where to hook up external tach????

LarryH

Registered User
OK, i've searched and tried to find it in the shop manual so please don't fry me. The only thing I can find is something about a tach test wire near the DIS. Can someone tell me where to connect a diagnostic tach? Thanks!
 
On the earlier model SC's that have the vehicle maintenance monitor in the dash on pin 10 is the tach signal. This where I tapped off on mine for my shift light.
 
Mike, thanks. Could you elaborate on the location of the monitor. I assume under the dash but where???? Thanks! Larry
 
The VMM is in the middle of the dash above the radio. It has a plug on the back of it. If you don't have a VMM and there is a blank panel in it's place there is still a plug for it somewhere behind the blank panel.
 
It's a dark green/yellow wire and it's on one of the corners on the same side as the little latch that locks the plug into the socket.
 
Just so you know, that "tach" wire is not a reliable source of a tach signal. Some cars will not run with a tap on that wire and others will give out a false signal. I have found it is better to tap an injector wire and set your tach for 1 cyl operation (if it has that option).
 
OK, that settles it. Off to sears for a plug wire sensed diagnostic tach. That's what i get for trying to use a 1973 needle tach meter for a points ignition based system. L
 
I pull my shift light signal from there and it always comes on at the shift point. I can adjust it to any rpm and it always triggers and does so consistently. At idle around 950 rpm or so it will occasionally false trigger the light though. I don't know if it's a quirk of the shift light or the signal that causes it but it could make a tach jumpy at idle.
 
I pull my shift light signal from there and it always comes on at the shift point. I can adjust it to any rpm and it always triggers and does so consistently. At idle around 950 rpm or so it will occasionally false trigger the light though. I don't know if it's a quirk of the shift light or the signal that causes it but it could make a tach jumpy at idle.

I'm not saying it doesn't work. I've used that connection many times myself. In fact I use that connection for all my datalogging on the literally 100's of SC's that I've worked on. But every now and then my datalogger won't get a signal from that wire. Occasionally it gets a bad signal. Then there are the times I get those emails from people saying "I hooked up my shiftlight/tach/etc like you said and my car won't run worth a crap!"

So I'm just saying that the circuit is sensitive and it may or may not work on any given SC with any certain tach or shift light. That's all. ;)
 
I got out my EVTM book and traced out the tach wire to the VMM. It comes from an ignition suppression resistor that has its other end connected to the coil. Another ignition suppression resistor is connected to the same point on the coil and goes to pin 4 of the EEC. If the input impedence of the tachometer were too low it could load up the tach signal in effect shorting it out causing a problem. The best way to check the input impedence of the tachometer is to measure its DC resistance to the ground connection. Although, it's not the same thing it will give you a good indication. If the resistance measures more than several 100K ohms, the higher the better, you're probably Ok. If it's less and in the kilo-ohm or lower range it could be a problem. Now that we've thoroughly confused you Larry, try it and see if it works Ok. It's easier than running a wire thru the firewall.
 
wow....and right when i thought I was going to have something intelligent to contribute. I'm taking my sears tachometer and going home now........
 
Anyone have any input on getting a signal if you dont have the wiring for the VMM. My 93 doesnt have the wiring behind the blank plate.
 
Circuit 11 Tachometer?

From E & V TM for a 1990SC, circuit 11 is shown on 5(five) pages attached below.
Looks like Tachometer is on circuit 11, and Pin 4 on the EEC?
.................................................(NOT pin 36, as I have seen on a posted pinout?)
 

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