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MG MGB Technical - Over heating 77 MGB

I have a 1977 MGB North American model - twin cooling fans GHN5UH425872G - engine no. 884AEL1879. Heritage says the engine and car numbers match.

I had an ignition problem and finally ran it down to a bad ignition rotor (Paul Hunt's pages on this problem are right on). This car has been converted from the OPUS 45D to a 25D distributor with points. It has an early model head without the coolant port for the ZS auto choke - it now has a Weber DGEV.

Now, the car runs hot, but the cooling fans do not turn on. Prior to replacing the ignition rotor, it ran on the cool side, the needle on the coolant guage never going more than half way across its sweep - even on an August Atlanta Georgia day.

I replaced the thermostat (from a 180 to a 165), then the fan switch in the radiator. The fuse to the radiator fans is good. The needle on he coolant guage still tries to peg on hot after warm up.

Is it possible that there is an issue with the pump impeller? Before I replace the pump, I just want to see if I have exhausted the possibilities.
B Dubuc

Number of issues here.
You changed the dizzie.............has the timing been re-set properly? Ignition too advanced will cause overheating as will too much centrifugal advance.
The carb has been changed............if the mixture is set too weak this will cause overheating.
Is your judgement of overheating, soley the dashboard gauge? If the fan and temp switch are sound and the fans not cutting in, then the gauge could be wrong.
An easy test of the fan circuit is simply to short out the two leads to the temp switch, with ignition on the fans should spin.
Allan Reeling

Thanks Allan:

The fans do work when the circuit is closed, they just don't kick on when the temp gauge indicates that the engine is hot - As for timing and dizzie change, this is the same distributor that I had on the car when I bought it - A PO changed it out. The overheating manifested itself after replacing the rotor button.

I will get the timing light out tonight and make sure that the timing is where is should be, but the car does not ping when under load.

B Dubuc

If you have the cooling outlet for the ZS carb, you have a '75 or later head. Check your coolant temperature with a mechanical or infrared gauge to determine if your dash temp gauge is giving you faulty readings. RAY
rjm RAY

If all this started after changing just the *rotor*, i.e. assuming the distributor conversion had been done earlier and was initially OK, then either it is coincidental or you disturbed something else. Regardless of the gauge the fans should definitely *eventually* come on if running at a fast idle unless Georgia has suddenly got very cold.

If the distributor had been changed immediately before this problem then the the timing *could* now be wrong, but that would just mean the fans should come on earlier than they used to.

These days I'm so suspicious of new parts that I'd suspect your new radiator switch was faulty even if the old one wasn't! First thing now is to measure the temperature of the head and radiator header tank with another instrument as advised. Don't keep swapping parts and hoping, as well as possibly introducing more problems its also expensive (but I'm just tight).
PaulH Solihull

Thank you everyone - the head has been changed out as it does not have a coolant port for a ZS.

Although Georgia hasn't gotten colder, it is November now and we have gotten a cold front through and the temps are down into the 30s at night.

I will have to see about obtaining an external instrument to measure the head temp.

B Dubuc

I would suspect timing although it would need to be noticeably wrong to cause your overheating. Is the new cylinder head gasket blowing - you know how to check. Radiator ? They don't last forever.
Lastly - when the prob is sorted, the V8 fan mod might be worthwhile in Georgia - just a switch on the dash and some wire - gives you fan control in the cabin.
R Walker

I've measured a lot of these miserable fan switches, and found them to vary randomly from around 150F to 240F at turn on, and anywhere from 5 to 100F in hysteresis to turn off - frequently on the same switch! New or used, And different at every run - DO NOT use switch activity as a meaningful fact! I wire them through a relay, switching earth through the switch, with a parallel lead to a dash switch so you can override the rad switch if it really is getting hot.

There are numerous aftermarket fan controls available, that probably actually work AND are settable for operating temp & range. I've sworn to never mess with another standard switch!

Also, DO NOT rely on the dash gauge as a real temperature, until you have verified its action.
Infrared temp indicators might work, but I use a thermocouple attached to the rad at the switch location, insulated from ambient air. However, a cooking thermometer in the rad is pretty good (or glued on and insulated for cars without rad caps).

And put the correct thermostat back in - 165F is too cold; besides poor running and fuel contamination of oil, the fans will never work correctly even if you find a good switch. Fans should come on several degrees above the stat temp, say 200-210F, which should be right on the lower edge of the "H" mark. With the correct cap and 50% AF, it will not boil until well on the high side of 250F.

FR Millmore

I'll say again, if timing faults are causing hot running then the fan stat will be operating to run the fans. If the fans aren't running then either there is a problem with the fan stat, or the coolant simply isn't as hot as you think it is from the gauge. Until you determine the temperature of the coolant independantly you won't know what to investigate first.

210 is only half way between the typical stat temp of 180 and the end of the scale of C/F gauges which is 230. CNH gauges could well be different, but a 180 stat equates to about just over the N on mine, and the switch-on point is of the fans should be 194 which is only about 1/3rd the way from N to H, and also about 1/3rd the way from 18-0 to 230 on C/F gauges.

Having a low stat will affect oil temperature as you say, but I can't see how this will affect the fans. Having a completely separate switch as long as they are immersed in coolant (which they do need to be) then the fans will come on if the coolant temp rises to the appropriate point. Which they should always do at a fast idle as there will be no air-flow through the rad.

Many V8 owners have a manual switch for the fans and indeed mine came to me with one. But as long as the thermostatic switch is working as it should they are pointless. If the switch isn't coming on when it should then there is a problem with the switch and it should be replaced with something that *does* work. If there is some other problem which means the fans aren't cooling as they should then a manual switch is adding nothing. Since I uprated the 12v and earth connections of my V8 fans I have never felt the need to use the switch, but I keep it as it is illuminated and tells me that the fan stat has operated.
PaulH Solihull

Do you have an opinion about whether the later fan switch with spade connectors (and which costs twice as much) is any better than the early switch with the "prong" connector? Are they interchangeable (assuming the proper electrical connections are made)?
B Dubuc

No opinion, but a question as to whether either were original or a copy. I replaced my V8 switch with an SU Burlen as the original Otter was no longer available (at the time anyway). The SU had a slightly lower switch-on point, maybe no bad thing but may just have been because the Otter was getting erratic anyway, but it's problem is a massive hysteresis that means the temp gauge gets nearly right back down to the N before it switches off, which means the fans are running for far more than they used to. Most of the time anyway, occasionally it has a spell of a very small hysteresis which means the fans are switching on and off every few seconds and the temp gauge hardly moves at all. But the real question, before contemplating buying yet another radiator switch, is what is the coolant temperature when the gauge is showing full scale? If the gauge is giving a false reading then spending more money on switches is a waste. Having said that even with a faulty gauge reading the fans should switch on eventually, unless the car happens to be sitting in a very cold air stream passing through the radiator.
PaulH Solihull

This thread was discussed between 03/11/2010 and 07/11/2010

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