DIY: Spridget starter motor troubleshooting and replacement

Ever since I bought the car I’ve noticed it would sometimes refuse to start, instead making a very loud mechanical grinding or screeching sound. The problem was worse on both very cold or very hot days, especially when the motor was hot. While planning a burger trip to visit some friends in Milton Keynes, the starter actually jammed in the flywheel ring gear. Here’s how to troubleshoot and replace it! Instead of going with a replacement or repaired Lucas inertia starter, I decided to upgrade to a modern “high-torque” Powerlite unit (found on eBay for £185 with free shipping!).

First, let’s dig into why the sound happens, and why it’s a very common issue. An expensive-sounding grinding noise is generally bad news, though it’s bound to happen at some point in Spridget ownership. The original starter in Sprites and Midgets is of the inertia type, commonly nicknamed a “crash” starter.

Inertia Starters

Inertia starters have a pinion gear assembly that is threaded onto the shaft and a bendix gear that sits on the end of the shaft. When the starter receives power and the shaft spins, the pinion doesn’t immediately spin and is pushed up the shaft by the bendix thread. The inertia (that is, reluctance to move) of the pinion gear gives the starters their name. The pinion is thrown into the flywheel’s ring gear, where it stops and will begin to be turned by the starter shaft, thus turning the car’s motor. Once the car’s motor is spinning faster than the starter motor, the pinion is threaded back onto the starter shaft. There’s a strong spring on the bendix to cushion the pinion’s return, as it is often very sudden and violent. The very forceful engagement and disengagement of the starter is why they’re commonly referred to as “crash” starters and causes increased wear on the pinion teeth and flywheel ring gear.

Occasionally, the pinion gear will get stuck in the ring gear and not turn the motor. In my case, the pinion was so stuck that it wouldn’t even disengage by rocking the car hard in gear. There’s an “unstick” bolt on the end of the starter which was rounded off and therefore useless. In the end, a long rod and sharp whack with a mallet freed the starter up and got me home.

Troubleshooting

A loud grinding sound can be caused by several things. First, check that the battery holds enough charge and that all the battery and ground connections are not loose, dirty or damaged (including the body-to-gearbox cable on the bottom of the car). A multimeter or jumper cables to a strong battery can be used. If the starter works fine repeatedly without grinding, the starter may not be receiving enough power to kick the pinion out properly.

If the problem persists, it’s likely the starter is dirty and sticking or damaged. The next step would be to remove the starter and check it over for dirt or damage. The motor tends to rest in the same two positions when turned off so the ring gear teeth may also be damaged at those two points.

If after removing the starter the flywheel and pinion gear look undamaged, try bench-testing the starter by hooking it straight to a battery. Using jumper cables, attach one cable to a mounting ear on the starter and to the battery negative terminal. The positive cable should be attached to the battery’s positive terminal and briefly held to the starter’s power connection. Make sure the starter isn’t run for more than 5 seconds at a time as it gets hot and can short or burn out.

If the starter spins fast and freely, the issue is likely to be with your electrical connections. If it spins slowly or the pinion doesn’t kick out fully, it will require dismantling and a good clean and brush replacement.

My starter ended up having a badly chewed up pinion gear as well as needing new brushes, so I opted for a replacement instead of a rebuild. A starter or alternator repair shop can easily repair these starters for very little cost.

Starter Removal

Removing the starter motor is very straightforward and can be done in 15 minutes. You’ll need an assortment of AF-sized sockets and wrenches to remove the nut on the positive terminal and starter mounting bolts (there are only 2).

Begin by jacking the car up and disconnecting the battery. Loosen or remove the alternator or dynamo so that it can be pushed up and out of the way, removing the belt. Follow the wire from the starter solenoid down to the starter and remove the nut that holds the wire to the power terminal. These starters are grounded to the motor and shouldn’t have a ground wire. Then remove the nut from the top and bottom mounting bolts and simply pull the starter out and through the opening by the alternator or dynamo. Replacing the starter is as simple as removing it.

The original Lucas crash starter next to the Powerlite gear reduction unit

The original Lucas crash starter next to the Powerlite gear reduction unit

Pre-engaged or Gear Reduction Starters

Like I wrote earlier, I decided to replace my old, beat up inertia starter with a more modern starter from Powerlite (http://www.powerlite-units.com/products/rac403/). The RAC403 starter is a direct replacement for the 1275 Spridget’s starter.

There are multiple benefits to a modern starter replacement. Firstly, Powerlite claims their units require 40% less power for cranking the engine. Second, they provide a much softer engagement than the inertia starter (nicknamed crash starters for a reason!). The pinion gear will spin slowly for a second or so until it engages the flywheel, at which point it will crank at full power. This leads to a huge reduction in both pinion and flywheel wear and gives the car a very luxurious starting sound. Fantastic for a daily driver like mine!

Lastly, the modern gear reduction starters give a much more consistent cranking speed over the course of the engine’s rotation. As the engine rotates and the pistons reach TDC (top dead center, i.e. when the piston is at the end of it’s compression stroke), the increased compression in the cylinder leads to a lot of resistance. Worn out inertia starters will slightly slow down at this point, making the car hard to start – especially when hot! Even though the overall cranking speed is almost the same for both starter types, the consistency throughout an engine’s rotation leads to much smoother starts.

With my car requiring anywhere between 4 and 8 starts a day, this definitely sounded like a great improvement!

Note: Fitting this starter is exactly like the old inertia type, however I had to climb under the car with a long socket and an extender to get the bottom mounting nut on – it’s conveniently blocked by the starter’s “hump”. This may not be a problem in right-hand drive cars.

The Results

Here’s a side-by-side video of the car starting with the old starter – note the free spinning at the start, and the new starter. Big improvement!

Update 17/03/2018: Driving to an apartment viewing, the car wouldn’t start after a 30ish minute drive. Turns out the starter failed! I had it sent back to Powerlite who repaired the unit and had it back in my hands in 3 days. Turns out one of the starter’s internal gears chewed itself up – likely caused by a drop during shipping. Powerlite were super helpful and responded immediately when messaged on Facebook. I’ve become quite adept at replacing Spridget starters now!

By | 2018-10-07T11:44:31+01:00 December 20th, 2017|DIY / Tech Tips|0 Comments

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