1G power transistor to blame?

[tsigrl2016G]

Yesterday morning, i start my car and the tachometer isn't working. The car sounds like it's idling normally, and when driving, it shifts fine, and the engine sounds normal...like the actual idle has not been effected, it's just that the tach isn't reading the idle.
As the day went on, the more i drove the car,,,the more the tach started to "wake up", but, it wouldn't read past 4,000 (even though i got on it and sounded like the engine had to have been up to 6,000-6,500 at least).
A friend suggested the power transistor...i was thinking actual electrical wiring though, but either way...we just took a look around the ig. coil to see if anything was loose, and nothing was.
We must have done somthing not realizing it, because when i started the car back up, the tach was working. All night long, it was fine.
Then, today i start the car and the tach reads 0. I'm drivin around and it creeps up to 2000, then i shift, but the tach doesnt move. When i pull to a stop, it stays at 2500 or wherever it was, and when i turned off the car a few times...it stayed where it was and didn't drop to zero. It seems though, as the more i drive it, the more it "fixes" itself.
I've looked at the factory manual's diagrams for the circuit for the combination meter, and sure enough, the tach's main source is the power transistor, of course it shares a connection to the computer with the stock boost gauge. The engine seems fine, and nothing else is being obviously effected....so i don't think it's the ecu.

everything else in the ignition system can cut in and out till it goes bad,,,i'm just not sure the power transistor does that too....it costs more than i'd like, but if someone ever had this prob. or knows more about it and could help me out... that'd be great!

Kylei

 

[icedearth15]

I will post my experiences I had with my 1990 Talon. I got the car and the tach didnt work. So at first I thought the cluster was bad....I changed that out with another cluster and it didnt help anything. The longer I had the car I realized that every once in a while the tach would jump around a little, but never reading the correct RPM. I then got a power transistor unit off of a buddy of mine who has a bunch of parts cars....thinking this was the problem I put it in a nothing was different. I then went to pointed the finger at the coil packs. ON the 90 this is a "tacho unit" or something similar to that anyway....which is bolted on the back of the coil packs. Adn this is where the 90's get the tach signal....this tach unit along with singnals from the coil packs create the tach signal. I later learned that the later dsm's (as far as I know anything past 90') dont have these "tach units" attached to the rear of the coil packs. Insteed the 91-94's use signals directly off of the coil packs. I know a limited amount about electronics, but from what I understand there is a unit inside 91-94 coil packs which send the tach signal out.

Also, so people have had the "noise filter" which is bolted onto the back of the intake manifold be unplugged which caused there tach not to work. But I am not sure if any cars past 1990 had them.

So you have some basic places to look....Tach in car (gauge cluster) could be bad, power transistor MAY be bad (I highley doubt that is your problem), "noise filter" (if you have one) could be unplugged, or my best guess, you need to buy some new/used coil packs.

 

[tsigrl2016G]

It might be the coil pack, they changed some of the relay locations in the 92 and 93s, the 91s as well. Of course the headlights changed. The sensors are the same in regards to their use, but they're not interchangable from a 90-91-92. The ignition system is set up the same on a 90 tsi awd and a 92 tsi awd, but the ig. is basically the same, with the ECU directly activating the power transistor for the ign. timing control.

Actually, the divide the power transistor to two- A and B.
pwr trans A goes to ig. coil A and then to sp. plugs for no. 1 and 4 cyl. And pwr trans B goes to ig. coil B and then to sp. plugs for no. 2 and 3 cyl.

Actually, i've been looking at the diagrams for it more, and there is a signal unit for the tach mounted on the back of the coil pack, i'm not sure if that's what you were talkin about or not but thanks for the advice...

kylei

that and the power transistor sends the signal to the tach, according to the way the diagram shows the current flow, i guess it could be either one. i'll try em out, thanks

The more i look at the factory manual, the more i'm thinking it's the "signal unit for the tach" that's on the ignition coil. it just lists that the pwr transistor is driven by an ECU signal that turns the primary current for the ignition coil on and off, it doesn't list that it sends a sig. to the tach.

It lists that the ignition coil, since it is two-type, has the unit that sends signals to the tach. it doesnt' look like that's available seperate from the coil pack, so, it looks like i'll be buying a new coil unless ne one else has ne ideas. hanks again for the help

 

[92awdgsx]

if you need coils i have the factory coils and igniter from my 92 gsx.

 

[tsigrl2016G]

icedearth: did u have to buy the coil or could u get the sending unit for the tach separate?

 

[icedearth15]

I got the coils from a friend ($20), which when he gave them to me the tach sending unit was attached.

 

[tsigrl2016G]

i got a coil pack from a friend and it doesn't look like the signal unit came with... my coil pack is the one with the triangle plug..not the square. I'm gonna switch the coil to see if it makes a difference. I didn't think that the coil itself sent the signal on my car...the factory service manual shows the unit...but the cd manual has classed together for 90-94 and then for the 2G. The only time the tach doesn't work is when i first start it up cold...then it wakes up. If it's not the coil pack...i don't know what it could be. Looking at my pack on my car..it doesn't look like there's the tacho unit there...but ..i'll switch em an see...thanx for the help/info.

 

[BrokenTsi]

this should help with diagrams and what not.

http://www.ecanfix.com/users/mdhamilton/ecuconversion.html

 

[steve]

The setup is different between the 90's and all the rest.

On the 91+ cars the tach signal comes from the Power Transistor Module, on 90's it comes from a tach module on the coils. So icedearth15 means well but his information doesn't relate to your car.

Steve