4.2l to 4.0l tach issue - Pirate4x4.Com : 4x4 and Off-Road Forum

locked up xj · 05-04-2006, 04:40 AM

I'm looking at buying a 90 YJ that had an 99XJ 4.0l swapped in and have a few questions for yall.

The guy said they never wired in the tach ... something about how it could mess up the computer? Is there something I do not know about wiring in a tach? Should I have any problems if I try to install an aftermarket tach? Is there a way to run the 90 tach?

matts88yj · 05-04-2006, 06:39 AM

There should be a signal wire coming from the MPI harness. When I swapped in my MPI kit, it was grey with a blue tracer. That is the signal wire. I had to then run the ground to the negative side of the coil to get a correct read out. When and if you have to do this, do it with the engine running since when you splice into the negative side of the coil, it can short out the computer and then it wont be able to run. I am running an Autometer Phantom Tach and it is a lot better than the factory stuff.

Matt

MattPascoe · 05-04-2006, 07:50 AM

And if you use the factory tach, you DO NOT use the MPI signal wire.

I'd have to look through my MPI notes, but you connect the factory tach to the coil IIRC.

Mohadeab · 05-04-2006, 04:04 PM

On the 4.0 just splice into the green wire on the coil to the hot lead to tach and then ground.

locked up xj · 05-04-2006, 05:06 PM

Quote:

Originally Posted by matts88yj

There should be a signal wire coming from the MPI harness. When I swapped in my MPI kit, it was grey with a blue tracer. That is the signal wire. I had to then run the ground to the negative side of the coil to get a correct read out. When and if you have to do this, do it with the engine running since when you splice into the negative side of the coil, it can short out the computer and then it wont be able to run. I am running an Autometer Phantom Tach and it is a lot better than the factory stuff.

Matt

Do it with the engine running? What about dissconnecting the puter before doing the splice? Just curious if that would be OK as well.

Thanks for all the help guys.

CJ5-Man · 05-04-2006, 05:56 PM

I think he means do it on a known running/functional vehicle, not during an engine swap since it could cause a no start problem....

the stock computer out works for 4 cylinder tach's. On mine I just tapped the secondary coil wire like everyone else on this thread so far.

05-04-2006, 04:40 AM	#1 (permalink)
locked up xj Registered User Join Date: Aug 2004 Member # 34440 Location: College Station, TX Posts: 142	4.2l to 4.0l tach issue I'm looking at buying a 90 YJ that had an 99XJ 4.0l swapped in and have a few questions for yall. The guy said they never wired in the tach ... something about how it could mess up the computer? Is there something I do not know about wiring in a tach? Should I have any problems if I try to install an aftermarket tach? Is there a way to run the 90 tach? __________________ noeither.com

05-04-2006, 06:39 AM	#2 (permalink)
matts88yj Rock God Join Date: Nov 2005 Member # 62615 Location: Lancaster, PA Posts: 1,121	There should be a signal wire coming from the MPI harness. When I swapped in my MPI kit, it was grey with a blue tracer. That is the signal wire. I had to then run the ground to the negative side of the coil to get a correct read out. When and if you have to do this, do it with the engine running since when you splice into the negative side of the coil, it can short out the computer and then it wont be able to run. I am running an Autometer Phantom Tach and it is a lot better than the factory stuff. Matt __________________ Unstacking Hardass

05-04-2006, 07:50 AM	#3 (permalink)
MattPascoe Registered User Join Date: Nov 2003 Member # 24805 Location: Lake Hodges, CA Posts: 80	And if you use the factory tach, you DO NOT use the MPI signal wire. I'd have to look through my MPI notes, but you connect the factory tach to the coil IIRC. __________________ '89 YJ

05-04-2006, 04:04 PM	#4 (permalink)
Mohadeab Registered User Join Date: Jun 2001 Member # 5344 Location: Aloha, Oregon Posts: 273	On the 4.0 just splice into the green wire on the coil to the hot lead to tach and then ground. __________________ If you don't learn from History, then you are doomed to repeat it

05-04-2006, 05:56 PM	#6 (permalink)
CJ5-Man Registered User Join Date: Apr 2001 Member # 3976 Location: College Station, Tx Posts: 1,475	I think he means do it on a known running/functional vehicle, not during an engine swap since it could cause a no start problem.... the stock computer out works for 4 cylinder tach's. On mine I just tapped the secondary coil wire like everyone else on this thread so far. Last edited by CJ5-Man; 05-04-2006 at 05:56 PM.

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