86 tach wiring

[Agreen]

I have searched for a while and I keep coming up dry. I'm looking for a schematic for the 86 2.8 v6 that shows the tach wiring. I found one on Tom Collins' page, but it just shows the tach signal coming off the coil; not the tach or where all it goes through. 

 

My issue:

-I bought this truck 2 weeks ago

-Tach didn't work, found a replacement in an ammo box behind the seat.

-I had to pull the dash bezel to fix the stero wiring hack job.

-Figured that I'm almost there, might as well swap it to see if it works.

-checked all the contacts and wiring behind the cluster, looks good.

-Swapped it, turned the key on and it went to almost 1000 rpm without running.

-Fired up and it pegged high. All other gauges still work (gas gauge is really bouncy, but is accurate if I'm stopped)

 

That's when I started looking for wiring diagrams but I'm coming up short. 

 

Or if there's a common fix for this that I'm not finding in my google-fu, I'd love to hear it.

[eaglescout526]

Yeah....Unless you have knowledge about the circuit board on the back of the tach itself, it probably won't work. That being said mind posting some pics of the tach? In the mean time I will go find my 86 wiring manual and take a pic of the circuit. 

[Agreen]

I'm just looking for the basic wiring from the coil to the tach. But I do have a lot of knowledge of electronics, so a simple tach such as this one shouldn't be that hard to figure out. I'm thinking it's probably another corroded connection somewhere along the lines, but what I'm finding is that the old AMC wiring diagrams are just so terrible.

 

The FSM for my 98 XJ has beautiful diagrams with connector pinouts and everything I've ever needed. 

 

Here are some pictures of the tach I pulled. Luckily I brought it with me to work tonight.

 

(I took those 2 nuts off the back, btw.  I'm going to see what I can do with the circuit)

 

 

[eaglescout526]

86 Wiring diagram for the cluster. Tach is on the second half.

 

 

Hopefully this comes across well. Tried to make it seamless.

[Agreen]

Well, that's good and bad news.  Looks like the power and ground for the tach is the same power and ground for all the instruments.  So I know I don't have a problem there.  Looks like 1 of 2 possibilities.  Either I have no tach signal coming from the coil or 2 bad tachometers.  

 

Thanks for the diagram!  Is that taken from the 86 FSM?  I might need to go ahead and get one of those.

[eaglescout526]

No it’s a separate electrical manual. The FSM doesn’t have any electrical diagrams. The bad part about the old tachs is they don’t have potentiometers like the later ones which is probably why they have components on the board that fails. I sent a couple of mine to a member on here to take a gander at them and see where a pot can be added but haven’t heard anything from them yet. 

[The86manche]

I actually had a similar issue with my tach that you have. Tried a few with similar results. I ended up changing one of the chips on the circut board and it started working properly.

It's the white chip you can see in one of your photos.

I had no idea how or if I could find one new so I ended up pulling a tach from a fiero that actually had a similar configuration and pulled the chip off it and soldered it onto mine. 

[JYau]

It could be that the one you found in the ammo box was the original, and the replacement tach is now bad as well. You could test the coil voltage with a meter, I believe it would be an AC signal but with the meter in DC mode it would low-pass the signal so you should at least measure a non-zero signal. A non-zero voltage would at least indicate that the coil wire isn't broken.

 

I don't know exactly how the tach circuit works but I would think that the signal that indicates RPM is a DC voltage, which is proportional to the coil's average voltage. This is likely accomplished by a low-pass filter within the tach itself (A low-pass is needed to bring the AC coil voltage down to a DC like voltage). With that said and since you have a knowledge of electronics, a good test would be to use a DC variable power supply and connect it to the coil signal input on the tach and slowly wind the voltage up from 0V to say 5V to be safe, and see if the tach needle moves linearly with the supply voltage. 

[Agreen]

A friend of mine keeps talking about how it's common for Fieros to have a bad tach filter, so I should be looking into that. Well, I don't see one on my coil which could be because it doesn't have one, it could be built into the circuit board, or it might not even have one? But that gives me hope that I can see of there's better info for a Fiero tach repair.

