Another Cooling System problem! '92 740

TomC · Post by **TomC** » 01 Jul 2008, 07:02

I'm having overheating problems with my '92 740, non-turbo.

The engine cooling fan is not operating. I jumped the leads going to the fan sensor and the fan comes on. I replaced the sensor, still will not start automaticly. It's my understanding that if the relay was bad, I wouldn't be able to jump start the fan.

Also, when the car overheats, the temp guage starts to jump around, rapidly. The guage never goes above the normal position and after jumping around between the off position and normal, it just stops working.

As a temporary fix, I'm operating the fan with a toggle switch and the car hasn't given me problem sense.

I have 2 questions;

Can a faulty temp guage sensor cause the cooling fan to not operate?

Can I just keep operating the fan manually or will that cause me more problems?

This is my first post here and I'm hoping someone on here can help me, Thanks

Post by **billofdurham** » 01 Jul 2008, 12:53

Welcome to Matthew's Volvo Site.

The cooling fan on the 740 is electronic and controlled by the fuel system Electronic Control Unit. If the temp sensor is giving a false reading the ECU may not switch on the fan.

On your car you have an on-board diagnostic unit which will show if the temp sensor has set a fault code. Instructions for using the unit are at: https://www.matthewsvolvosite.com/chat/p ... c&start=12. The method is the same as for the turbo but some fault codes may be different. Anything you find that you don't understand post back.

As for the toggle switch it is OK as a stop gap but I would not rely on it for too long as you may be sending false info to the ECU.

Bill.

TomC · Post by **TomC** » 02 Jul 2008, 09:29

Thanks for the information.

I looked for the diagonistic unit you described and I can't find it, could it be in another location?

Post by **billofdurham** » 02 Jul 2008, 11:29

It should be behind the left suspension turret as on all the 740s. Somebody may have moved it but I can't see why.

Bill.

TomC · Post by **TomC** » 02 Jul 2008, 12:31

There is a un-painted metal box, about the size of a pack of cigarettes,
but it can't be opened without unbolting it from it's mounting position.

Thanks for your help

Post by **billofdurham** » 02 Jul 2008, 15:24

I can't think what the metal box is but this is the location of the diagnostic box:

Bill.

TomC · Post by **TomC** » 18 Jul 2008, 08:51

Since my last post, Ive encountered much more serious problems .

The car is overheating again. I discovered that I'm having a coolant circulation problem. I changed the thermostat and the water pump.
I flushed out the radiator(flow seemed good).

Still not working. No circulation. I'm stumped.

I still have not been able to locate the Diaignastic box

The metal box that's there has a vacuum line going to the intake manifold and a electrical connection. I can send a picture if you like,

Thanks for any advice.