m37 ignition switch

[nutsfor59]

hi,need help,my old switch was a 4 wire and the replacement switch is a 2 wire with a spliter on it and 2 leads after that,is there anything i need to know or do the numbers still match up or ??thanks

[cuz]

You'll have to tell us what wire numbers are on the new switch leads.

[nutsfor59]

on the new switch 2 leads exit the switch,one is #11,the other turns into a double connector with numbers 12 and 27,thanks and let me know,this trucks wiring issues are very discouraging !!!,wish i knew someone who could come and check it out or diagnose it, !!

[MSeriesRebuild]

on the new switch 2 leads exit the switch,one is #11,the other turns into a double connector with numbers 12 and 27,thanks and let me know,this trucks wiring issues are very discouraging !!!,wish i knew someone who could come and check it out or diagnose it, !!

#11 is the switch feed, or power input. #12 is the power feed to the distributor, while #27 is the power feed to the gauge spider harness when the ignition switch is turned on. This switch is ok if you don't have any added accessory circuits, however your description sounds like it is the old (early) style. The later style (the one you removed) with 4 pins has 2 input leads and 2 output leads, creating 2 totally separate circuits. This allows for a single isolated circuit for wire #12, then a separate circuit for other needs; such as a heater fan, etc. This is the better switch to use if your truck has extra accessories, less chance of overload and resulting problems.

[nutsfor59]

hi,yes i do have extra stuff and thanks for the heads up,i will look for a correct 4 wire switch and not use the 2 wire,i will sell it or trade it since it's new,thanks

[JBizal]

I have to correct Charles' response. The original M37 switch was a 4 wire unit. 11 in, 27, 12 and 85 out. 85 was an accessory wire which in most cases had a dummy plug end on it. The switch Charles is referring to is the later Packard rubber connector style switch. These switches do require 2 hot leads in to get 2 hot legs out. It does not allow for accessory wire unless you split one of the outputs. Unless you are running a heater, or need an accessory power point, the switch you have will work perfectly. Most M37's had 3 circuit breakers in them. Most M37's only used 2 of them. #15 and #25, which left the 3rd open for accessory power also.
John

[MSeriesRebuild]

I have to correct Charles' response. The original M37 switch was a 4 wire unit. 11 in, 27, 12 and 85 out. 85 was an accessory wire which in most cases had a dummy plug end on it. The switch Charles is referring to is the later Packard rubber connector style switch. These switches do require 2 hot leads in to get 2 hot legs out. It does not allow for accessory wire unless you split one of the outputs. Unless you are running a heater, or need an accessory power point, the switch you have will work perfectly. Most M37's had 3 circuit breakers in them. Most M37's only used 2 of them. #15 and #25, which left the 3rd open for accessory power also.
John

We've used both types numerous times. My intended point was that an accessory could be added without it being in the #12, distributor feed circuit. This is not possible with the early 2-wire switch. I like the later 4-pin style packard much better. It was a definite upgrade to the system; that is why it was incorporated into the trucks at rebuild time or when switch replacement became necessary. The fact that it has 2 wires in / 2 wires out splits electrical load and spreads it over more than just 2 wires as with the early switch. I have seen where people who had a couple of accessories tied into the 2 wire switch have caused over heating of the wires melting the insulation. My point is this, spliting off the second leg with 27/85 for instruments and moderate draw accessories is better. I'd rather lose the accessory alone if a wire issue occured, and still have power to the distributor. Leaving the #12 on a circuit alone stands a much better chance that a power interuption going to the distributor will not occur leaving you stranded with a stalled engine. We've done lots of changes that lessen the chances of issues coming into play. Because it is 1 way on the military diagram does not mean it is necessarily the best way it could have been done. Heavy current draw accessories should be connected to the 3rd breaker or additional circuits with separate breakers or fuses added.

[nutsfor59]

well guys thanks for all this input !! and john bizal my parts master i am bob from pennsylvania ! in regards tyo this switch the one i have has 4 leads with apparently 4 rubber to metal adapters on it,only reason i got a new one was i knocked off the locating pin ,the replacement i got from you was a 2 wire,anyways i am going to split the 12 wire with a doble connector and make to 12's since i have now installed the arctic heater in the right fender and i also have a cab heater i got from you all outstanding stuff by the way !!,i'll need to look again but i am sure i'm running the 3 breakers you guys mention,i just want to get this thing charging correctly and certainly not burn up any wires from overload as mentioned,i have never really been good with wiring tech and am doing my best,also noone near knows 24volt and 100 amp servicing,sooooo i will read this advice over and over a few times and see what happens !!! thanks john and the rest of you so helpfull along the way.