Posted: Wed Jul 24, 2024 8:46 pm Post subject: Internal distibutor block high resistance
Just got this question in a PM from wrprc and I feel all should benefit from it's discussion.
Quote:
The distributor cap (inner) that is on my '53, M38A1 has around 5K Ohms of resistance between 3, of the rotor contacts to their corresponding spark plug wire socket contacts and about 15K Ohms on the remaining rotor contact to spark plug wire socket contact. I haven't been able to find any info that would tell me if this apparent built in resistance is normal or if it is a sign of a defective distributor cap. The engine does appear to be run okay. Does anyone have any info regarding this?
Thanks for the quick reply.
My meter was on auto ranging and the contacts do not appear to be burned or corroded. I'm not sure if this makes any difference but but I've had the Jeep since the late '70s (sitting idle the majority that time) and the distributor cap hasn't been replaced since I've had it. Before I toss the cap (unnecessarily?), I'm curious if something closer to zero ohms is recommended for optimum ignition system performance? Having one measurement that's 3 time greater than the other 3 does seem odd even if "some" resistance is normal. I'm hoping that someone who may have a M38A1 dist cap sitting on the shelf will be wiling to share their resistance test results for comparison?
Thanks for the reply. I'm fairly confident in the meter and the readings. I've had a long career of working with avionics and associated test equipment but even so I'm going to take your suggestion and try another meter (analog) to rule it out.
My first thought was that the resistance should be essentially 0 ohms, but since I don't know much beyond the basics of ignitions systems, I'm keeping an open mind about what's considered normal or not.
This is my first post, however for years I've relied on the amazing wealth of knowledge and advice that is so generously shared on this site. It's gotten me past a number of pot holes on the road to getting my Jeep back on that road. (It's almost there. I hope).
Joined: Oct 02, 2014 Posts: 1990 Location: South Carolina, Dorchester County
Posted: Thu Jul 25, 2024 5:31 pm Post subject:
I agree the resistance from any top contact to the corresponding bottom contact (straight through) should be near zero ohms.
Besides the high resistance you measured, I also see no logical reason why one of the four contacts would read differently than the other three.
I assume the cap is clean (sprayed with contact cleaner to remove carbon and dirt) and carefully inspected for cracks or other defects.
(The engine running good seems to eliminate this possibility.)
I don't have a 24 volt system or cap to check for you, but from what I know about distributor caps of any type --- there should be no resistance.
Caps can fail in odd ways (even intermittently) so maybe the best bet is to buy a quality replacement to compare with.
Keep us posted what you find.
Good luck! _________________ Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51
“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari
The resistance on two caps I had on the shelf was between 2 and 4.5 ohms each post. I've checked the Tm 9-1825 and it offers no basic figure to work with. As Ron said I find that outlandish reading extremely odd. Again you did not comment on my question about corrosion and burned contacts. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
I think you've definitely confirmed that my distributor cap is at least degraded. (FYI - I did confirm similar resistance readings using an analog meter and after recleaning the contact surfaces.) I guess I can't complain about replacing a part that's probably over 50 yrs old. Thanks again for the help with this question and the others that I've had but never posted about.
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