Hello all, I've serached this forum high and low and have not found this question asked yet, so here goes:

My 1970 Ser II OTS starts up and runs fine for abour 40 mins. But when the water temp gets to just before the red line the fans (dual fans) will kick on and then the car runs extremely rough and will stall out if I don't keep on the gas. Once the fans kick back off the car runs fine and smoothly again and idles perfectly. When the fans cycle back on the car runs roughly again and will stall. It doesn't seem that the car is actually overheating, and the fans kick on as they should and then kick off again when temp lowers sufficiently. I'm sctaching my head on what could be happening. I have had a new alternator, fuel pump and battery installed. The car is on a battery tender when parked. The Volt meter reads about 13.7 - 14 when running the car without the fans on, but when the fans kick on the voltage drops to just under 13 and the car runs rough. When the fans kick off again the voltage goes back to the 13.7-14 range and the car runs smoothly. Any ideas on what could be happening? Thanks in advance for any input you might have.

 

You might check the voltage at the (+) plus side of ignition coil to ground, while manually jumping fans to come on.... Do you have points distributor or a electronic version?
If you can duplicate the problem by manually jumping the fans; with engine running, add a electrical jumper from battery (+) plus terminal to the (+) plus side of ignition coil, and jump fans again. If this helps, then you can added a slave relay into the ignition circuit for permanent fix..
Rgds
David

 

It sounds like a load on the alternator may be causing the idle drop, particularly if you installed a higher capacity alternator. A slight dip in the voltage with the added electrical load is normal but what voltage doesn't tell you is the amp draw, which is what puts load on the alternator. If one of the fan motors is failing and has high resistance it can cause a high amp drain and add considerable load on the alt. See what happens of you unplug each of the fans (one at a time) when they're running. If the engine idle becomes normal then the fan motor you just unplugged has a problem. If that fails to tell you anything then the resistance may be in the wiring so closely check all the connectors and relays for corrosion, and see if the fan relay gets really hot when the fans are on. If the fan motors are good you're looking for a dodgy connector or ground that is causing excess load. If still no luck then the next test is to load up as much power demand as possible when the engine is running and the fans are not (headlights, blower motor, A/C if you have it, flashers/brakes etc.). If the engine acts up with all that load then you've got an issue with the alternator loading up too much (did you change the pulley size?) or a problem with the engine taking the load.

 

David and Irgridley - excellent ideas and suggestions. David- I have the points version.

Will begin troubleshooting and loop back with any further Q's.

Thank you!

CM

 

Only put the jumper from the Battery to the + side of the ignition coil with the engine running. If you leave battery voltage to Ignition coil with the engine not running you can burn the points.
Rgds
David

 

I think it sounds like the load on the alternator dropping the idle speed as described above. The voltage readings you gave are very normal - the readings from the guage are only an indication anyway. They really just show if the voltage is dropping a little or steady. If you put a multimeter across the battery when the fans kick in I bet you'd see an entirely different reading to the one you are shown in the car. It could be your carbs/timing isn't set up quite right, and this is shown up when the alternator load kicks in and effects the tickover speed.