Weird tach issue, need advice

[spook302]

So here it is. Wen I start the car in the morning the tach stays at zero. 15 minutes into my morning drive it start to work but reads too high and sometimes stays high while at idle. Ill get to work turn the car off and work for three hours and take a lunch. When I go to lunch it works fine, for the rest of the day it will work just fine. Then the next morning zero again till later. Why? I first thought something under the hood could be affected by heat. Like it doesnt work till its warm. That is not the case. i let it warm up fully this morning before driving it and still nothing. I checked the wire connect at the coil pack, its fine. The connection at the module is fine as well. None of the other gauges are affected just the tach. Any advice would be great. I would really like to resolve this problem as it is the last real problem since I rebuilt the car. i dont know if this was a problem since I rebuilt the engine cause i bought the car with a slipped belt and bent valves

 

[boostedjay32]

have you checked the cars speed sensor in the tranny? may be going bad on you and making the cable turn when ever it wants to therefore you are seeing the tach go all crazy on you

(sorry for the run on
i just woke up)

 

[spook302]

The speed sensor I think is fine. There is no cable for the tach its all electric. The speed sensor is for the speedo

I just read up on the same problem somebody else had but like allot on this site once people fix the problem they move on and dont post what it was.

 

[spook302]

Could my computer be the problem here? Could it be that when the ecu is cold I am having this problem?And when it warms up it starts to work.

 

[eclipzed]

It could be your power transistor. I ruined my power transistor by runing th MSD DIS2 without the proper tach adaptor. Power transistors are not cheap though.

 

[96eclipzGsT]

Could my computer be the problem here? Could it be that when the ecu is cold I am having this problem?And when it warms up it starts to work.

Maybe it is the gauge cluster. There is a way to check it with a voltmeter under the hood, but I forget which connector you hook into. It might be near the ground wire for the timing. Maybe someone else can chime in on exactly how to do it?

 

[spook302]

That would be great. I have tried to look at old post but I am not getting anywhere. I have another computer I ould use I guess to eliminate the option of the ecu. Its just weird that it will always start to work.

 

[spook302]

Is the trnasmitter on the back of the intake manifold on the support bracket for the intake?

 

[MichaelSuch]

The power transistor is the black "box" next to the coil pack on the intake. Just curious if you found a solution for this problem? Mine started doing the exact same thing last year, took it down for a rebuild and still does it now. Only does it when it's cold out though, it was perfectly fine during the summer.

 

[-red97rum-]

Bringing this back from the dead.

My tach does the same shit, it's annoying as all hell. Everything else works fine. I swapped in a different ecu that was eprom and it still acts the same so it's not an ecu issue.

I'm going to check the wiring to the coil pack and power transistor next. Anyone have any ideas?

 

[-red97rum-]

Okay, well we can rule out the power transistor unit now too (at least for my car). I pocketed one from a local junk yard this weekend and it didn't change the behavior of my stock tach. I'm guessing the wiring may be corroded or it's the tach in the cluster itself.

 

[maxeclipse23]

SPOOK- I saw that you have a new top end listed in your mods. Is that refering to a new head(just to clearify) and if so was your car having this problem prior to this?


REDRUM- Was the power transistor you got from the junkyard off a known running vehicle? Or did you just pull it off? Also check your wires in the connector for that plug in case one may have been accidently been pulled out. Any other symptoms going on with your car? Was there anything worked on prior to this problem occuring? The more information the merrier.

 

[-red97rum-]

REDRUM- Was the power transistor you got from the junkyard off a known running vehicle? Or did you just pull it off? Also check your wires in the connector for that plug in case one may have been accidently been pulled out. Any other symptoms going on with your car? Was there anything worked on prior to this problem occuring? The more information the merrier.

