Tech Tips
TechTips is a monthly feature in AutoInc. Each month, specialists who field calls for autoline Telediagnosis® , a technical hot line service, share some of the most common vehicle problems and their solutions.
My Engine Has 4 volts
Recently, I received a Ford hot line call on a 1984 Bronco II for a no-spark condition. The technician working on the vehicle told me that he had replaced the TFI ignition module, Profile Ignition Pickup (PIP) and the ignition coil before contacting us. He also said he was convinced that Fords can't be fixed!For the most part, no-spark conditions are viewed as a "walk- in-the-park" diagnosis. But sometimes, you don't see the forest for the trees. I asked the technician to back-probe the very top wire of the module's connector (the PIP wire) to check the voltage while cranking the engine. He said during cranking that he was seeing about 8.5 VDC. Since PIP is a zero to batt voltage digital square wave with about a 50 percent duty cycle, a typical DVOM will average the signal to around 5-7 volts (depending on whether the signal is being tested during cranking or running). Since 8.5 VDC is not close to half of cranking voltage, I knew the no-spark problem was definitely related to this signal.
The only two things that can cause this problem are a faulty PIP sensor or a poor ignition ground. The PIP sensor was new so it was probably OK. I asked the technician to back-probe the bottom wire on the module's connector (ignition ground) to check the voltage while cranking. It read 4 VDC! The bottom wire of the module connector gets its ground from the engine through the module and distributor. The PIP sensor uses this ground to pull PIP low, to zero VDC.
I asked him to attach the positive lead of his DVOM to the engine block and the negative lead to battery negative, and then to crank the engine while looking for voltage. Sure enough, he had 4 VDC at the engine block! Because of this, the PIP voltage could only cycle from 10.5 VDC to 4.0 volts. The fact is, with a poor ignition ground, the module doesn't charge the coil. The result is a no-spark condition.
What we found was a voltage drop between the battery negative post and the engine block due to a poor connection between the block and the battery negative cable. With the starter engaged and loading the battery, the poor connection was adding a major resistance to the circuit.
Then I asked the technician to run a jumper cable between the engine block and battery negative. I heard the engine roar to life. The technician told me that before fixing the no-spark condition, the vehicle also had problems with the starter dragging. But no more!
Case in point: throwing parts at a vehicle can get expensive. Don't overlook the basics - they're your most valuable tool!
By Mitch Belew, autoline Ford specialist
(18 years experience).No-charge Condition On Early Honda Accords
Yet another charging problem! Recently I ran across a no- charge condition with the charge light lit up on a carbureted 1984 Honda Accord after a binding throttle cable had been replaced. Pretty routine stuff here, but something went haywire because of a small oversight in reinstalling the air cleaner housing.In the air cleaner housing there is a two-wire air temp switch. This switch has male and female bullet connectors for a red wire that goes to the emissions control unit and a black wire that is a dedicated ground. In the same vicinity, the choke heater also has two-wire bullet connectors. There you'll find a white wire with blue tracer that goes to the alternator, where the voltage regulator is the source of voltage to the choke heater after the engine starts. The other wire is a red wire, which is the ground side of the choke heater. This red wire has a parallel circuit. One leg goes to the air temp switch, another leg goes to a resistor by the connector for the firewall emissions box, then it ties into the fuel ECU. After the car is started, the alternator supplies voltage on the white/blue wire to the choke from the voltage regulator and the choke warms slowly with the red wire going through the resistor. Then when the intake air temp switch reaches 65' F, it closes and the choke is grounded directly, warming up much faster now until it opens completely.
Imagine if those two sets of bullet connectors are switched during reassembly. Now we have the harness side wires for the choke heater plugged into the air temp switch. With a warm engine and the air temp switch closed, the white/blue wire from the alternator that was meant to power the choke is taken straight to ground. Since this white/blue wire is not fused after the regulator, the regulator is "terminated."
In 1984 there was a recall on the voltage regulators on early production Accords. After pulling one alternator out of the car to replace the regulator for 1.2 hours of warranty time, I knew there had to be a faster way. I quickly learned how to replace the regulator and brushes in the car. With the oil filter out of the way and the stay that supports the intake manifold from the back of the block removed, the harness, output insulator and rear cover come right off. The whole thing takes about 15 minutes.
By Marlowe Peterson, autoline Asian import specialist
(27 years experience)© 1996, Automotive Information Systems.
Autoline Telediagnosis® with its 29 phone specialists handles over 12,000 phone calls per month from shops all over North America. As an ASA member, you're already signed up to use the hotline service at a discounted rate. Call (800) 288-6210, Monday - Friday, 7 a.m. to 7 p.m., Central Standard Time.
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AutoInc. Magazine ®, Vol. XLIV No. 1, January 1996