Is this the reason for hot start problems?

[JSV798]

Most of us will have experienced the hot start problem where after letting the car stand for 5 minutes or so when the engine is hot you get nothing when you turn the key to start, except for the sound of the fuel pump running. If you are lucky the starter motor will turn on the second or third go. The starter solenoid on my car (1978 SC) was replaced in 1997 and in recent years the hot start problem has re-appeared. Never having had the starter off on this car I put the job off until now. I had already bought a 10 mm allen socket and used this to remove the lower barrel nut. However, when reaching round the top of the gearbox I couldn't get it on the top one. Eventually, I found by touch that the top fixing was a standard nut. At this point I thought that it was going to be an engine out job but to my surprise I was able to get a 17 mm open-ended spanner on it and remove it along with a spacer. The arrangement is shown below (note the new solenoid):



The reason for this odd arrangement becomes clear when you look at the next photo.



If you look closely you can see that the top stud is extra long as is another one lower down which holds the gearbox on and is packed out with 3 extra nuts. I assume the reason for these extra long studs might be because the car was originally a sportomatic but was converted to manual before my ownership.

The next photos show the top of the old solenoid and after the cap has been de-soldered:






Here you are looking at the upturned cap on the left and the top of the solenoid (with the cap removed) on the right. There are 2 contacts on the left; the top one is almost completely blackened while the lower one is still bright copper. The right hand shows the corresponding contacts and the one that connects to the blackened is also tarnished but not as badly.
I am indebted to T77911S for his write up of how to dismantle the solenoid and should be read alongside this at:
SOLENOID and STARTER rebuild......finally

Considering how badly these contacts are tarnished it's a wonder the solenoid worked at all. I had already bought a new solenoid for £25 (about $30) which I thought good value and probably better than trying to fix the old one. The new one is not a Bosch part and looks lower quality; the nut seen on where the battery/alternator cables are attached has been replaced by a thin disc and this caused a problem when I was trying to tighten the second nut (not shown) onto the cables as the spanner hit the centre protrusion. I had to find a spare 13 mm nut, clean it up and put it on before the cables in order to be able to tighten the nut.
However, I followed the instructions give by T77911S and have cleaned up the contacts and soldered the old one back together and now wish I hadn't bought the new one! I left am wondering why just one set of contacts was tarnished. It is the one to which the battery cable is attached; the other one connects to the starter motor.

So, if you are having hot starting problems, replace or repair the solenoid. I'll report back on how this repair goes on as I have not tested it on a hot start as yet.

[jakz]

great infoand post, I need to dive into mine hot start is the only real issue my 911 has

jake

[pete3799]

I cleaned the contacts on my old solenoid and got a few months out of it. In the end i put a high torque starter on and haven't looked back. Hope yours lasts longer.

[Targa4now]

You may have unearthed the issue with my car. Any recommendations on a preferred replacement starter would be appreciated. Thank you Pelican Parts

[coldstart]

8 years ago, I put a high torque starter on my 911 and it solved all my starting issues. The original starter had a 'hot relay' wiring that gave the original starter a few more years. My mechanic told me that it wasn't the solenoid.

I think it really great that you took the time to figure out the issue and fix it.

[JSV798]

Not sure that the "hot relay" will help since as I discovered the problem is burned contacts in the solenoid. Not sure either why a high torque starter is better: does high torque mean even more current through the solenoid? In which case it will burn out sooner. Perhaps someone can chime in here as to why a high torque starter is needed.
In my case the starter is 42 years old and now on its second solenoid. Turns the motor over really briskly and so far hot start problem has not re-appeared. While I had the starter off the car I checked the brushes and they were fine. My test is to drive the car until the oil is warm and the come home and park the car in the garage. Try starting after 5, 10 and 15 minutes and started every time. It failed this test before I changed the solenoid. Now starting really well.

[Targa4now]

Is there an easy diagnostic test to rule out the ignition switch over the solenoid/starter, if I may ask?

[boyt911sc]

Originally Posted by Targa4now

Is there an easy diagnostic test to rule out the ignition switch over the solenoid/starter, if I may ask?

Targa,

Test the ignition switch using pin #1 (yellow wire) in 14-pin connector located in the engine bay and at the solenoid terminal. When the yellow wire from the ignition switch is energized, the yellow wire to solenoid (starter) will cause the starter to spin and turn the motor. Either the solenoid is not getting power or it could be defective are the most common culprits. Test and confirm.

