What during an alternator replacement would cause the engine to overheat?

[Tommy Z]

I just recently pulled the alternator from my 79' SC to have the VR replaced. Re-installed it today along with a new fan belt. Wound up with 1 shim on the fan side and 5 on the pully side to get the belt tight enough and took it for a test drive. Alternator is pumping a solid and steady 14+ volts so all is well there, but the engine is running hotter than normal. The needle on the gauge hit roughly the middle between the 9:00 position and the red zone. It dropped a little as I cruzed along at a slow pace, but it very rarely runs above that 9:00 position normally.

On my test drive, I never saw any indication that the alternator was not charging the battery which led me to believe that the fan is turning as it should at operating speed. I checked the fan belt tension when I got back to the house and it is tight, bordering on too tight. Would an overly tight fan belt cause the engine to run hot? Is there anything else during the alternator removal and re-installation process that I may have botched that would result in the engine over heating? I left no rags or obstructions on top of the engine, behind the alternator. The only thing I may possibly have done is perhaps changed the orientation of the fiberglass wind deflector behind the alternator. It was the one photo I did not take so I couldn't be sure. Would that affect the passage of fresh air over the engine?

Your patience and guidance is appreciated as always, as I learn how to wrench. Thanks!

[unclebilly]

Higher voltage so your gauge is reading differently now.

[Tommy Z]

Originally Posted by unclebilly

Higher voltage so your gauge is reading differently now.

Interesting theory. I'm not sure that's the case though. The normal output for my alternator has always been in the 14V +/_ Range and with the VR replaced, it is now back in that normal 14V range. Why would the new regulator cause the temp gauge to read differently now than before when the VR was operating properly? Let's say it's a correct theory and the engine temp is no different and it's only the gauge, would that mean it is a gauge issue? Engine definitely appeared to be hotter than normal supporting the accuracy of the gauge.

[mike sampsel]

Not sure I believe the temp gauge is affected by the variation in voltage from the alternator. Would think the Porsche temp gauge design system is impervious to voltage swings. I would expect it reads a constant temp for voltage at the battery from 11.5 to 14 or so.

The alternator shroud channels the fan air to various locations. If the shroud were not in correctly it might affect cooling. Not sure this is even possible though, fins up might be the only way to install it. It should be fins towards the sky, I believe though.

[mike sampsel]

Originally Posted by Tommy Z

Interesting theory. I'm not sure that's the case though. The normal output for my alternator has always been in the 14V +/_ Range and with the VR replaced, it is now back in that normal 14V range. Why would the new regulator cause the temp gauge to read differently now than before when the VR was operating properly? Let's say it's a correct theory and the engine temp is no different and it's only the gauge, would that mean it is a gauge issue? Engine definitely appeared to be hotter than normal supporting the accuracy of the gauge.

Are you sure your fan belt is snug and not slipping, a slipping belt can reduce the fan speed. Thought the shims were to be 3 and 3 not the 1 and 5 you state?

[mike sampsel]

One other strange thing which I've experienced. If the woodruf key falls out of the slot, the alternator will run fine, but not the fan. Easy to test as the fan will rotate easily by hand and if the key fell out (because it spins on the shaft of the alternator rather than with it). This would definitely make it run hot. Seems unlikely worth a check though.

[Tommy Z]

Originally Posted by mike sampsel

Are you sure your fan belt is snug and not slipping, a slipping belt can reduce the fan speed. Thought the shims were to be 3 and 3 not the 1 and 5 you state?

The fan belt is definitely tight. The 1 and 5 shim placement means it might even be too tight. The fan does not spin freely independent of the belt.

[Tommy Z]

Originally Posted by mike sampsel

One other strange thing which I've experienced. If the woodruf key falls out of the slot, the alternator will run fine, but not the fan. Easy to test as the fan will rotate easily by hand and if the key fell out (because it spins on the shaft of the alternator rather than with it). This would definitely make it run hot. Seems unlikely worth a check though.

I don't think the woodruff key came out at all but absent of any other ideas it's worth a check. Thanks. I cant imagine the belt being too tight would interfere with the fan function would it? I'll probably shift a shim to the inside tomorrow just to see if it makes a difference. Thanks everyone for your suggestions.

[Showdown]

Did you bump your dizzy at all… check your timing… that could cause higher temps.

[Tommy Z]

Originally Posted by Showdown

Did you bump your dizzy at all… check your timing… that could cause higher temps.

I don't think so but it could be possible.

[Ian Comerford]

It has already been mentioned but have you checked your alternator shroud was installed correctly?

[proporsche]

just a wild guess-have seen it before.By any chance any rug was left behind and slipped onto the oil cooler?

Ivan

[Tommy Z]

Originally Posted by turbofan

Yeah, an overly tight fan belt can make the engine run hotter. I had a similar issue with my old truck where a too-tight belt caused overheating because it put extra strain on the water pump. Also, that wind deflector can make a difference in airflow if it’s not positioned right. Maybe try loosening the belt a bit and checking that deflector.

I’ll try loosening up the belt but as we all know there is no water pump in our air cooled cars 😄

[Tommy Z]

Originally Posted by proporsche

just a wild guess-have seen it before.By any chance any rug was left behind and slipped onto the oil cooler?

Ivan

Thanks Ivan. Yes, I’m sure but I’m going to disassemble this morning anyway and double check. I never had any kind of rag or anything in there while I was working. I’m going to check that air deflector behind the alternator as well.

[blucille]

you should be able to shine a light in there without removing the fan to see if the air deflector is installed correctly or otherwise.

[pmax]

You don't even have to remove the fan, just the passenger side blocking plate will do, 10 min job to take a look "inside".

[MBAtarga]

You can't install the fan air diverter "wrong" - if keyed incorrectly, you won't be able to insert it into the shroud.

[Tommy Z]

Quick update for anyone interested. No obstruction or foreign objects so that wasn't it. Woodruff key was in place. Decided to move one shim from the outside to the inside for 2 shims in and 4 out. Ran it for a solid hour and ran it hard on the highway and the temp never went above normal. I can't tell you I understand the mechanics of it, but it would appear that the overly tight fan belt contributed towards the overheating issue and now that I have loosened the belt a bit, the engine is running nice and cool as it did before the alternator replacement.

[pmax]

Originally Posted by Tommy Z

... I can't tell you I understand the mechanics of it, but it would appear that the overly tight fan belt contributed towards the overheating issue and now that I have loosened the belt a bit, the engine is running nice and cool as it did before the alternator replacement.

Either the fan's rotating in "lockstep" with the engine or the belt's squealing.

Something else is the cause of the overheating.

[Klax]

What oil are you running? I've been using VR-1 20-50 for years and I think it runs hotter than the synthetics I used to use.

I'm growing more convinced that the air here in Colorado is just different, meaning far more variable in temp and humidity level than our friends at/near sea level, and that can cause fairly large swings in running temps from day to day and elevation to elevation. A quick weather check says 36% humidity today vs 9% yesterday. When I lived in Ohio, my car never, never, made it to 9:00 on the gauge, but here I'm always above 9:00 and approaching 10:00 in summer unless I'm coasting down the mountain.