6.9 Starter Motor Wiring?

[pal]

Hi Guys,

I have a question I'm hoping you can help me with a starter motor issue.

My 6.9 starter motor got progressively worse over the last month and so I've had it rebuilt. The electrics guy replaced the solenoid and also bushes. The starter was tested on the bench at the garage and it tested out ok.

I fitted this back to the car and the issue appears to be the same. The car won't start (cold or hot) unless the starter body is gently tapped with a spanner. When tapped the starter sometimes sparks before the car starts. Could this be a wiring fault? This is a hard fault now and happens everytime it starts.

When starting with the lights on the lights don't dim much and there is no solenoid click. The fuel pump starts and makes a noise and the dash lights are on. The flywheel is new as it was replaced only a couple of years ago, at the same time the garage replaced the starter. 

There are four wires on the starter:
1) Small Brown (earth)
2) Small Black (iginition voltage?)
3) Big Red (12v)
4) Small Red (12v?)

At the moment the big red and the little red are wired to the same large terminal.
The Brown and black are connected to the solenoid screw terminals respectively.

Does this sound ok? Should both the red wires be connected to the same terminal?
I don't really understand what all of these wires do. I did have a look at the wiring diagram but it confused me even more.

Any help as always appreciated, I'm hoping I've made a simple error.

Thanks
- Pal

[Bandolero]

The wiring sounds ok, otherwise the car wouldn't start properly at all. If you put the wrong wires onto the connections, you will F*** it up.

Sounds to me like a solenoid problem. If it is, don't go to the Merc dealer for a new solenoid as it is an ordinary Bosch one. I had one fitted to my 6.9 and it was only AU$90 for the part.     (Over $600 at the "Stealership.")

[koan]

A motor that needs a "tap" to get it running is usually an indication of warn brushes but that's been done, maybe the wrong brushes have been fitted or incorrectly fitted.

The sparks, where do you see sparks?

If at the terminals something is loose or dirty and needs attention - disconnect battery first.

Looking at the solenoid connections from the front of the car there are two big terminals top (lead goes into starter) and bottom (to battery) and two smaller terminals top-right (to start position ignition switch) and bottom left (to ballast resistors near coil).  If you look closely you should see the ends of some wires soldered to metal tabs on the right hand side - in case solenoid is on upside down.

koan

[pal]

Cool, ok. I'll trace the wires back to the ballast resistor. Recently did some bodywork there, maybe some wires are disturbed or loose. The sparks are sometimes seen between the spanner used for tapping and the starter or solenoid body. After this the starter jumps into life.

If the wires are ok I'll go back to the repair shop with the starter.
Cheers.

A motor that needs a "tap" to get it running is usually an indication of warn brushes but that's been done, maybe the wrong brushes have been fitted or incorrectly fitted.

The sparks, where do you see sparks?

If at the terminals something is loose or dirty and needs attention - disconnect battery first.

Looking at the solenoid connections from the front of the car there are two big terminals top (lead goes into starter) and bottom (to battery) and two smaller terminals top-right (to start position ignition switch) and bottom left (to ballast resistors near coil).  If you look closely you should see the ends of some wires soldered to metal tabs on the right hand side - in case solenoid is on upside down.

koan

[koan]

The sparks are sometimes seen between the spanner used for tapping and the starter or solenoid body. After this the starter jumps into life.

That sounds like the starter is not grounded properly.

Are the two starter mounting bolts tight?

There should be a ground strap somewhere there, between the starter motor or engine and the chassis of the car. It looks like a flat woven braid of fine copper wires, about an inch wide.  Possibly it is dirty and not making contact or is loose, or even missing.

!!! NOTE !!!

Please be careful where you are waving that spanner!  Lead acid car batteries can supply thousands of Amps through the heavy cables to the starter, instantly welding spanners and screwdrivers on contact.

When that happens the battery can boil or rupture spewing acid everywhere.

Always disconnect the battery, starting with ground lead then the positive lead, and re-connect in the reverse order.

koan

[WGB]

I would check that earth strap between motor and chassis.

If there is a fault in it or it has been disconnected inadvertantly several hundred amps have to find another way back to the battery and they can damage other components.

I have seen a diff pinion chewed up (Unlikely on an M-B due to the rubber couplings) due to excess current tracking back to earth through the driveline plus strange things happening to throttle linkages.

I can also bear testimony to Battery power with a scar on my left wrist where I arced my watchstrap between Battery positive and earth (on an A95 of all cars) - the watch strap was glowing red hot and the clasp had welded itself shut permanently.

Take care.

Bill

[pal]

Thanks guys, I will be very careful. I normally wear rubber gloves anyway just in case (just as well).
I will report back on my findings tomorrow after I trace the wires back and check the earthing.

Cheers
Pal.

[jjccp]

Could it be the neutral safety switch?

A while back, I had an intermitent starter problem on the 280 SE, so I replaced the starter. Still had the problem. It turned out that it was the neutral safety switch all along.