Tony

[Michael at Pelican Parts]

Originally Posted by Targa4now

You may have unearthed the issue with my car. Any recommendations on a preferred replacement starter would be appreciated. Thank you Pelican Parts

If you're looking for a high-torque starter, we have a couple of options available from IMI Performance and WOSP in our catalog.

We also have a tech article explaining the installation of the starter motor.

[Targa4now]

I appreciate everyone’s responses. Looks like an afternoon of training to be a contortionist is in my future, ugh

[GH85Carrera]

Originally Posted by Targa4now

I appreciate everyone’s responses. Looks like an afternoon of training to be a contortionist is in my future, ugh

Getting the starter off of a 915 transmission car is easy, but dirty. Just "bear hug" the transmission and you can get the wrenches on there easily to remove the starter by feel. Obviously step one is disconnect the battery, and the electrical connections on the starter.

My story, I had the original Bosch starter work fine for years. Only after a long drive would the starter get heat soaked and just lock up. I took it to a local rebuilder, and he said the grease was dry, and the bearing worn. So I felt confident for the next long trip. Same issue, hot Bosch starter, it locks up until cooled off. It was fine for local trips and even hour long trips, just not an all day trip.

I took it back, and he said I must have a bad connection and the starter was in perfect shape. So I said screw it, and put on the high torque starter. It has been on my car for years, and had many 12 hour days of driving in HOT summer days, and it always spins fast and hard.

If someone wants a good Bosch starter I have mine in the attic. Just let me know, I will sell it cheap.

It does sound "funny" compared to the Bosch starter. But I am use to that now. I just like that it always works like I want.

[Targa4now]

Thanks for the starter offer but I already placed a PP order earlier today. This nagging issue has left me stranded at least 5 times over the last 18 months and calling it frustrating would be an understatement.

I almost always resist buying parts before a complete diagnosis but this has to be the culprit. I also recall the solenoid making a loud metallic click sound every 10th start or so but it mysteriously sorted itself out. I guess this is the price of turning a blind eye.

Thanks again Pelicanites

[JSV798]

Originally Posted by GH85Carrera

Getting the starter off of a 915 transmission car is easy, but dirty. Just "bear hug" the transmission and you can get the wrenches on there easily to remove the starter by feel. Obviously step one is disconnect the battery, and the electrical connections on the starter.

My story, I had the original Bosch starter work fine for years. Only after a long drive would the starter get heat soaked and just lock up. I took it to a local rebuilder, and he said the grease was dry, and the bearing worn. So I felt confident for the next long trip. Same issue, hot Bosch starter, it locks up until cooled off. It was fine for local trips and even hour long trips, just not an all day trip.

I took it back, and he said I must have a bad connection and the starter was in perfect shape. So I said screw it, and put on the high torque starter. It has been on my car for years, and had many 12 hour days of driving in HOT summer days, and it always spins fast and hard.

If someone wants a good Bosch starter I have mine in the attic. Just let me know, I will sell it cheap.

It does sound "funny" compared to the Bosch starter. But I am use to that now. I just like that it always works like I want.

Did your mechanic change the solenoid? In my experience it is the solenoid that burns out. In my case the original starter worked for nearly 20 years without trouble so until your high torque starter has been working for over 20 years it's not possible to know if is better than the original.

[JSV798]

Originally Posted by Targa4now

Is there an easy diagnostic test to rule out the ignition switch over the solenoid/starter, if I may ask?

I noticed that I could hear the fuel pump running which is also powered when the key is turned to start on a yellow wire. I think that this is a safety feature although I have heard that cars earlier than my 1978SC had the fuel pump running when just the ignition was on. Once the engine starts the microswitch on the air flow sensor keeps the fuel pump running: if the engine stops the switch opens and the fuel pump stops.
So you should be able to hear the fuel pump run if the ignition switch is OK.

[911pcars]

On some starters, the armature is a tight fit rotating within the stationary field coils in the motor housing. When heat soaked, the armature can expand. This decreases the rotating clearance which can stall the motor. During cranking, electrical current (~60-150A or more) flows into motor, more if the armature can't rotate. This increases the temperature and exacerbates the issue.

