No crank on a 71 850 spider

L

Loren Field

True Classic
Hi guys – I can use some help with my 71 850 spider. I was trying to track down why the charge light on the stays on when the engine is revving, and after swapping out the alternator, the engine stopped cranking (the indicator lights on the dash still come on and the gauges energize when I turn the key to on position, but nothing happens when I advance the key to the start position. I pulled the red wire from pin #50 on the back of the ignition switch and jumped it directly to the battery, and the starter spun as usual which I think indicates that the circuit from the ignition switch to the starter is OK. Since the gauges and indicator lights on the dash come on when the key is turned to on, I think that the circuit from the positive terminal of the battery to pin #30 on the back of the ignition switch is also OK (that would be the rosa/pink wire in the wiring diagram for the car). Given this, I am thinking that the ignition switch is bad. Can you think of anything else I should check before getting a new ignition switch? It is kind of odd that it stopped working when I was mucking with the alternator, but it seems that the diagnosis is straightforward. On the bright side it was quite convenient for it to crap out in my garage rather than 50 miles down the road. As always, many thanks for your thoughts and for the help keeping the beast running. Best, Loren in Indy.
 
Have you measured through the ignition switch itself? When in Start, you should read a connection from the VBatt input contact to the switch to the Start output contact of the switch. In this way, you could make sure whether the switch is still functional in Start.
 
A good suggestion indeed, no connection between VBatt input and Start output when the key is turned to start. Time to replace the switch. Thanks again, Loren
 
Loren Field said:
A good suggestion indeed, no connection between VBatt input and Start output when the key is turned to start. Time to replace the switch. Thanks again, Loren
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You might consider a starter relay after the switch replacement to ensure good current flow and save your future switch.

I know the 124 and X switches can be taken apart with effort and cleaned, if yours isn’t in bad condition otherwise might this be a good alternative.

You could also consider a “concealed” momentary switch for the starter with a relay, it could be only energized when the ignition switch is in ‘run’ position.
 
So the car was running but the red light was staying on, you changed the alternator & now the car won't start, & so the ignition switch is being blamed?

I'd be re-checking the wiring connections to the starter motor, as they're likely to have been knocked around when you changed the alternator - particularly the wire with the female spade terminal on it.
 
Hi Karl - thanks for the thoughtful suggestions - all make sense :). Best, Loren
Jeff Stich said:
So the car was running but the red light was staying on, you changed the alternator & now the car won't start, & so the ignition switch is being blamed?

I'd be re-checking the wiring connections to the starter motor, as they're likely to have been knocked around when you changed the alternator - particularly the wire with the female spade terminal on it.
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Hi Jeff, I suspected the ignition switch after first checking the wiring to the starter, and then by showing that the car would start if I simply bypassed the switch by disconnecting the wire that attaches to terminal 50 on the back of the switch and jumping it directly to the battery. khnitz suggested removing the switch and directly checking for continuity between the VBatt input (terminal 30) and the Starter output (terminal 50) when the key is advanced to start; I did that and there was no continuity through the switch. This seems to indicate that the switch is not working, unless I am messing up on the terminals that I am checking. Let me know if I've got it wrong. Thanks, ljf
 
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