[pal]

Hmmm perhaps but the NSS was changed out about 2-3 years ago. In any case knocking the starter gets it going so the NSS is probably ok. I did make sure that the NSS connections were in ok and I compared it to my 350SEL and all looks fine.

Interesting test results today (followed all suggested safety precautions).

I disconnected the starter and tested the four wires as follows:

1) Connected the thick red wire the smaller red wire together as per the starter connection. This then allowed the ignition light on the dash to come on. Of course I was very careful to mind that there was no way that any wires would touch the body or each other.

2) Next I proceeded to test the wires one by one, firstly with ignition off and then on.

3) The large red/small red wires always registered a steady 12ish volts.

4) The brown wire registered 0 with ignition off and then 8.6v with ignition on.

5) The black wire (actually red with white stripe but extended using black) registered as earth. This was also confirmed using a continuity tester and registered as zero.

6) I looked for a copper mesh earth between the starter body and the chassis - this was absent so I used a 25 amp wire and then made a temporary connection between the starter body and the negative terminal of the battery. This didn't make any difference.

So ignition on sends the solenoid out (is 8.6v enough?) to engage the 12v supply from the battery to spin the gear and crank the car.

I reconnected the starter and then left it away from the ring gear pointing downwards (taking extreme care to ensure that no live connections could touch the body or other connections - I used thick old plastic to seperate the terminals).

The solenoid didn't even click out when the ignition was turned on!

I'll be taking this starter back to the electro guy and then get him to retest it.

Thanks for all your help, I will report back when I get the starter back.

Cheers
Pal

[koan]

Don't give up on the starter yet, I think you still have wiring problems. I have wiring diagrams for USA cars but your colours don't match, I can't work out what is what.

The colours from the wiring diagram are:

Heavy wire from battery   Black
Wire to alternator              Red/White
To ignition switch             White/Violet  (to pin 1 of relay code 4 in fuse box)                   
To ballast Resistors          White/Red/violet (to junction of 2 resistors)

A bit of 25 Amp wire is no good for the earth strap it needs to carry about 700 Amps when cranking, Don't do anything more till you install an earth strap between the engine and chassis, you should see signs of where it was. Read WGB's post, the current has to go somewhere, you could end up with a burnt wiring loom.

Now from what you describe:

The thick red is the supply from battery positive, the small red probably goes to the alternator B+ terminal and should bring on the light on the dash which it does.

The brown wire with 8.6 V is what I would expect from the ballast resistors and goes to the bottom left small terminal of the solenoid.

The remaining black wire that you say is earth can only be the ignition switch start position and connects to top right small terminal of the solenoid.

You are probably seeing earth (actually low resistance not earth) through the cold start switch and solenoid. This wire goes to pin 1 of relay code 4 in fuse box (the number is on the wiring). You should be able to measure continuity between the relay socket and the black wire at the starter motor.

Again, don't do anything more until you get an earth strap, if you are not confident of my advice (I'm not!) let the electrician fix it.

koan. 

[pal]

Yep, there is a big thick earth strap under one of the transmission bolts. I guess this should provide the earth I need.

[pal]

Interesting results today  The starter guy put me straight  There's a first time for everything and I think I know a little more today.

I had photographed the starter connections before removal and being a careful sort. I made sure that these went on in exactly the same way. Today I've been shown that these were wrong! The two connections to the solenoid screw terminals for ignition and the ballast connection were the wrong way round. Tapping the starter allowed the solenoid to make enough contact to shoot out even though they were incorrectly wired.

I didn't understand how the old starter could have ever worked. Apparently the old solenoid may have been incorrectly wired but was moving far enough on ignition to make internal contact for the battery supplied 12v to engage.

How weird is that?

Also, of the tests I was doing missed one important part was to test the wire voltages past ignition lights on. I didn't do this before.

This evening I tested the starter on the bench. I secured it (kickback) and then applied 12v +ve to the large terminal. I applied -ve to the starter body. I then made a loop and attached it between the large terminal and the adjacent screw terminal (ignition brown) and hey presto the solenoid clicked out and the gear worked a treat.

After all this - it was probably something simple - Doh! Thanks for all your help, I'll let you know how I get on.

Pal
(terribly bad amateur auto electrician)

[koan]

That's good news - but what about the sparks ?

Something I discovered when cleaning the engine compartment is that solvents can remove the colour stripes from the wires - this doesn't help when trying to work out what goes where.

koan.

[pal]

Um... not sure but if the solenoid wiring is wrong could it be that the main 12v is trying to get across the solenoid the wrong way - it only happens when tapped? Could this be the cause of the sparks I wonder?

I'm dropping the kids to school in a bit and will fit the starter before going into work.

Back soon with the results
Cheers.

[WGB]

Sounds like to work at all there must have been a short when it was tapped - that would explain the sparks.

I assume you have checked for any cracked insulation or other condition such as damp dirt that would allow a short.

Bill