Electrical guru needed!

[Backlash2032]

Okay. So I've been living without a tach ever since I swapped in the GTE. My first attempt was at a resistor swap. It went according to plan, except i noticed that the tach was always 4-500 rpm off. Sometimes even 1k rpm off. (idled at 500, went down the highway at 2000 instead of 2500, rev limited at "5500") So I bought a couple tachs off of a guy. Turns out the 89 tach he had worked the exact same way. He also sent me a pre 89 tach, which worked perfectly except for the fact I have no cruise control. The pre 89 tach was spot on. And i figured out why it was different. One resistor is different between the two driver boards.

So, I figured i'd take the pre 89 driver board and solder on the post 89 cruise control unit. I did that and took it out for a spin. The thing is so damn erratic. Idles at 700 rpm. Rev limits sometimes at 6000, sometimes at 5500. Cruises down the highway at 60 at 2000, sometimes 2500. It will randomly jump to ~3500 rpm, then go back down to actual rpm. AND my cruise control STILL doesn't work!

So I have a CPS out of a pre 89 car, and I figured thats why the pre 89 tach is spot on, and the post 89 not so accurate..

So what I'm asking is.. Why is the tach so erratic? And why doesn't my cruise control work??

 

[toyotanos]

The tach runs off of the ignitor's tach/RPM/IG- signal. The ignitor creates that signal based off of what the ECU is commanding it to do. The circuit is exclusive to the tacho, so the cruise has nothing to do with it. You need to check to check the wiring to the check connector, ignitor, and combination meter to make sure the circuit doesn't have resistance or voltage when it shouldn't have it. Other than that, the ignitor and tachometer are the only other components in the system!

crude text diagram:


[ECU]
| (trigger pulses)
|
V
[Ignitor]--->--->---[tachometer]
(IG- signal)

 

[Backlash2032]

Well there has to be a difference between pre 89 and post 89 ignitors, otherwise there wouldn't be the resistance change in the driver board, correct? Assuming that's correct, there must be some wiring fault...

But what that doesn't explain is why it was rock solid when I had the original tach in the car, with the resistor swap done. It was perfect.. except for it being off.

 

[toyotanos]

Not sure if the ignitors changed during that period, I own a 91 so take that with a grain of salt. Were it me diagnosing the issue in the shop, I'd think it might be an issue with the GE-GTE swap. Some spot in the wiring that works sometimes, but not all the time. I've also had issues with tachometer needles having been removed and inaccuracy following. I've never had intermittant operation, though.

 

[Dan_Gyoba]

I have an '89 NA. I swapped in an '87 GTE, and have had exactly the same symptoms with the tach. I'm using the '89 face, but the '87 driver board. I've been meaning to do a more thorough diagnosis.

The tach circuit is actually a pretty simple one. It takes voltage from the gauge fuse, and uses a divider network to make a fixed current based on the tach signal. The instrument itself is essentially an ammeter.

If you are getting erratic behavior, then you probably have a poor connection somewhere, resulting in inconsistent signal.

The cruise control light is on the tach face, but that shouldn't affect the cruise control itself. I've been able to use mine with and without that part connected.

I've been looking at how to fix the tach. I'm looking at the resistor board operation. I think that if I can raise the signal current by a percentage, I should be able to make the circuit work perfectly.

 

[Backlash2032]

So.. theoretically if I got a 90 ignitor and a 90 tach it should work correctly?

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[Poodles]

So far as I can tell, the ignitors are the same. Honestly gauges are fairly sensative so you may have either damaged them or thrown them out of wack (if it's 400rpm off, you probably moved the needle by accident)

 

[Dan_Gyoba]

I didn't think that the ignitors were different, but I do think that there's something different with the gauge calibration.

I don't think that the needle is off. It reads low, but it will come off the peg when cranking, which is much less than what it typically reads low by. I hit the rev limiter at an indicated 5500 RPM, so that's gotta be ~7000. Idle looks okayish, maybe a bit low, but not bad.

The fact that the resistors are different tells me that the gauge itself is possibly different. Maybe the '89+ gauge allows for/needs higher current through the ammeter portion.

What I'd love to see is a comparison of the pre and post '89 driver boards. The resistor colour codes would be enough to tell me what would need to be changed.

 

[Backlash2032]

I'll snap a few pictures later today.

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[egesledder]

You could run a test wire from the diagnostic plug straight to the plug on the back of the gauge cluster. That might at least eliminate a bad connection somewhere. I'm modifying my stock tach in a toyota pickup (swapped 7mgte to replace the old 4cyl) with a potentiometer across the M+ and M- terminals in order to adjust it. I'm guessing if you can get yours to work consistently and it is just off, you could do something similar. I've got a temporary external tach to help with adjusting it accurately.

 

[Backlash2032]

Which wire do I need to hook into to get the tach signal?

 

[egesledder]

In the check connector, its the IG- terminal. For a more direct route, the black IG- wire coming off of the igniter is where the signal comes from. It goes directly from here to both the check connector and the tachometer, no where else. If you have corroded wires or connectors, I could see that changing the signal your tach sees. Check the wire for continuity with ground while both the gauge cluster and igniter are unplugged, resistance should be infinite. Also, check the resistance of the wire between the igniter and gauge cluster plug, it should be ~zero.

If you don't have a multi-meter, buy one! Its a cheap tool that should be in every mechanic's tool box.

 

[Backlash2032]

Okay. Cliffs for today. Hooked up the signal wire direct from the IG- in the check connector, still intermittent operation. Hooked up a different tach to power, ground, and signal (via IG- in check connector) same result. Same reading infact, even though it was an 89 gauge (driver board in the car is out of a pre 89) I got so pissed off at the thing that I took my dads "engine analyzer" and hooked it up to the IG- in the check connector, and to my surprise... It was accurate. (Well, to my ear) So I just moved the needle on the gauge to where it should be. Doesn't go all the way to zero when you turn it off, but oh well....


Then. On my drive home.. I had a brain wave. I feel absolutely retarded now, but it makes perfect sense. It was even mentioned in this thread. Removing the needle will cause inaccuracy. I always just shrugged that off as they didn't put the needle back on right.. But I have a feeling it actually damages the gauge. Here's why I think that way.

The gauge was never really that far off from idle-~2000 rpm, and that's where it started going hay wire. My cruise RPM is above 2000 rpm. When I go wide open it is the most inaccurate thing in the world.. When I am just driving normally I notice that it will climb much faster than the engine is from 2500-3000 rpm, sometimes a little higher. And I know its not signal because I ran it straight from the check connector, and I checked the wire from the ignitor to the check connector and it was fine. So it has to be in the gauge. And that's the only thing I can come up with.

SO. Since I just LOVE my new red needles.. I think what I'm going to have to do, is buy a tach off of somebody on the forums, and instead of pulling the needle off, I need to crack the plastic so it just.. falls off. Then put my nice red needle on it, and call it DONE.

 

[Poodles]

How did you make the needles red? If they're too heavy, there's your issue...

 

[egesledder]

Not sure how different they are, but I have a tach out of an '87 I would sell you. I know it works.

 

[Backlash2032]

I took a q-tip and dipped it into paint thinner and took the yellow paint off the bottom. Then took a red touch up pen and put a coat of paint on the bottom.

For the speedo and tach I painted the centers black.

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