 

The one in the box had PAP (Pull-A-Part) in paint pen on it. I'm guessing he pulled it and never got around to installing it because of how much disassembly goes into pulling thr instrument cluster.

 

I think first I want to verify +12v, gnd, and the coil pulse at the cluster. I'm thinking it might have an open in the signal wire. Anyway, I have an oscilloscope so I can see if I'm getting it or not. I just wish I had a signal generator.

 

Edit: Look what I found!

http://www.fieros.de/en/articles/tach.html

[Agreen]

For future reference:

 

http://www.howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm

 

I still haven't gotten the tach fixed yet.  I made a quick attempt a few days ago, but it was a spectacular failure.  The page I just shared has info on that specific chip (apparently all it is is a bank of resistors).  So with that knowledge, checking it should be a lot easier.  

 

Also, a tach from a 93 will not work, even for testing.  I jumpered +12v, ground, and tach signal from the 86 dash to the 93 tach and it almost turned the engine off.  

[Minuit]

Those look like carbon composition resistors, which have a bad habit of drifting (usually upwards) in value as they age. Might be worth checking their actual value with a meter against what they're supposed to be (see resistor color chart). Shouldn't need to pull them out of circuit to do so unless they're in parallel.

 

[Eagle]

6 hours ago, Agreen said:

For future reference:

 

http://www.howtoalmanac.com/kevin/projects/automotive/tachfixtruck.htm

 

I still haven't gotten the tach fixed yet.  I made a quick attempt a few days ago, but it was a spectacular failure.  The page I just shared has info on that specific chip (apparently all it is is a bank of resistors).  So with that knowledge, checking it should be a lot easier.  

 

Also, a tach from a 93 will not work, even for testing.  I jumpered +12v, ground, and tach signal from the 86 dash to the 93 tach and it almost turned the engine off.  

 

There are two diagnostic connectors under the hood, one of which has ports for inserting probes to test the tach signal. You can use any aftermarket tachometer or a shop idle tach to run the tests. The electrical manual will have pinouts of tose two diagnostic connectors to show you what ports to use for the tach test.

 

The '93 tach should work.

[Eagle]

If it'll post, here's the pinout for the diagnostic ports:

 

[Agreen]

I don't think (could very well be wrong) that the 86 has a diag connector. It has a 2bbl 2.8 v6 and GM HEI ignition.

 

I really REALLY appreciate everyone who gave input. I took both of my tachs to work with me last night and traced out the points at which I soldered the resistor on. I was way off. That "how to almanac" really changed the game when I found it. It showed the resistor chip internal diagram, and that the resistor between pins 4 and 10 usually go bad. Sure enough, it was open. And the first page I linked to didn't describe that. At the time, having only that to go off was difficult, and I ended up soldering the resistor across the wrong pads. 

 

So yeah! It's fixed now! Now to adjust the resistor and get my interior back together!

[Eagle]

Sorry --

 

The '86 2.5L had the diagnostic connectors, but you're right -- the 2.8L did not. My bad.

 

Good job on diagnosing the circuit board. That would have been completely over my pay grade.

 

[eaglescout526]

I wonder if you can change out that resistor for a potentiometer so you can adjust it for a 4 or 6 cyl. 

[Eagle]

10 hours ago, eaglescout526 said:

I wonder if you can change out that resistor for a potentiometer so you can adjust it for a 4 or 6 cyl. 

 

I wondered the same thing. Man, that could be the ultimate CC DIY upgrade.

[eaglescout526]

8 minutes ago, Eagle said:

 

I wondered the same thing. Man, that could be the ultimate CCDIY upgrade.

I’d be doing it to every full 84-86 cluster that way folks can have the blue cluster as an option rather than the other two options. I might have a spare old tach I can do that too. I sent mine off to another CC member to do some digging on these to see if it was possible and haven’t heard back due to their busy schedule. 
 

@scaleless

[Minuit]

I see no reason why you wouldn't be able to. You'd need to calibrate it, but that's easy enough with the appropriate equipment.

[Minuit]

Anyone want to send me one of these tachs for research purposes?