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OBDII 4.0L swap tach wiring

Furious

New member
City
Gig Harbor
State
WA
New member here. I have a ‘85 CJ7 that I did a 4.0L OBDII swap into and I’m trying to wire up my CJ tach. The engine was out of an early model 2000 TJ and still has a coil and dist ignition. I have an OMIX or Crown tach in it and I’ve tried just about everything. Last wire I tapped was the A7 GY Ignition Coil Driver wire and the tach didn’t budge. I just did a full frame off restoration and this is the last electrical gremlin. Anyone have any idea of whether I can get this tach to work with the OBDII wiring harness or am I stuck with buying aftermarket like speedhut? Thanks.
 

Randyzzz

Blown Budget
BENEFACTOR
Gold Member
Lifetime Member
SOA Member
City
Redmond
State
OR
No answer to your question, but wanted to say “welcome”!
 

bigwalton

Alaskan Postal nutjob
FORUM MANAGER
SOA Member
City
Dexter
State
MI
New member here. I have a ‘85 CJ7 that I did a 4.0L OBDII swap into and I’m trying to wire up my CJ tach. The engine was out of an early model 2000 TJ and still has a coil and dist ignition. I have an OMIX or Crown tach in it and I’ve tried just about everything. Last wire I tapped was the A7 GY Ignition Coil Driver wire and the tach didn’t budge. I just did a full frame off restoration and this is the last electrical gremlin. Anyone have any idea of whether I can get this tach to work with the OBDII wiring harness or am I stuck with buying aftermarket like speedhut? Thanks.
I missed this, did you ever manage to figure it out? I never had a tach on my OBDII swap before I went to the Speedhut tach, so I don't know how helpful I can be there. Can you find the speedhut instructions and see what they suggested? Being digital, it could be a completely different. I have a thread on here about my Speedhut install and I might have mentioned where I got tach signal in my thread.
 

gr8dain

Hater of Electronics
BENEFACTOR
Gold Member
Lifetime Member
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City
Ashburn
State
VA
Welcome as well. And I am interested in this as well. Trying to get my 4.0 swap buttoned up and would love to know how to feed my aftermarket tach as well.
 

Dave The Sparky

Scrambling Across The Pond
Member
City
Halifax
State
UK
Usually your coil has 2 connections, one is an ignition feed which will be 12Volts when ignition is on and the other cable is the one that signals to fire the coil and pulses with the revs of the engine as its told to fire the spark..
Its the pulsing one you want, i hope it is anyway as thats the one i am going for in my Chevy swap 😁
 

CJ-8_Jim

Scrambler Junkie
Lifetime Member
City
NW Chicago Suburb
State
IL
New member here. I have a ‘85 CJ7 that I did a 4.0L OBDII swap into and I’m trying to wire up my CJ tach. The engine was out of an early model 2000 TJ and still has a coil and dist ignition. I have an OMIX or Crown tach in it and I’ve tried just about everything. Last wire I tapped was the A7 GY Ignition Coil Driver wire and the tach didn’t budge. I just did a full frame off restoration and this is the last electrical gremlin. Anyone have any idea of whether I can get this tach to work with the OBDII wiring harness or am I stuck with buying aftermarket like speedhut? Thanks.
Welcometo the board

OEM CJ tachometer won't work as is with later MPI electronics... Something about an eletronic conversion that is needed. That's why HESCO includes a tach in their MPI package

10+ years ago there was an aftermarket seller that was selling converted oem tachs (as he could find cores or you supply him with a core) on ebay for outrageous prices.
 

bigwalton

Alaskan Postal nutjob
FORUM MANAGER
SOA Member
City
Dexter
State
MI
Welcometo the board

OEM CJ tachometer won't work as is with later MPI electronics... Something about an eletronic conversion that is needed. That's why HESCO includes a tach in their MPI package

10+ years ago there was an aftermarket seller that was selling converted oem tachs (as he could find cores or you supply him with a core) on ebay for outrageous prices.
Wondering if they're firing the plug on the exhaust stroke to burn off any remaining fuel... I know that became a thing to help with emissions. Maybe that's screwing it up.
 

Dave The Sparky

Scrambling Across The Pond
Member
City
Halifax
State
UK
Not the 4.0 but i thought i would check my tacho would actually work with my 2001 Chevy 4.3 engine, it may help others..
Here is the Crown tacho i bought and its instructions.


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Dave The Sparky

Scrambling Across The Pond
Member
City
Halifax
State
UK
The Chevy manual says the 0.5mm white and black (cable 1847) is the tacho (firing signal) so i dug it out of the harness and stripped some insulation off making sure i didn't actually cut it. If this works it will get a soldered joint.

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Dave The Sparky

Scrambling Across The Pond
Member
City
Halifax
State
UK
So after giving the tacho 12volts and temporarily wrapping the green tacho signal wire around the Chevy white and black wire it works fine although it does seem to be a constant 100 RPM out with what the OBD reader is saying but thats a cheap thing as well and i can live with 100 RPM.
It appears to be constant throughout the revs so not sure why?


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Last edited:

Furious

New member
City
Gig Harbor
State
WA
Sorry I haven’t responded sooner. Been busy moving and working on the Jeep. We’re now in Soithern Italy, in COVID lockdown at the moment but I still haven’t gotten the tach to work. Think I’m going to break down and get a new speedhut tach. When can get out I’ll post some pics of the Jeep driving the Amalfi Coast.
 

Scramblin Man

Beyond help
Lifetime Member
City
Woodway
State
Tx
I have heard that if you convert the original 4.2 to a Mopar MPI kit, you have to use a 4cyl tach to make it work correctly. Not sure if that would apply to a 4.0 ??
 

Rosco1974

Scrambler Junkie
BENEFACTOR
Gold Member
Lifetime Member
City
West Haven
State
UT
I have the same tach on my ODBI 4.0L and the RPMs are off, but it fills the hole and "works"
 

gr8dain

Hater of Electronics
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Lifetime Member
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City
Ashburn
State
VA
I am wondering if I look for an aftermarket tach for a 4.0 Cherokee, will it work? And what the instructions say for wiring it up. I really want a working tach for some reason. 😜
 
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