Stevep Posted February 22, 2023 Report Share Posted February 22, 2023 A customer sent a 48 Desoto to me that has been sitting for a while. I got it running well but the transmission doesn't automatically upshift. It has a new transmission harness but there is no fuse in it and I was wondering if it is a special fuse and if so, where can I get one? Thanks in advance Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted February 22, 2023 Report Share Posted February 22, 2023 The trans relay fuse is a SFE30...a 30 amp fuse ..It's right about 1-3/8" long. Note...That 1948 Chrysler M-5 transmission really doesn't even need any 6V electrical power just to be able to upshift .... It the transmission has enough oil in it and even with out any of the wiring hooked up it should imediately upshift when letting off the gas. But also know the transmission cannot downshift until at a dead stop without 6V control...45lbs oil pressure needs to bleed off at a dead stop. The 1946-48 M-5 transmissions need the 6V electrical wiring and controls only to tell it when to and when not to upshift or downshift. The electrical system for this M-5 transmission only controls oil pressure flow to the upshift piston. Just some excess info for you!? 1 Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted February 22, 2023 Report Share Posted February 22, 2023 This must be the fuse a the kickdown relay. Dodgeb4ya beat me to it. My fuse holder was bent, thus no power. Ii straightened it. Sometimes you can find the right amperage fuse, but it is too short to make a good connection. Probably because my fuse holder had been bent. So I added a spacer the same diameter as the fuse. Here's the relay, looking from across the engine. The tube at the upper right is the fuse holder. Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted February 22, 2023 Report Share Posted February 22, 2023 Donald, that wiring terminal block to the right of your relay is an add on, right? Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted February 22, 2023 Report Share Posted February 22, 2023 Right. I have eight spots on a junction block connecting most of the engine wiring. I have a scheme where I can quickly switch from my Pertronix system back to the breaker point system if necessary. It's a bit over the top, I know. I have more confidence with the Pertronix system than when some people reported failures some time ago. The failures may have has something to do with not putting that resistor in the kickdown circuit. But now that I think about it, since I'll be going to Canada for the DeSoto Convention this summer, I may want to buy a spare Pertronix unit, or pack the old distributor parts and coil. When things went awry in the past I've ben flat-bedded home, but never yet across an international border. 2 Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted February 23, 2023 Report Share Posted February 23, 2023 (edited) What the M-5 transmission KD relay looks like inside ... One other thing... there is a short card board like insulator tube stuffed down inside the relay fuse holder... with out it the fuse will burn out immediately. The fuse is a 1-7/16" SFE 30....30 amp fuse. Edited February 23, 2023 by Dodgeb4ya 1 Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted February 23, 2023 Report Share Posted February 23, 2023 Do they still make that SFE 30 amp fuse? I've never seen the inside of a trans. relay. Back in the 80s, which I think of as the days of cheap NOS parts, I bought a NOS M-5 trans. relay as a backup to my original. Quote Link to comment Share on other sites More sharing options...
Dodgeb4ya Posted February 23, 2023 Report Share Posted February 23, 2023 Evidently SFE fuses are still available... 1 Quote Link to comment Share on other sites More sharing options...
MarcDeSoto Posted February 23, 2023 Report Share Posted February 23, 2023 I should buy a few in case they stop production! 1 Quote Link to comment Share on other sites More sharing options...
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