No the transistor was not off a known running car, it was in the junkyard.
But in all seriousness, that is a good point, I'm going to see if I can swap one from my buddy's running car. I also suspected the wires that fit into the plug but after checking them I can't see anything out of the ordinary. The car has done this as long as I've owned it. Swapped coil packs too with no change. The car runs fine and the RPM displays perfectly in the logger. I'm not really that worried about it because it's not like I rely on the stock tach. It's just one of those little annoying things, you know? Hmm lets see.. In the last year I've changed 3 different turbos, done a head gasket job and numerous maintenance items. I actually just swapped in my built motor this month. I was hoping this would lead to someone having the same issue and posting up a quick fix. Thanks for the suggestions though, if my buddy's transistor unit fixes it I'll post it up.

 

[90moneypit]

If your logger is displaying the correct rpm's then the problem isn't going to be with the ecu. The 'black box' that fixed the OP's problem is the tach driver for 1990 cars. I'd hazard a guess that it was on the edge of going out but the heat from running was closing a gap inside the module. They internalized it on 91+ cars so it's just another of the many 'quirks' that a 90 has. It sounds like the problem is with your cluster. I don't think it'll be as easy to swap out but it just may help you narrow it down between physical or electrical problem.

 

[90moneypit]

it sounds like your speed sensor is going out man those suck

Um, the speed sensor doesn't drive the in dash tach. It's either a bad motor in the cluster or a loose/broken connection on the PCB. While it's possible to fix those, it's usually cheaper to buy a used cluster.

 

[spook302]

well I just realized I have another cluster from the stripped down lazer I have. I am going to swap them out and see what happens. My car was purchased with a slipped tb. So I dont know if the problem was there before I tore the motor out of it or not. Now the weird thing is it acts like it needs to warm up.

 

[Justin_l_]

well I just realized I have another cluster from the stripped down lazer I have. I am going to swap them out and see what happens. My car was purchased with a slipped tb. So I dont know if the problem was there before I tore the motor out of it or not. Now the weird thing is it acts like it needs to warm up.

Did you ever get a chance to swap out clusters? The car I just picked up is doing the same thing.

 

[Sam@RRE]

The signal for the Tach comes directly from the white wire on the transistor pack. On my car with AEM EMS the tach goes stupid on the two step and rev limit due to it being a ignition cut limiter and not fuel so the tach signal from the transistor pack gets cut and a 0RPM signal is sent out but the engine is really spinning at 5500rpm or 8000rpm depending if its my rev limit or 2step. Its just really stupid to see that tach sweep from 8K to 0 and back up to 8 and 0 in less than a second. My solution for this was to cut the white wire off the transistor pack and jump the CAS yellow wire (crank signal) to the transistor pack white wire. That way it just goes off the crank signal. It is a direct square wave signal from the transistor pack out to the cluster and the CAS is a square wave output that is mechanically driven.

 

[Justin_l_]

That's great to know. I'm going to have to look into that when I get this thing back together.

 

[WizardTSi]

Great tip Sam!
Took me a whole 2 minutes to fix mine

 

[spook302]

It was my cluster. The old one would stick at 0 when it was cold at night. i could tap on it and it would start to jump up slowly. it would still act weird until it got warm enough. The lower mile one is good but on a very very cold night it will act up when i first start it but by the time i come back to it to leave in the morning its fine.

its just how they act when they get old

 

[NeMiZiS]

My solution for this was to cut the white wire off the transistor pack and jump the CAS yellow wire (crank signal) to the transistor pack white wire. That way it just goes off the crank signal. It is a direct square wave signal from the transistor pack out to the cluster and the CAS is a square wave output that is mechanically driven.

This will fix the tach on any 90?
I think I may give this a try when I get the Talon running again.
My stupid tach has never worked and I've already replaced the "noise filter" and swapped clusters.

 

[obsidian]

Sam, I see the yellow wire on my stock unit but where is this white wire you're talking about? From the crank angle sensor or the cam angel sensor and did you cut it completely and go to the yellow or did you splice the yellow in conjunction with the white wire?