LT1 to Datsun tachometer wiring

[wheelman]

I'm working on my wiring harness right now and need some info regarding how others have wired their LT1 PCM or OptiSpark outputs to the Datsun tachometer.

 

I have a 280Z tach I'll be using as well as the JTR manual that explains the 15K ohm resister to keep it from jumping around so I think I have that part covered.

 

My question is which of the tach signals have you guys used?

The low resolution signal from the OptiSpark, Red PCM connector - pin 13

or

The PCM tachometer output, Black PCM connector - pin 2

or

something else.

 

If I use the tach output will I still need the 15K ohm resistor?

 

One other question, can I combine all the ground connections going to the PCM into one main ground with the overall vehicle ground or are there some that must remain separate to create digital vs analog grounds?

 

Same question regarding power? I know there are switched and non-switched direct battery connections but can they be combined into switched and non-switch power buses so to speak.

 

Wheelman

[80LS1T]

I would use the tach out put signal from the PCM(RED CONNECTOR...A13)( I think you got your connectors mixed up....A13 is the tach output and B2 is the low res. signal from the opti). It should be a white wire(it is for a 95' Z28 LT1) I dont know if you will need the resistor or not since I am using an Autometer tach?

 

As far as I know a ground is a ground. But power I would wired up to different fuses. That way it will be easier to diagnose problems later. Not to mention not all the powers are rated at the same ampres. Like the injector fuse is rated at 7.5A and the oxygen sensors have a 10A rating. You dont want to protect a circuit that is rated at 10A max with a 7.5A or vise versa.

 

Guy

[flibuoy]

I asked the same question some months ago and received the same answers you have received so far!

 

Both points you mention are sources...the aftermarket harness I have has a dedicated "tach" wire that comes off the red 13 and is noted as the "cleanest" signal". I am prepared to add/delete resistance and adjust through slot in back of tach...I doubt it will miraculously be correct and steady.

 

Again, addressing grounds...ALL grounds in the entire aftermarket harness are joined to a single ground within the harness, to then be a single ground to battery ground...just be sure chassis/engine/pcm grounds are all good then the single point is apparently OK idea.

 

There are some really well versed folks on the list...LT1s are becoming more common as is the LS....it is just that sometimes they miss the post..or figure somebody will answer John

[wheelman]

Guy,

Good catch, you're correct I did list them wrong in my post. I was looking at a pin out list and trying to keep it straight in my head while I typed and crossed them.

Maybe I'm a bit dyslexic or something.

 

I plan on putting the PCM in the passenger footwell and I'm trying to rewire the system in a way that reduces the number of wires going through the firewall. I'll have to look at the different circuits and determine whether each power connection can be sourced from a line already in the engine bay or behind the dash and then put in whatever fuses I need to protect everything. I have the relays from the donor car so I'm going to build a board for those as well.

 

Flibuoy,

I suspected the grounds were all the same but thought I read somewhere that it was a good idea to keep the analog and digital grounds separate to prevent dirty signals. Anyway I guess it doesn't matter so I'll combine them all into one and use the Datsun ground wire that is already routed through the firewall.

 

I have a bunch of amphenol 13 pin connectors that I'm thinking of using as a means to route most of the PCM circuits through the firewall, what do you guys think of the idea? If your not familiar with these connectors I can post some pictures but they are military grade sealed locking round connectors that can handle wire guage of about 20. I think I can route all the PCM connections using 4 of these and still have some pins left over but my concern is that the wires won't be large enough.

 

Wheelman

[MikeJTR]

The coil on the LT1 is the same as on TPI and TBI engines from 1987 through 1995 cars and trucks. The LT1, however, does not have the extra wire for the tachometer hook-up.

To connect the tachometer to the LT1, get a connector from the coil of a 1987-1995 car or truck.

[theBrit]

I dont suppose anyone has a part number or a specific vehicle I should reference for the truck connector mentioned by MikeJTR ?

I am guessing that with that info I could go to the GM counter and get one.

 

Thanks Mike

[MikeJTR]

1987-1992 V8 Camaro's have the coil connector with the tachometer wire.

[240playtoy]

my '93 LT1 also has the wire, along with something that lookes like a capacitor added in line on it's way to one of the harness connectors. But my tach cuts out when it approaches 3000 rpm. (have the 15k equivalent in line)

[240playtoy]

By wire, I mean a wire coming from that 4th post on the coil.

[Guest 73LT1Z]

(1995 LT1)

I wired my tach to the 4th terminal & got nothing. I spliced into the white/black wire to the coil & the tach seems to work OK (jumps around at low RPM but I need to add the resistor).

[wheelman]

73LT1Z,

I should have updated this thread with what I ended up doing to get my tach to work.

I connected the tach input to A13 and adjusted the pot in the tach as the JTR manual suggests and it's a smooth as a baby's butt. No 15K resistor needed. I ended up using a scan program on my laptop to match the tach reading to the actual RPM but it was very close using the JTR adjustment method.

 

Wheelman

[Guest 73LT1Z]

Excellent info- thanks a lot!