During bench testing, inspect the armature and field coils for signs of friction (rubbing interference) and discoloration from overheating.

generic starter assy.:


In addition, check the solenoid plunger contact disc as described in the preceeding posts.

Sherwood

[JSV798]

Originally Posted by Targa4now

I appreciate everyone’s responses. Looks like an afternoon of training to be a contortionist is in my future, ugh

I put off this job for years but it was much easier than I thought. I have a (very) shallow pit that I can run the car over which gives me just enough room to crawl under and lie under the gearbox with my feet sticking out the rear. One nut is easily reached but to get to the other one you have to reach round with your right hand over the top of the gearbox and find the nut by feel. You will need a 10 mm allen socket of decent length for the barrel nuts; the one I bought wasn't long enough on its own but had an extension that helped do the job. I can get the starter off in less that 10 minutes and back on in a similar time but it is heavy. And it can get dirty if you have had oil leaks. I had my car cleaned up with a steam clean after fixing oil leaks but there was still oil on the top of the gearbox.

This is what I used. The allen socket is just over 4 inches but could do with being longer. It has a 3/4 inch drive so had to use a 3/4 inch extension and the a 3/4- 1/2 inch converter to fit my 1/2 inch drive ratchet (I've had my 1/2 inch drive socket set now for over 50 years!)

[boyt911sc]

Originally Posted by jsv798

i noticed that i could hear the fuel pump running which is also powered when the key is turned to start on a yellow wire. I think that this is a safety feature although i have heard that cars earlier than my 1978sc had the fuel pump running when just the ignition was on. once the engine starts the microswitch on the air flow sensor keeps the fuel pump running: If the engine stops the switch opens and the fuel pump stops.
so you should be able to hear the fuel pump run if the ignition switch is ok.

JSV,

When the AFS (air flow sensor) switch is closed or grounded the fuel pump will not run. You can verify this by testing terminal #85 of the fuel relay. When the engine stops, the air flow sensor plate will drop down and close the ground switch unless the air flow switch has been disconnected or disabled.

Tony

[JSV798]

Originally Posted by boyt911sc

JSV,

When the AFS (air flow sensor) switch is closed or grounded the fuel pump will not run. You can verify this by testing terminal #85 of the fuel relay. When the engine stops, the air flow sensor plate will drop down and close the ground switch unless the air flow switch has been disconnected or disabled.

Tony

Yes I have this back-to-front in my analysis of the wiring diagram, see below:



I found the workings of the fuel pump relay a challenge. Just to see if I have this correct, the diagram shows pin 87a connected to pin 30 which means that when the ignition is on (run position) the fuel pump will be running so AFS switch will be open (AFS plate raised by intake air flow). That is, diagram shows state when engine is running. With ignition on but engine not running pin 85 is grounded via AFS switch and relay flips to connect pin 87 to 30, and since pin 87 is not live (yellow wire) pump does not run. But when key is turned to start, yellow wire becomes live and pump runs. Is this correct?

[Flat6pac]

On a 77 and later S or SC if the fuel pump runs with the ignition on, the air flow sensor and the cold start switch are reverse plugged. The blue and green plugs, generally looking the same color, to the front of the injection are reversed.
The cold start plug should ohm with the temp connector on the right chain cover.
Bruce

[boyt911sc]

Originally Posted by JSV798

Yes I have this back-to-front in my analysis of the wiring diagram, see below:



I found the workings of the fuel pump relay a challenge. Just to see if I have this correct, the diagram shows pin 87a connected to pin 30 which means that when the ignition is on (run position) the fuel pump will be running so AFS switch will be open (AFS plate raised by intake air flow). That is, diagram shows state when engine is running. With ignition on but engine not running pin 85 is grounded via AFS switch and relay flips to connect pin 87 to 30, and since pin 87 is not live (yellow wire) pump does not run. But when key is turned to start, yellow wire becomes live and pump runs. Is this correct?

JSV,

Search the thread by DKLEVER48 regarding FP relay & socket operation. Everything you need to know were discussed in the thread and will give you a better understanding how they work in conjunction with the ignition switch. All the 5 terminals were discussed in that thread and will be a good reading specially if you are working with CIS:
30
87a
87
86
85

If you have any question, just ask. There are plenty of help available in this forum.

Tony