 4 cylinders. (Thats easy). Just trying to figure out some symptoms of a coil/igniter going bad. I have replaced the plugs, caps and rotors within the last month to help solve this issue, but had no luck. The wires are about 2 years old and they all were within spec (ohms). There wasn't any damage to the wires that I could see.

flipmode64

Did you ever figure this out? I seem to be having the same issue. Please see this video.

dicer

Tach signal originates at the crank position sensor then ecu then igniter, haven't spent to much time studying it.