Slow RPM on starter motor

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Model T Ford Forum: Forum 2016: Slow RPM on starter motor
Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Thursday, June 09, 2016 - 05:25 pm:

My 1926 starter motor seemed to be cranking at a slow rpm. So with a new 6 volt battery I checked the voltage drops around the loop. Battery to starter switch, across the switch, switch to starter AND the ground return. I have 1/0 cables from the battery to the switch and then on to the starter. I had 0.75 volts between the battery Positive and the starter, mostly at the switch and another 0.9 volts between the starter and the battery negative. Leaving only 4.35 volts at the starter.

A good cleaning and tightening of all the connections didn't result in much improvement. Then I was thinking about the return ground path. How does the starter current get back to the battery? The battery is tied directly to the frame with a large strap, but the motor to frame is just through various metallic contacts. So I made a motor to frame copper ground strap, connected directly from one of the starter mounting screws to the frame. This ground strap reduced the ground voltage drop to 0.2 volts. Quite and improvement and a noticeable rpm improvement also. For what it's worth. Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jason Given - St. Paul, MN on Friday, June 10, 2016 - 09:56 am:

I had a similar issue with my 26. It ended up with a bad ground. I removed a number of mounting bolts, between the starter to the battery, following the path the ground would need to take to get to the battery. I cleaned up the metal, and bolt to remove rust and paint. When I installed a star washer between the component and the bolt, to bit in to give a good connection. This made a huge difference. I also replace my grounding strap. It looked ok, but once I installed a new one, all was perfect.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Friday, June 10, 2016 - 10:30 am:

Mike,

All that extra grounding should not be necessary. The T engine has very good grounds at both the pan ears and the nose pan bearing. Perhaps someone installed insulating material between the engine and the frame, or mistakenly painted the pan snout or the inside of the bearing surface?

The pan snout area and the inside of the bearing need to be bare metal, and should be assembled with grease so that the pan snout stays rust free and can easily rotate as the frame flexes. The fact that you saw an improvement by adding ground straps proves that you have problems in all of the normal grounding paths.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Friday, June 10, 2016 - 10:43 am:

Jason, I spent quite a bit of time cleaning and tightening connections. A few hundred amperes through a bad connection and the starter can lose a lot of voltage.
Royce, you are probably correct, but a little help is never a problem. A good copper jumper from the frame to the starter will parallel those connections you mentioned and most likely continue to work as those steel connections rust and corrode.
I also tested my starter. With a new battery and spinning freely it would draw 70 amperes and 240 amperes cranking the engine. So I believe my old starter is not perfect, but pretty good. Anyway, it should work fine through this driving season. Thanks Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jason Given - St. Paul, MN on Friday, June 10, 2016 - 11:48 am:

I know on mine, the guy I bought it from, had never stared the engine while on the chassis. He panted the frame then started to put it together. In my case there was a lot of resistance due to the paint.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Semprez-Templeton, CA on Friday, June 10, 2016 - 12:28 pm:

I commend Mike for his understanding of ohms law. Some people will solve the problem by increasing the voltage in the starter circuit (6V to 12V) when the better solution is to reduce the resistance in the circuit. Watts is Watts.


Top of pagePrevious messageNext messageBottom of page Link to this message  By David Dewey, N. California on Friday, June 10, 2016 - 12:50 pm:

I have been told for years to use 2/0 cables. Maybe 1/0 is just borderline?
Decades ago I had a '46 Ch***y for an everyday driver. Only problem was that when warm, it didn't like to turn over. Rebuilt starter with the High Torque fields (we actually had an old-time electrical shop in town then that had such stuff in stock--AND knew about it!), these field were used in the automatic transmission cars. That didn't help, so we just lived with it for a while. Then one day I was checking over adding ground straps, etc. and when I took off the ground cable from the chassis, realized the area still had nice shiny paint. Hmm, cleaned off to bare metal. Problem solved! Over restoration was the situation!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dave Hjortnaes, Men Falls, WI on Friday, June 10, 2016 - 01:51 pm:

Mike

You should show us a picture. I assume the motor is back in the car. You got a lot done yesterday while I was hanging out in Oconomowoc.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Zahorik on Friday, June 10, 2016 - 02:31 pm:

Yes it is, but not completely finished. It's too hot today! Mike


Top of pagePrevious messageNext messageBottom of page Link to this message  By john kuehn on Friday, June 10, 2016 - 03:57 pm:

When I restored my Touring I happened to remember to have a good ground for the starter. I cleaned all the newly painted areas the starter would get ground.
I sanded the paint off the transmission cover where the starter mounts and the starter flanges that bolt to the cover.

Lastly the battery ground cable that bolts to the frame

So far so good on the starter I rebuilt and installed on the car. It turns over pretty good.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Robert Skingley ......Westland, Michigan on Saturday, June 11, 2016 - 06:39 am:

If the starter has just been rebuilt you might bench run it to see if it runs slow. I have seen new nose bushings that needed to be reamed...to reduce the friction. The shaft could also be bent.


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