Tach adapter needed? • The Dodge Challenger Message Board

by **freddesjunne** » 03 Sep 2007 21:16

Hi,

my engine builder is about to mount the ignition system, an all MSD system (6AL, coil and distributor). I have two questions.

1. The MSD box produces a 12 volt square wave signal with a 20% duty cycle. Does anyone know if this is close enough to trigger my ralley dash tach? MSD says a "current triggered tach" needs a tach adapter. I guess the opposite would be a voltage triggered one?

2. The engine builder says he's not too sure about using the old ralley dash tach together with this modern ignition system, and therefore he suggests I should use a modern tach to be sure not to damage the MSD box in case the tach short circuits. MSD has kept me in the phone queue for 45 minutes now, from overseas, so I figured I'd ask you guys instead; MSD wouldn't build a box as sensitive as that, would they? I'd rather use my old tach and not litter the coupe too much with more gauges.

Thanks in advance
/Fredrik

by **dave-r** » 04 Sep 2007 8:13

Stock tachs do not work with MSD units. I think they do an adapter or something though that allows this?

by **dave-r** » 04 Sep 2007 8:20

Yes. It says in the instuctions for mine (digital 6 plus) that you need adaptor PN8910 if your original tach was originally triggered from the coil -ve and you are using the white wire from the MSD unit.

by **drewcrane** » 27 Oct 2007 17:13

well i did the swap and i think there are 2 adaptors, i have them both i cant get the stock tach to work,it might be the tach. but i did get it to work on a sun tach. an after market unit. however my stock tach might be broken. you might have to play around with it but you might have to spend alittle money to find out what works ill look in my garage now to see what i have

by **drewcrane** » 27 Oct 2007 17:20

ok the adaptor i am currently using is msd tach adaptor part numbor 8920 that is what im using with 6al msd and a small sun tach, as far as the stock tach try it i doubt it will hurt the msd ive abused mine and it still works,i do have a spare fro test reasons but they are pretty durable make sure youe stock tach works with the stock system first if you can , and then switch over.

by **spitfire9137** » 28 Oct 2007 13:02

Doesnt that 6AL box have an output wire for a tach? I was under the impression those wires didnt need any adapters.

by **dave-r** » 28 Oct 2007 14:35

Ref: my answers above.

Many aftermarket tachs do not require an adaptor. However stock tachs usually do. Mopar tachs are in that group that do. Adaptor PN8910 to work with the MSD I have.

by **Jon** » 28 Oct 2007 14:46

You might check with Greg over at RT Engineering. He knows alot about Mopar electronics.